The Reproductive System: Embryology and Human Development

28 The Reproductive System: Embryology and Human Development

PowerPoint® Lecture Presentations prepared by Steven Bassett Southeast Community College Lincoln, Nebraska

• Development involves: • Differentiation of cells • Reorganization of cells • Development can be characterized by different periods of time • Prenatal development • Embryology • Postnatal development

• Development can be characterized by different periods of time • Prenatal development • Conception to delivery • Involves embryology (development during the prenatal period) • Postnatal development • Development from birth to maturity

• Fertilization is the joining of two haploid cells to create a diploid cell • Function of the haploid cells • Spermatozoon • Delivers the paternal chromosomes to the ovum • Ovum • Provides the maternal chromosomes • Provides nourishment for embryonic development

• Fertilization occurs in the ampulla of the uterine tube • 200 million sperm cells enter the vaginal canal • Only about 10,000 make it to the uterine tubes • Less than 100 actually contact the egg • Only one will fertilize the egg

• Fertilization details • When the egg is ovulated, it is surrounded by the corona radiata, which protects the egg as it is being ovulated • Numerous sperm cells release hyaluronidase, from their acrosomal cap, in an effort to decompose the corona radiata

ANIMATION Fertilization and the Preparations for Cleavage

Oocyte at Ovulation Fertilization and Oocyte Activation Pronucleus Formation Begins Ovulation releases a secondary oocyte and Acrosomal enzymes from multiple sperm create The sperm is absorbed into the cytoplasm, the first polar body; both are surrounded by gaps in the corona radiata. A single sperm then and the female pronucleus develops. the corona radiata. The oocyte is suspended makes contact with the oocyte membrane, and in metaphase of meiosis II. membrane fusion occurs, triggering oocyte activation and completion of meiosis. Corona First polar radiata body Fertilizing Second polar Nucleus of Female spermatozoon body fertilizing pronucleus spermatozoon

Zona pellucida

Cytokinesis Begins Amphimixis Occurs and Spindle Formation and Cleavage Begins Cleavage Preparation The first cleavage division nears completion The male pronucleus develops, and spindle roughly 30 hours after fertilization. Further Metaphase of first fibers appear in preparation for the first events are diagrammed in Figure 28.2 cleavage division cleavage division.

Male Female pronucleus pronucleus

Blastomeres

Events at fertilization and immediately thereafter

Zona pellucida

Events at fertilization and immediately thereafter

Male Female pronucleus pronucleus

Blastomeres

Events at fertilization and immediately thereafter

• Prenatal development is gestation period (9 months) • Prenatal development is divided into trimesters • First trimester (rudiments of all organs appear) • Second trimester (fetus looks like a human) • Third trimester (organs become functional, rapid growth)

• The First Trimester • 1 to 12 weeks • Four events within the first trimester • Cleavage (sequence of cell reproduction) • Implantation (implantation into endometrial lining) • Placentation (formation of the placenta) • Embryogenesis (development of the embryo)

• Cleavage and Blastocyst Formation • Cell division results in the formation of blastomeres • A solid ball of cells eventually develops – this is a morula • Some cells migrate to one “edge” of the morula creating a mass of cells and a hollow cavity called the blastocoele • The ball of cells is now called the blastocyst • The outer layer of the blastocyst consists of cells called the trophoblast

Blastomeres

Polar bodies 4-cell stage 2-cell Early morula stage

DAY 1 DAY 2 DAY 3 Advanced DAY 4 morula First cleavage division DAY 0: Inner cell Fertilization mass DAY 6

Ovulation Blastocoele

DAYS 7–10: Implantation in Trophoblast uterine wall Blastocyst (See Figure 28.3)

• Cleavage and Blastocyst Formation (continued) • Trophoblast cells provide nutrients to the developing embryo • The inner cell mass consists of stem cells that will develop into all the cells of the body

• Implantation • Upon contact with the endometrial lining, the trophoblast cells divide rapidly • The trophoblast cells “fuse” with the endometrial lining forming a syncytial trophoblast • This layer of cells releases hyaluronidase to erode away more of the endometrial lining so the mass can implant

• Implantation (continued) • Upon implantation, the inner cell mass separates from the trophoblast area • When the inner cell mass separates from the trophoblast, two cavities form: • Amnionic cavity • Blastocoele cavity

ANIMATION Implantation

DAY 6 FUNCTIONAL LAYER UTERINE OF ENDOMETRIUM CAVITY

Uterine glands Blastocyst

DAY 7

Trophoblast

Blastocoele Inner cell mass

DAY 8

Cellular Syncytial trophoblast trophoblast

DAY 9 Hypoblast Blastodisc Epiblast

Developing primary villi Amniotic cavity

Lacuna

• Formation of the Blastodisc • A layer of cells forms between the amnionic cavity and the blastocoele cavity • The layers are called • Epiblast • Hypoblast

Hypoblast of blastodisc

Lacunae Yolk sac

Blastocoele

Uterine lining DAY 10

The blastodisc begins as two layers: the epiblast, facing the amniotic cavity, and the hypoblast, exposed to the blastocoele. Migration of epiblast cells around the amniotic cavity is the first step in the formation of the amnion. Migration of hypoblast cells creates a sac that hangs below the blastodisc. This is the first step in yolk sac formation. © 2012 Pearson Education, Inc. Prenatal Development

• Gastrulation and Germ Layer Formation • Eventually some cells of the epiblast move toward the center of the blastodisc creating a primitive streak • This movement is called gastrulation • As the cells move toward the primitive streak area, they begin to migrate between the epiblast and hypoblast layers • This creates three distinct layers of cells

• Gastrulation and Germ Layer Formation • The three layers of cells are: • Ectoderm (derived from the epiblast layer) • Mesoderm (new layer between the epiblast and hypoblast) • Endoderm (derived from the hypoblast layer)

ENDOMETRIUM Syncytial trophoblast Amniotic cavity Cellular trophoblast Epiblast of Ectoderm blastodisc Blastodisc Hypoblast of blastodisc Primitive Lacunae Yolk sac streak

Blastocoele

Uterine lining Mesoderm Endoderm DAY 10 DAY 12

The blastodisc begins as two layers: the epiblast, Migration of epiblast cells into the facing the amniotic cavity, and the hypoblast, exposed region between epiblast and to the blastocoele. Migration of epiblast cells around hypoblast gives the blastodisc a the amniotic cavity is the first step in the formation of third layer. From the time this the amnion. Migration of hypoblast cells creates a sac process (gastrulation) begins, the that hangs below the blastodisc. This is the first step in epiblast is called ectoderm, the yolk sac formation. hypoblast endoderm, and the migrating cells mesoderm.

• The ectoderm, mesoderm, and endoderm are collectively known as the germ layers • Each layer will form specific tissues and organs of the body • Germ layers will also form structures involved in embryonic survival called extraembryonic membranes

• Formation of Extraembryonic Membranes • There are four major extraembryonic membranes • Yolk sac • Amnion • Allantois • Chorion

• Formation of Extraembryonic Membranes • Yolk sac • This is a pouch that extends from the hypoblast cells into the blastocoele • It is the early site for blood cell formation • Amnion • Amnionic fluid fills the amnion cavity, which surrounds and cushions the embryo and fetus

• Formation of Extraembryonic Membranes (continued) • Allantois • Eventually gives rise to the urinary bladder • Chorion • The mesoderm and trophoblast layers form together to form the chorion • The chorion will eventually develop extensions into the endometrium

Migration of mesoderm around the inner surface of the trophoblast The embryonic disc bulges into the amniotic cavity at the head creates the chorion. Mesodermal migration around the outside of fold. The allantois, an endodermal extension surrounded by the amniotic cavity, between the ectodermal cells and the mesoderm, extends toward the trophoblast. trophoblast forms the amnion. Mesodermal migration around the endodermal pouch creates the yolk sac. Amniotic cavity (containing Amnion amniotic fluid)

Syncytial Allantois trophoblast Head fold Yolk of embryo Cellular sac trophoblast Chorion Chorion Mesoderm Syncytial trophoblast Yolk sac Chorionic villi Blastocoele of placenta

The developing embryo and extraembryonic membranes bulge The embryo now has a head fold and a tail fold. Constriction of into the uterine cavity. The trophoblast pushing out into the uterine the connections between the embryo and the surrounding lumen remains covered by endometrium but no longer participates trophoblast narrows the yolk stalk and body stalk. in nutrient absorption and embryo support. The embryo moves away from the placenta, and the body stalk and yolk stalk fuse to form an umbilical stalk. Tail fold

Uterus Body stalk Myometrium Yolk stalk Decidua basalis Yolk sac

Umbilical stalk Embryonic gut Placenta Yolk sac Embryonic head fold Chorionic villi of placenta

Decidua capsularis

Decidua parietalis

Uterine lumen Decidua parietalis

Decidua basalis

Umbilical cord

Placenta

Amniotic cavity

Amnion The amnion has expanded greatly, filling the uterine cavity. The fetus is connected to the placenta by an elongated Chorion umbilical cord that contains a portion of the allantois, blood vessels, and the Decidua remnants of the yolk stalk. capsularis

• Placentation • The placenta begins to form when the chorion produces villi (chorionic villi) that extend into the endometrial lining • The body stalk connects the embryo to the chorion • As the fetus develops and moves farther into the uterus, it obtains its nutrients via the umbilical cord

The developing embryo and extraembryonic membranes bulge into the uterine cavity. The trophoblast pushing out into the uterine lumen remains covered by endometrium but no longer participates in nutrient absorption and embryo support. The embryo moves away from the placenta, and the body stalk and yolk stalk fuse to form an umbilical stalk. Uterus Myometrium Decidua basalis Umbilical stalk Placenta Yolk sac Chorionic villi of placenta

Decidua capsularis Decidua parietalis Uterine lumen Decidua parietalis

Decidua basalis Umbilical cord

Placenta

Amniotic cavity

Amnion The amnion has expanded greatly, filling the uterine cavity. The fetus is connected to the Chorion placenta by an elongated umbilical cord that contains a portion of the allantois, blood vessels, and the Decidua remnants of the yolk stalk. capsularis

Decidua capsularis Umbilical Yolk Amnion cord (cut) Placenta sac

Decidua basalis Chorion

Umbilical vein

Umbilical arteries

Chorionic villi

Area filled with maternal blood

Decidua parietalis

Myometrium Uterine cavity

Cervical (mucous) plug in cervical canal Maternal blood vessels External os Cervix Amnion Trophoblast (cellular Vagina and syncytial layers)

For clarity, the uterus is shown after the embryo has been removed and the umbilical cord cut. Blood flows into the placenta through ruptured maternal arteries. It then flows around chorionic villi, which contain fetal blood vessels. Fetal blood arrives through paired umbilical arteries and leaves through a single umbilical vein. Maternal blood reenters the venous system of the mother through the broken walls of small uterine veins. Maternal blood flow is shown by arrows; note that no actual mixing of maternal and fetal blood occurs.

• Embryogenesis • Shortly after gastrulation, embryogenesis begins • By week 4, a head fold and tail fold develop • The embryo is separated from the blastodisc area

The embryo now has a head fold and a tail fold. Constriction of the connections between the embryo and the surrounding trophoblast narrows the yolk stalk and body stalk.

Tail fold

Body stalk Yolk stalk

Yolk sac

Embryonic gut

Embryonic head fold

• Embryogenesis • At about 12 weeks, organs begin to form • This is called organogenesis

Medulla

Ear

Pharyngeal Forebrain arches

Eye

Heart Somites

Body stalk Arm bud Tail Leg bud

Fiber-optic view of human development at week 4

Chorionic villi

Amnion

Umbilical cord

Placenta

Amnion

Umbilical cord

• Second Trimester • Fetus is covered by the amnion sac • Fetus grows faster than the placenta • Third Trimester • All fetal organs become functional

• Changes in the uterus during gestation • Uterus will increase in length from 7.5 cm to 30 cm • Contains almost 5 L of fluid • The uterus and contents weigh about 22 pounds • Abdominal organs are pushed out of their normal positions

Placenta

Uterus Umbilical cord Amniotic fluid Fetus at Cervix 16 weeks Vagina

9 months 8 months After 7 months dropping, 6 months in preparation 5 months to delivery 4 months 3 months

Liver Small intestine

Stomach Pancreas Transverse colon

Aorta Fundus of uterus

Common iliac vein

Umbilical cord

Placenta Cervical (mucus) plug in cervical canal

External Urinary bladder os Pubic symphysis Vagina Rectum

Urethra

• The goal of labor is parturition (expulsion of the fetus) • There are three stages of labor • Dilation stage • Expulsion stage • Placental stage

• The dilation stage • The fetus begins to move down the cervical canal • The cervix dilates • The amnion sac breaks • The expulsion stage • The birth of the child • The placental stage • Ejection of the placenta

Umbilical The Dilation Stage Placenta cord Sacral Pubic Cervix Vagina promontory symphysis

Cervical canal

Fully developed fetus before labor begins The Expulsion Stage

The Placental Stage

Uterus Ejection of the placenta

Umbilical The Dilation Stage Placenta cord Sacral Pubic Cervix Vagina promontory symphysis

Cervical canal

Fully developed fetus before labor begins

Uterus Ejection of the placenta

• This is the period from birth to one month • Events that occur from birth to one month are: • Lungs fill with air • Blood circulation changes with the closing of the ductus arteriosus and the foramen ovale of the heart • Heart rate drops from 120–140 beats per minute to about 70 • Breathing rate drops from 30 per minute to the normal rate

• Events that occur from birth to one month are (continued): • Kidneys filter the infant’s own blood • Digestive system becomes active • Metabolic rate is increased to maintain warmth for a few days after birth

• Overview of Embryology • The development of the: • Integumentary System • Endocrine System • Skull • Heart • Vertebral Column • Cardiovascular • Appendicular Skeleton System • Muscles • Lymphoid System • Nervous System • Respiratory System • Spinal Cord • Digestive System • Brain • Urinary System • Special Sense Organs • Reproductive System

• The Development of the Integumentary System • 1 month: epithelium overlies mesenchyme • 3 months: epithelium multiplies to form layers; mesenchyme develops into connective tissue thus forming the dermis • 4 months: epidermis begins to grow into the dermis thus forming columns of cells; hair follicles, sweat glands, and sebaceous glands develop from these columns

1 MONTH At the start of the second month, the superficial ectoderm is a simple epithelium overlying loosely organized mesenchyme.

The Development of the Integumentary System

Germinative cells

Connective tissue Epithelial Blood vessel column 3 MONTHS Mesenchyme Over the following weeks, the epithelium becomes stratified 4 MONTHS through repeated divisions of During the third and fourth months, the basal or germinative cells. small areas of epidermis undergo The underlying mesenchyme extensive divisions and form cords of differentiates into embryonic cells that grow into the dermis. These connective tissue containing are epithelial columns. Mesenchymal blood vessels that bring cells surround the columns as they nutrients to the region. extend deeper and deeper into the dermis. Hair follicles, sebaceous glands, and sweat glands develop from these columns.

• The Development of the Integumentary System (continued) • Skin: at 3 months, the epithelium continues to thicken • Melanocytes migrate into the stratum basale layer • Nails: develop from the thickening of the epidermis

The Development of the Integumentary System NAILS

Ectoderm Nail field

SKIN Loose Fingertip connective tissue As basal cell divisions continue, the epithelial layer thickens and the basal 4 MONTHS Melanocyte lamina is thrown into irregular folds. Nails begin as thickenings of the epidermis Germinative Pigment cells called melanocytes near the tips of the fingers and toes. These cell migrate into the area and squeeze thickenings settle into the dermis, and the between the germinative cells. The borderline with the general epidermis becomes epithelium now resembles the distinct. Initially, nail production involves all of epidermis of the adult. the germinative cells of the nail field. Dermis The embryonic connective tissue Dense differentiates into the dermis. Fibroblasts Nail root Eponychium Nail bed Nail connective and other connective tissue cells form tissue from mesenchymal cells or migrate into the area. The density of fibers increases. Loose connective tissue extends into the ridges, but a deeper, less vascular region is dominated by a dense, irregular Subcutaneous collagen fiber network. Below the dermis layer the embryonic connective tissue develops BIRTH into the subcutaneous layer, a layer of loose connective tissue. By the time of birth, nail production is 4 MONTHS restricted to the nail root.

The Development of the Integumentary System

HAIR FOLLICLES Hair Sebaceous gland

Hair column Sebaceous gland

Papilla

5 MONTHS A hair follicle develops as a deep BIRTH column surrounds a papilla, a At birth a hair projects from the small mass of connective tissue. follicle, and the secretions of the Hair growth will occur in the sebaceous gland lubricate the epithelium covering the papilla. hair shaft. An outgrowth from the epithelial column forms a sebaceous gland.

The Development of the Integumentary System EXOCRINE GLANDS

Epithelial column Duct of sweat Mesenchyme gland

5 MONTHS BIRTH A sweat gland develops At birth, sweat gland as an epithelial column ducts carry the secretions elongates, coils, and of the gland cells to the becomes hollow. skin surface.

The Development of the Integumentary System

MAMMARY GLANDS Hollowing Epidermis nipple

Epidermal thickening Branching duct Developing Fat duct 5 MONTHS BIRTH Mammary glands develop in At birth, the mammary glands a comparable fashion, but the have not completed their epidermal thickenings are development. In females, further much broader and extensive elaboration of the duct and gland branching occurs. system occurs at puberty, but functional maturity does not occur until late in pregnancy.

• The Development of the Skull • 5 weeks • Cartilage begins to appear • Cartilage within the brain area enlarges to form the chondrocranium • Skull walls and floor are beginning but no roof • 9 weeks • Endochondral ossification appears; frontal and parietal structures appear

© 2012 Pearson Education, Inc. Embryology Summary 28.2 The Development of the Skull (Part 1 of 7) The Development of the Skull Pharyngeal cartilages Second arch (hyoid) First pharyngeal arch (mandibular) Arches 3, 4, 6 Brain

Eye Nose

5 WEEKS After 5 weeks of development, the central nervous system is a hollow tube that runs the length of the body. A series of cartilages appears in the mesenchyme of the head beneath and alongside the expanding brain and around the developing 5-WEEK nose, eyes, and ears. These cartilages are shown in light blue. EMBRYO Five additional pairs of cartilages develop in the walls of the pharynx. These cartilages, shown in dark blue, are located within the pharyngeal, or branchial, arches. (Branchial refers to gills—in fish the caudal arches develop into skeletal supports for the gills.) The first arch, or mandibular arch, is the largest. © 2012 Pearson Education, Inc. Embryology Summary 28.2 The Development of the Skull (Part 2 of 7) The Development of the Skull

Chondrocranium

Brain

Eye

Nasal capsule

Vertebrae

8 WEEKS The cartilages associated with the brain enlarge and fuse, forming a cartilaginous chondrocranium (kon-dro-KRA- ne-um; chondros, cartilage  cranium, skull) that cradles the brain and sense organs. At 8 weeks its walls and floor are incomplete, and there is no roof. © 2012 Pearson Education, Inc. Embryology Summary 28.2 The Development of the Skull (Part 3 of 7) The Development of the Skull

Frontal bone

Occipital Sphenoid bone

Maxilla

The mandible forms as dermal bone develops Hyoid bone around the inferior Larynx portion of the mandibular arch. 9 WEEKS During the ninth week, numerous centers of endochondral ossification appear within the chondrocranium. These centers are shown in red. Gradually, the frontal and parietal bones of the cranial roof appear as intramembranous ossification begins in the overlying dermis. As these centers (beige) enlarge and expand, extensive fusions occur. © 2012 Pearson Education, Inc. Embryology Summary

• The Development of the Skull (continued) • 10 weeks • Mandibular arch fuses with the chondrocranium • Hyoid arch begins to form • 12 weeks • Ossification is well under way • Birth • Cranial roof remains incomplete • The anterior and posterior fontanels are present

© 2012 Pearson Education, Inc. Embryology Summary 28.2 The Development of the Skull (Part 4 of 7) The Development of the Skull The dorsal portion of the mandibular arch fuses with the chondrocranium. The fused cartilages do not ossify; instead, osteoblasts begin sheathing them in dermal bone. On each side this sheath fuses with a bone developing at the entrance to the nasal cavity, producing the two maxillae. Ossification centers in the roof of the mouth spread to form the palatine processes and later fuse with the maxillae.

Parietal bone Frontal bone

Maxilla

Mandible

10 WEEKS The second arch, or hyoid arch, forms near the temporal bones. Fusion of the superior tips of the hyoid with the temporals forms the styloid processes. The ventral portion of the hyoid arch ossifies as the hyoid bone. The third arch fuses with the hyoid, and the fourth and sixth arches form laryngeal cartilages. © 2012 Pearson Education, Inc. Embryology Summary 28.2 The Development of the Skull (Part 5 of 7) The Development of the Skull

Parietal Frontal bone bone Occipital bone

Maxilla Zygomatic arch

Mandible

Temporal bone

12 WEEKS After 12 weeks ossification is well under way in the cranium and face. (Compare with Figure 5.6, p. 123.) © 2012 Pearson Education, Inc. Embryology Summary 28.2 The Development of the Skull (Part 6 of 7) The Development of the Skull

Frontal Parietal bone bone

Mandible Occipital bone

BIRTH The skull at birth; compare with the situation at 12 weeks. Extensive fusions have occurred, but the cranial roof remains incomplete. (For further details, see Figure 6.19, p. 165.) © 2012 Pearson Education, Inc. Embryology Summary 28.2 The Development of the Skull (Part 7 of 7) The Development of the Skull

Nasal septum

Palatine arch

Normal

Abnormal

Cleft palate

Bilateral cleft lip and palate

If the overlying skin does not fuse normally, the result is a cleft lip. Cleft lips affect roughly one birth in a thousand. A split extending into the orbit and palate is called a cleft palate. Cleft palates are half as common as cleft lips. Both conditions can be corrected surgically.

• The Development of the Vertebral Column • 4 weeks • On either side of the developing spinal cord are blocks of mesenchymal tissue called somites • The medial side of the somites produces the vertebral column • 8 weeks • Vertebral cartilage forms around the spinal cord • The sacrum and coccyx are fused together

• The Development of the Vertebral Column (continued) • 12 weeks • Ribs and sternum undergo ossification • The tips of the longer ribs do not ossify; they remain as cartilage thus forming the costal cartilages

© 2012 Pearson Education, Inc. Embryology Summary 28.3 The Development of the Vertebral Column (Part 1 of 6) The Development of the Vertebral Column Pharyngeal arches Ear Somites Spinal cord Somite Eye Sclerotome Heart Notochord

Tail The developing spinal cord lies posterior to a longitudinal rod, the notochord (NO-to-kord; noton, back  chorde, cord). 4-WEEK In the fourth week of development, mesoderm on either side of the spinal EMBRYO cord and notochord forms a series of mesenchymal blocks called somites (SO-mits). Mesenchyme in the medial portions of each somite, a region known as the sclerotome (SKLER-o-tome; skleros, hard), will produce the vertebral column and contribute to the floor of the cranium.

The Development of the Vertebral Column Spinal cord Mesenchyme Neural arch Tubercle Spinal cord in of somite Spinous process of rib spinal canal Tubercle of rib Muscles of back Head of rib Transverse Centrum of process vertebra Ventral Ossification Cartilaginous body centers rib cavity 8 WEEKS 12 WEEKS The cartilages of the vertebral centra About the time the ribs separate from the grow around the spinal cord, creating vertebrae, ossification begins. Only the a model of the complete vertebra. In shortest ribs undergo complete ossification. the cervical, thoracic, and lumbar In the rest, the distal portions remain regions, articulations develop where cartilaginous, forming the costal cartilages. adjacent cartilaginous blocks come Several ossification centers appear in the into contact. In the sacrum and coccyx, sternum, but fusion gradually reduces the cartilages fuse together. the number.

BIRTH At birth, the vertebrae and ribs are ossified, but many cartilaginous areas remain. For example, the anterior portions of the ribs remain cartilaginous. Additional growth will occur for many years; in vertebrae, the bases of the neural arches enlarge until ages 3–6, and the spinal processes and vertebral bodies grow until ages 18–25. © 2012 Pearson Education, Inc. Embryology Summary 28.3 The Development of the Vertebral Column (Part 4 of 6) The Development of the Vertebral Column Sclerotome Notochord Intersegmental mesenchyme

Somites

Cartilage of vertebral body 4 WEEKS 6 WEEKS Cells of the sclerotomal The migrating cells differentiate segments migrate away into chondroblasts and produce from the somites and a series of cartilaginous blocks cluster around the that surround the notochord. notochord. These cartilages, which will develop into the vertebral centra, are separated by patches of mesenchyme.

Intervertebral disc

Vertebra

Nucleus pulposus 8 WEEKS ADULT Expansion of the vertebral centra eventually eliminates the notochord, but it remains intact between adjacent vertebrae, forming the nucleus pulposus of the intervertebral discs. Later, surrounding mesenchymal cells differentiate into chondroblasts and produce the fibrous cartilage of the anulus fibrosus. © 2012 Pearson Education, Inc. Embryology Summary 28.3 The Development of the Vertebral Column (Part 6 of 6) The Development of the Vertebral Column

8 WEEKS 9 WEEKS Rib cartilages expand away from the developing transverse processes of the vertebrae. At first they are continuous, but by week 8 the ribs have separated from the vertebrae. Ribs form at every vertebra, but in the cervical, lumbar, sacral, and coccygeal regions they remain small and later fuse with the growing vertebrae. The ribs of the thoracic vertebrae continue to enlarge, following the curvature of the body wall. When they reach the ventral midline, they fuse with the cartilages of the sternum. © 2012 Pearson Education, Inc. Embryology Summary

• The Development of the Appendicular Skeleton • 4 weeks • Ridges appear along the length of the embryo • An accumulation of ridges at the superior end and the inferior end form pairs of limb buds • 5 weeks • Limb buds develop a core of cartilaginous tissue • Cartilaginous models develop between 5 and 8 weeks • 10 weeks • Ossification occurs

Cartilage primordia

Notochord

Cartilaginous core of limb bud

Limb buds Mesenchyme

4 WEEKS 5 WEEKS In the fourth week of development, After 5 weeks of development, ridges appear along the flanks of the the pectoral limb buds have a embryo, extending from just behind cartilaginous core and scapular the throat to just before the anus. cartilages are developing in the These ridges form as mesodermal mesenchyme of the trunk. cells congregate beneath the ectoderm of the flank. Mesoderm gradually accumulates at the end of each ridge, forming two pairs of limb buds.

The formation of the pelvic girdle and legs closely parallels that of the pectoral complex. But as the pelvic limb bud enlarges, the apical ridge rotates medially rather than laterally. As a result, the knee joint faces posteriorly, 1 5 /2 WEEKS while the elbow faces anteriorly.

Pelvic girdle

5 WEEKS Lower limb

8 WEEKS 7 WEEKS

By week 8, cartilaginous models of all of the major skeletal components are well formed, and endochondral ossification begins in the future limb bones. Ossification of the hip bones begins at three separate centers that gradually enlarge. © 2012 Pearson Education, Inc. Embryology Summary 28.4 The Development of the Appendicular Skeleton (Part 3 of 4) The Development of the Appendicular Skeleton

Cartilaginous Apical Humerus core of scapula ridge Mesenchyme of pectoral girdle

1 5 /2 WEEKS As the limb bud enlarges, bends develop at the future locations of the shoulder 7 WEEKS and elbow joints. Two The hands originate as cartilages form in the paddles, but the death forearm, and a lteral of cells between the rotation of the apical phalangeal cartilages ridge places the elbow produces individual in its proper orientation. fingers. 5 WEEKS

Cartilage Humerus Joint cavity

Ossified bone Scapula

Joints form where two cartilages are in contact. The surfaces within the joint cavity remain cartilaginous, while the rest of the bones undergo ossification. © 2012 Pearson Education, Inc. Embryology Summary 28.4 The Development of the Appendicular Skeleton (Part 4 of 4) The Development of the Appendicular Skeleton

BIRTH 10 WEEKS The skeleton of a newborn infant. Ossification in the embryonic Note the extensive areas of cartilage skeleton after approximately (blue) in the humeral head, in the 10 weeks of development. The wrist, between the bones of the shafts of the limb bones are palm and fingers, and in the hips. undergoing rapid ossification, Notice the appearance of the axial but the distal bones of the skeleton, with reference to the two carpus and tarsus remain previous Embryology Summaries. cartilaginous. © 2012 Pearson Education, Inc. Embryology Summary

• The Development of Muscles • 4 weeks • Mesoderm on either side of the notochord forms somites • The medial portion of each somite forms skeletal muscles; this region is called the myotome • 6 weeks • Myotome area develops into posterior muscles called epaxial muscles and anterior muscles called hypaxial muscles

The Development of the Muscles

Mesoderm from the parietal portion of the lateral plate Near the head, mesoderm forms and the adjacent myotome skeletal muscle associated with forms the limb buds. the pharyngeal arches. Myotome Sclerotome Gut Pharyngeal Migrating mesodermal arches Limb bud cells (arrows show directions of movement) Lateral plate Eye mesoderm Lateral plate (parietal layer) (visceral layer) Heart Coelom Somites The ventral mesoderm does not form segmental masses, and it remains as a sheet called the lateral plate. Somites A cavity appears within the Umbilical lateral plate of the chest and stalk abdomen; this cavity is the After 4 weeks of development, mesoderm coelom. Formation of the on either side of the notochord has formed coelom divides the lateral somites. The medial portion of each somite plate into an inner visceral will form skeletal muscles; this region is layer and an other parietal called the myotome. layer.

4 WEEKS

Hypaxial mesoderm in the trunk grows around the body wall toward Eye the sternum in company muscles with the ribs. This creates a mesodermal layer that extends from the chin to Epaxial muscles the pelvic girdle. Hypaxial muscles

Extensors Lung

Arm Rib bud Flexors

Heart Each limb bud has a flattened distal The hypaxial mesoderm Sternum tip, with a thickened near the sacrum migrates apical ridge. As caudally to produce the cartilages appear muscles of the pelvic in the limb buds, floor. surrounding Myotomal muscles organize mesodermal cells around the developing from the lateral vertebral column in two plate and myotomes groups, one dorsal (epaxial differentiate into muscles) and the other myoblasts. ventral (hypaxial muscles).

6 WEEKS

• The Development of Muscles (continued) • 7 weeks: muscles of the face, back, and abdomen develop • 8 weeks: limbs enlarge and muscles continue to develop

Muscles forming Mesoderm of the hyoid (second) Epaxial muscles remain arranged at the pharyngeal arch migrates over the lateral and in segments. These deep muscles arches are ventral surfaces of the neck and include the intervertebral muscles. associated with the surfaces of the skull to form Superficial epaxial muscles form the the head and the muscles of facial major muscles of the erector neck. The muscles expression. spinae group. of mastication develop from the mesoderm Mesoderm of the third, fourth, surrounding the and sixth pharyngeal arches Intervertebral muscles mandibular arch. forms the pharyngeal and Erector spinae intrinsic laryngeal muscles. Extensors

Pharyngeal myoblasts Flexors form a superficial layer Eye that later subdivides to muscles Migration of myoblasts create the trapezius and over the dorsal surface sternocleidomastoid of the trunk creates muscles. limb extensors; migration of ventral myoblasts produces the flexors.

Quadratus lumborum Transversus abdominis

Internal oblique Stomach External oblique Rectus abdominis The oblique, transverse, and rectus muscle groups 7 WEEKS develop in the hypaxial layer.

Flexors

Extensors

Flexors 8 WEEKS While the limb buds enlarge, BIRTH additional myoblasts invade the limb from myotomal segments Rotation of the arm and leg buds nearby. Lines indicate the produces a change in the position boundaries between myotomes of these masses relative to the providing myoblasts to the limb. body axis. © 2012 Pearson Education, Inc. Embryology Summary

• The Development of the Nervous System • 2 weeks • Somites appear on either side of the notochord • A neural plate is formed • A crease develops along the length of the neural plate • The edges of the crease are the neural folds • The neural folds “fold” to form a tube (neural tube) • 3 weeks • The neural tube becomes the CNS • Axons extending from the neural tube become the PNS

Neural Neural plate plate After two weeks of development, somites are appearing on either side of the notochord. The ectoderm near the midline thickens, forming Notochord an elevated neural plate. The neural plate is Somite largest near the future head of the developing embryo. 20 DAYS Neural groove A crease develops along the axis of the neural Neural fold plate, creating the neural groove. The edges, or neural folds, gradually move together. They first contact one another midway along the axis of the neural plate, near the end of the Neural tube third week.

Where the neural folds meet, they fuse to form a cylindrical neural tube that loses its connection with the superficial ectoderm. The process of neural tube formation is called 21 DAYS neurulation; it is completed in less than a week. The formation of the axial skeleton and that of the musculature around the developing neural tube were described on pages 770 and 774.

The Development of the Nervous System

Neurocoel Cells at the tips of the neural folds do not participate in neural tube formation. These cells of the neural crest Head Neural at first remain between the dorsal surface of the neural crest tube and the ectoderm, but they later migrate to other locations. The neural tube becomes the CNS. Axons from neurons within the neural tube and the axons of neural crest cells form the PNS.

Schwann cell

Sensory neurons The first cells to appear in Autonomic the mantle differentiate into motor Somites Neural crest neurons, while the last cells neurons to arrive become astrocytes Ependymal layer and oligodendrocytes. Further development of the Mantle layer CNS CNS and PNS will be found Marginal layer neurons in the Embryology Summaries later in this chapter. 23 DAYS Astrocytes and Ependymal cells oligodendrocytes

The neural tube increases in thickness as its epithelial lining undergoes repeated mitoses. By the middle of the fifth developmental week, there are three distinct layers. The ependymal layer lines the enclosed cavity, or neurocoel. The ependymal cells continue their mitotic activities, and daughter cells create the surrounding mangle layer. Axons from developing neurons form a superficial marginal layer.

• The Development of the Spinal Cord • 22 days • The neural tube is closed thus producing three layers: • Ependymal layer • Mantle layer • Marginal layer • Neurons develop in the mantle layer • Axons grow from the mantle layer to the marginal layer

• The Development of the Spinal Cord (continued) • 23 days • Axons form bundles and tracts in the spinal cord • 7 weeks • Ventral roots and dorsal roots develop • Glial cells develop • Spinal and cranial meninges develop • Spinal nerves develop

Ectoderm Neural crest Neural tube Neurocoel Ependymal layer Mantle layer Marginal layer By the end of the fifth Neuroepithelial developmental week, the (ependymal) neural tube is almost layer completely closed. In the 23 DAYS spinal cord the mantle Eventually, the 22 DAYS layer that contains growing axons will Mantle layer developing neurons and form bundles, or neuroglial cells will tracts, in the marginal Marginal produce the gray matter layer, and these tracts layer that surrounds the will crowd together in neurocoel. As neurons the columns that form develop in the mantle the white matter of 28 DAYS layer, their axons grow the spinal cord. toward central or peripheral destinations. The axons leave the mantle layer and travel toward synaptic targets within a peripheral marginal layer.

The Development of the Spinal Cord, Part I

By this time, cells of the neural crest have migrated to either side of the spinal cord and have formed the dorsal root ganglia. The neural crest cells become sensory neurons Roof plate and glial cells (Schwann cells and satellite cells). Processes from these sensory neurons Dorsolateral plate grow both into the periphery, to contact Dorsal root receptors, and into the CNS via the dorsal roots. Dorsal root ganglion Ventrolateral plate In each segment the axons of developing motor Floor plate neurons form a pair of ventral roots that grow away from the spinal cord.

Distal to each dorsal root ganglion, the motor efferents of the ventral root and the sensory afferents of the dorsal root are bound together into a single spinal nerve. Along much of the spinal cord, these nerves share a stereotyped pattern of peripheral branches, and the pattern 7 WEEKS accounts for the distribution of dermatomes. As the mantle enlarges, the neurocoel becomes laterally compressed and relatively narrow. The relatively thin roof plate and floor plate will not thicken substantially, but the dorsolateral and ventrolateral plates enlarge rapidly. Neurons developing within the dorsolateral plate will receive and relay sensory information, while those in the ventrolateral region will develop into motor neurons.

In addition to forming dorsal root ganglia and associated glial cells, neural crest cells migrate around the central nervous system and develop into the spinal and cranial meninges.

Larynx

Teeth

Dorsal root ganglia

Suprarenal medulla Meninges

Autonomic ganglia

Melanocytes

7 WEEKS (Distribution of neural crest cells) Neural crest cells aggregate to form autonomic ganglia near the vertebral column and in peripheral organs. Migrating neural crest cells contribute to the formation of teeth and form the laryngeal cartilages, melanocytes of the skin, the skull, connective tissues around the eye, the intrinsic muscles of the eye, Schwann cells, satellite cells, and the suprarenal medullae. © 2012 Pearson Education, Inc. Embryology Summary 28.8 The Development of the Spinal Cord, Part II (Part 2 of 3)

The Development of the Spinal Cord, Part II

Cranial nerves and ganglia

Eye

Cervical plexus Brachial plexus Spinal nerves

Lumbosacral plexus

7 WEEKS (Peripheral nerve distribution) Several spinal nerves innervate each developing limb. When embryonic muscle cells migrate away from the myotome, the nerves grow right along with them. If a large muscle in the adult is derived from several myotomal blocks, connective tissue partitions will often mark the original boundaries, and the innervation will always involve more than one spinal nerve.

Spina bifida Neural tube defect Spina bifida (BI-fi-da) results when A neural tube defect (NTD) is a the developing vertebral laminae fail condition that is secondary to a to unite due to abnormal neural tube developmental error in the formation formation at that site. The neural arch of the spinal cord. Instead of forming is incomplete, and the meninges a hollow tube, a portion of the spinal bulge outward beneath the skin of cord develops as a broad plate. This the back. The extent of the abnormality is often associated with spina bifida. determines the severity of the defects. Neural tube defects affect roughly In mild cases, the condition may pass one individual in 1000; prenatal unnoticed; extreme cases involve testing can detect the existence of much of the length of the vertebral these defects with an 80–85 percent column. success rate.

• The Development of the Brain • 23 days • The cephalic extension forms three brain vesicles • Prosencephalon • Mesencephalon • Rhombencephalon

The Development of the Brain, Part I

Before proceeding, Mesencephalon Rhombencephalon Neurocoel briefly review the summaries of skull formation, vertebral Prosencephalon column development, and development of the spinal cord in the previous Embryology Summaries. The initial cephalic expansion occurs Cephalic as the neurocoel enlarges, forming area three distinct brain vesicles: (1) the prosencephalon (pros-en- SEF-a-lon) or ―forebrain,‖ (2) the mesencephalon or ―midbrain,‖ and (3) the rhombencephalon Neural (rom-ben-SEF-a-lon) or ―hindbrain.‖ tube The prosencephalon and rhombencephalon will be subdivided further as development proceeds.

Even before neural tube formation has been completed, the cephalic portion begins to enlarge. Major differences in brain versus spinal cord development include (1) early breakdown of mantle (gray matter) and marginal (white matter) organization; (2) appearance of areas of neural cortex; (3) differential growth between and within specific regions; (4) appearance of characteristic bends and folds; and (5) loss of obvious segmental organization.

23 DAYS

• The Development of the Brain (continued) • 4 weeks • The rhombencephalon subdivides to form: • Metencephalon • Myelencephalon • The prosencephalon subdivides to form: • Telencephalon • Diencephalon

The Development of the Brain, Part I

Mesencephalon Metencephalon The rhombencephalon Myelencephalon first subdivides into the metencephalon (met- en-SEF-a-lon; meta, after) and the myelencephalon Diencephalon (mi-el-en-SEF-a-lon; myelon, spinal cord). Telencephalon

The prosencephalon forms the telecephalon (tel-en-SEF-a-long; telos, end  enkephalos, brain) and the diencephalon. The telencephalon begins as a pair of swellings near the rostral, dorsolateral border or the prosencephalon. 4 WEEKS

• The Development of the Brain (continued) • 5 weeks • Mesencephalon develops • Cranial nerves begin to develop • 8 weeks • Choroid plexus develops • Cerebral hemispheres expand • 11 weeks • Cerebellum develops

The Development of the Brain, Part I

N IV N V N VII Developing ear N III Cranial nerves develop Myelencephalon as sensory ganglia and link peripheral receptors Development of the with the brain, and motor mesencephalon fibers grow out of produces a small developing cranial nuclei. mass of neural tissue Special sensory neurons with a constricted of cranial nerves I, II, and neurocoel, the aqueduct VIII develop in association of the midbrain. with the developing receptors. The somatic motor nerves III, IV, and VI As differential growth proceeds and the grow to the eye muscles; N IX N X N XI N XII the mixed nerves (V, VII, IX, position and orientation and X) innervate the Pharyngeal of the embryo change, a pharyngeal arches arches series of bends, or (page 768). flexures (FLEK-sherz), appears along the axis of the developing brain.

5 WEEKS

The Development of the Brain, Part II

N III Cephalic N IV flexure The roofs of the Pontine flexure diencephalon and As growth continues myelencephalon N X and the pontine flexure fail to develop, develops, the brain leaving a thin N XI becomes more compact. ependymal layer The expanding cerebral in contact with hemispheres now the developing N XII dominate the superior meninges. Blood and lateral surfaces of vessels invading the brain. Migrating these regions neuroblasts create the create areas of the cerebral cortex, and choroid plexus. Cervical underlying masses of flexure gray matter develop into the basal nuclei. N I N VI N VIII N IX 8 WEEKS N II N VII

The Development of the Brain, Part II

Cerebral hemisphere (telencephalon) Cerebral Diencephalon hemisphere Mesencephalon

Cerebellum

Pons

Medulla Pons oblongata Cerebellum

Medulla Spinal oblongata cord Cranial nerve XI After 11 weeks, the expanding cerebral hemispheres have overgrown the diencephalon. At the metencephalon, cortical formation and CHILD expansion produce the cerebellum, which overlies the nuclei and tracts of the pons.

11 WEEKS

• The Development of Special Sense Organs • 4 weeks • Optic bulges appear in the lateral walls of the prosencephalon • The bulges indent forming the optic cup • The epidermis covering the optic cup forms the lens • 5 weeks • A pair of thickened areas in the front of the prosencephalon form the olfactory receptors • When nerves innervate the tongue, the epithelial cells differentiate to form gustatory cells

• The Development of Special Sense Organs (continued) • 3 weeks • Otic regions begin to appear on the lateral sides of the rhombencephalon • 4 weeks • Deep pockets form in the otic regions, creating otic vesicles • 6 weeks • The vesicles form the bony labyrinth • This process continues until the end of the third month

The Development of Special Sense Organs, Part I

All special sense organs develop from the interaction between the epithelia and the developing nervous system of the embryo. VISION Neurocoel Choroid Prosencephalon Epidermis Retina Optic cup Optic vesicle Lens Lens placode N II Optic stalk Sclera Lens vesicle

The first indication of These bulges become indented, Mesoderm aggregating around optic development forming a pair of optic cups, this complex forms the choroid appears as a pair of which remain connected to the and scleral coats. The anterior bulges called optic diencephalon by optic stalks. and posterior chambers develop vesicles in the lateral The epidermis overlying the optic as cavities appear within the walls of the cup responds by forming a lens mesoderm. prosencephalon. These placode, which thickens and extend to either side creates another vesicle. This like a pair of dumbbels, lens vesicle becomes the lens. each containing a cavity continuous with the neurocoel.

The Development of Special Sense Organs, Part I

OLFACTION

Nasal placode

Eye

External nares

5 WEEKS 10 WEEKS Olfactory receptors begin as Over time, the nasal placodes a pair of thickened areas in Nasal placode are enfolded and protected by front of the prosencephalon developing facial structures. during the fifth developmental (Development of the face was week. The thickenings are discussed in the previous called nasal placodes. Embryology Summary of the Skull.)

The Development of Special Sense Organs, Part I

GUSTATION Epithelial Sensory cells neuron Taste buds

Gustatory receptors When the nerve endings are the least specialized contact epithelial cells, the of any of the special epithelial cells differentiate sense organs. Taste into gustatory cells. If the buds develop as sensory nerves are cut, the sensory fibers grow taste buds degenerate; if into the developing the sensory nerve is moved, mouth and pharynx. it will stimulate the development of new taste buds at its new location.

The Development of Special Sense Organs, Part II

Neural groove EQUILIBRIUM Late in the third week of AND HEARING Otic placode development, a pair of otic placodes appears on either side Pharynx of the rhombencephalon.

Otic placode 3 WEEKS

The otic placodes form deep Neural tube pockets that subsequently lose their connection with the Otic vesicle epidermis, creating hollow Epidermis otic vesicles. Tail

4 WEEKS These vesicles gradually change shape, forming the membranous labyrinth. This Developing membranous process has essentially labyrinth been completed by the end of the third developmental month. Ganglia of N VIII

External pharyngeal Pharyngeal pouch groove

6 WEEKS

The Development of Special Sense Organs, Part II Thickened portions of the otic vesicles differentiate into the spiral and vestibular Developing ossicles ganglia, and their sensory terminals grow toward the Vestibular ganglion developing hair cells. Spiral ganglion External acoustic meatus Cartilage Middle ear cavity Auditory tube

7 WEEKS As these developments are under- Semicircular way, the surrounding mesenchyme ducts begins to differentiate into cartilage. Auricle This cartilage will later ossify to form the bony labyrinth. Auditory ossicles

External acoustic meatus Cochlea Tympanic membrane Temporal bone Middle ear cavity FULL TERM

• The Development of the Endocrine System • Week 5 • Five pharyngeal pouches are formed • The pouches form the thymus gland, parathyroid gland, and thyroid gland • The pituitary gland forms as a pocket in the pharyngeal area; this ectodermal pocket breaks away from the pharyngeal area and develops just superior to the thyroid gland area

• The Development of the Endocrine System • Week 5 (continued) • After the formation of the neural tube, some neural cells migrate away from the CNS and form an aggregate of cells that become the suprarenal glands

As noted in Chapter 3, all secretory glands, PARATHYROID GLANDS AND THYMUS whether exocrine or endocrine, are derived from epithelia. Endocrine organs develop from epithelia (1) covering the outside of the embryo, (2) lining the digestive tract, and (3) lining the coelomic cavity. Ectoderm Neural tube

First pharyngeal Pharynx pouch Developing ear I Pharynx First pharyngeal The dorsal masses of the third II Parathyroids cleft and fourth pouches form the Pharyngeal Pharyngeal III parathyroid glands. The ventral arches clefts masses move toward the midline IV and fuse to create the thymus gland.

V–VI Cells originating in the walls of the Thyroid small fifth pouch will be incorporated Endoderm into the thyroid gland (see below), where Thymus they will differentiate into C thyrocytes.

In sectional view, five pharyngeal pouches extend laterally toward the pharyngeal clefts. The first pouch lies caudal to the first (mandibular) arch. Pharyngeal pouches 5 and 6 are very small and are interconnected. WEEK 5 Endoderm lining the third, fourth, and fifth pairs of pharyngeal pouches The pharyngeal region of the embryo forms dorsal and ventral masses of cells that migrate beneath the plays a particularly important role in endodermal epithelium. endocrine development. After 4–5 weeks of development, the pharyngeal arches are well formed. Human embryos develop five or six pharyngeal arches, not all visible from the exterior. (Arch 5 may not appear or may form and degenerate almost immediately.) The five major arches (I–IV, VI) are separated by pharyngeal clefts, deep ectodermal grooves.

The Development of the Endocrine System, Part I

THYROID GLAND Endoderm Ectoderm

Ventral Thyroid pocket gland

The thyroid gland begins as a pocket in the Thyroid WEEK 5, Mid-sagittal section ventral midline. As this pocket branches C thyrocytes The boundary between ectoderm and slightly, its walls thicken, and the paired endoderm lies along the line formed masses lose their connection with the surface. As the embryo enlarges and by the circumvallate papillae of the changes shape, the thyroid shifts tongue (see Figure 18.7, p. 477). This caudally to a position near the line roughly corresponds to the middle thyroid cartilage of the larynx. of the mandibular (first) arch. The On its way, the thyroid gland thyroid gland forms here in the incorporates C thyrocytes from ventral midline. the walls of the fifth pouch.

The Development of the Endocrine System, Part II

PITUITARY GLAND Developing Hypothalamus Anterior Posterior pituitary gland Ectodermal lobe lobe pocket

The pituitary gland has a compound As these cells undergo division, the central origin. The first step is the formation chamber gradually disappears. This of an ectodermal pocket in the dorsal endocrine mass will become the midline of the pharynx. This pocket adenohypophysis (anterior lobe) of the WEEK 5, Mid-sagittal section loses its connection to the pharynx, pituitary gland. The neurohypophysis creating a hollow ball of cells that lies (posterior lobe) of the pituitary gland The pituitary gland forms in the dorsal inferior to the floor of the diencephalon begins as a depression in the hypothalamic midline above the forming thyroid gland. posterior to the optic chiasm. floor and grows toward the developing adenohypophysis.

SUPRARENAL GLANDS

Neural crest Pharyngeal cell mass Mesothelium arches Sympathetic Lining of preganglionic coelomic cavity fibers

Spinal cord Migrating neural Suprarenal crest cells medulla Dorsal root Suprarenal ganglion cortex Overlying epithelial cells respond by Sympathetic chain ganglion undergoing division, and the daughter cells surround the neural crest cells Future suprarenal to form a thick suprarenal cortex. medulla

Digestive tube

Each suprarenal gland also has a compound origin. Shortly after the formation of the neural tube, neural crest cells migrate away from the CNS. This migration leads to the formation of the dorsal root ganglia and autonomic ganglia. On each side of the coelomic cavity, neural crest cells aggregate in a mass that will become a suprarenal medulla. WEEK 5

For additional details concerning the development of other endocrine organs, refer to the subsequent Embryology Summaries on the Lymphoid, Digestive, Urinary, and Reproductive systems. © 2012 Pearson Education, Inc. Embryology Summary

• The Development of the Heart • Week 2 • The heart begins as a pair of tubes in the pharynx area • Week 3 • The heart begins to pump • The tubes fuse forming a one-chambered heart • Two large veins bring blood to the heart • One large artery carries blood away from the heart; the artery is called the truncus arteriosus

The Development of the Heart Mesoderm Pharynx Pharynx Future pericardial Neural groove cavity WEEK 3 Heart tubes Truncus arteriosus Future pericardial WEEK 2 cavity Future pericardial cavity The lateral plate mesoderm Ventricle LATERAL VIEW in this region has already Left atrial During the second split into parietal and visceral layers, creating a primordia developmental week, VENTRAL VIEW the heart consists of space that will eventually a pair of thin-walled, form the pericardial cavity. By the third week, the heart is pumping muscular tubes beneath and circulating blood. The cardiac tubes the floor of the pharynx. have fused, producing a heart with a single central chamber. Two large veins bring blood to the heart, and a single large artery, the truncus arteriosus, carries blood to the general circulation.

• The Development of the Heart (continued) • Week 5 • The interatrial and interventricular septa begin to form, ultimately forming the four chambers of the heart • The foramen ovale and ductus arteriosum form • 1 year • The foramen ovale and ductus arteriosum have closed (at birth) • The remnant of the foramen ovale is the fossa ovalis • The remnant of the ductus arteriosum is the ductus ligamentum

The Development of the Heart Pharynx Right Left atrium atrium Truncus arteriosus Aortic Future interatrial arches septum Ventricle Opening of sinus venosus Atrium Sinus venosus Future WEEK 5 WEEK 4 interventricular In week 5, the interatrial septum The heart elongates as the embryo and interventricular septa grows larger. It curves back upon begin to subdivide the itself, forming an S-curve that interior of the heart. gradually becomes more pronounced. The atrial and ventricular regions already differ in thickness.

The Development of the Heart

Interatrial Foramen Left septa atrium ovale Fossa ovalis Left ventricle

Right atrium At birth, the foramen Two interatrial septa develop, ovale closes, separating one overlapping the other. A gap the pulmonary and between the two, called the Right systemic circuits in the foramen ovale, permits blood ventricle heart. A shallow depression, flow from the right atrium to the the fossa ovalis, remains left atrium. Backflow from left to AGE through adulthood at the right is prevented by a flap that 1 YEAR site of the foramen ovale. acts as a one-way valve. Until birth, (Other cardiovascular this atrial short circuit diverts changes at birth are detailed blood from the pulmonary circuit. in Figure 22.27, p. 600.)

• The Development of the Cardiovascular System • Week 4 • Several aortic arches form and fuse together to form the dorsal aorta • Some of the aortic arches disintegrate leaving ultimately just one aortic arch

THE AORTIC ARCHES

I Aortic arches An aortic arch carries arterial blood through each II of the pharyngeal arches. In III the dorsal pharyngeal wall, IV these vessels fuse to create the dorsal aorta, which V distributes blood throughout VI the body. The arches are Left dorsal aorta usually numbered from I to Right dorsal aorta VI, corresponding to the pharyngeal arches. VENTRAL VIEW Fused dorsal aorta

Dorsla aorta

Aortic arches

Yolk sac

4 WEEKS We will follow the development of three major vessel complexes: the aortic arch, the venae cavae, and the hepatic portal and umbilical systems. (Arteries are shown in red and veins in blue regardless of the oxygenation of the blood they carry.) © 2012 Pearson Education, Inc. Embryology Summary 28.16 The Development of the Cardiovascular System (Part 2 of 6) The Development of the Cardiovascular System

Right common External carotid arteries Internal carotid artery Left common carotid carotid artery artery Brachiocephalic Common carotid arteries trunk Left subclavian Aortic arch Right subclavian artery artery As development proceeds, some Ligamentum of these arches disintegrate. The arteriosum ductus arteriosus provides an Ductus arteriosus external short-circuit between the Pulmonary pulmonary and systemic circuits. The left half of arch IV artery Pulmonary ultimately becomes the Most of the blood entering the right artery atrium bypasses the lungs, passing aortic arch, which carries Descending aorta instead through the ductus arteriosus blood away from the left or the foramen ovale in the heart. ventricle.

Dorsal aorta

Aortic arches

Yolk sac

• The Development of the Cardiovascular System • Week 4 (continued) • Venous circulation begins with the following vessels draining the tissues: • Anterior cardinal veins • Posterior cardinal veins • Subcardinal veins • These veins are paired and each pair fuses to form: • Superior vena cava • Inferior vena cava

The Development of the Cardiovascular System

Dorsal aorta THE VENAE CAVAE Aortic arches Anterior cardinal veins

Heart

Posterior cardinal veins Subcardinal Yolk sac veins

4 WEEKS DORSAL VIEW We will follow the development of three The early venous circulation draining the major vessel complexes: the aortic arch, tissues of the body wall, limbs, and head the venae cavae, and the hepatic portal centers around the paired anterior and umbilical systems. (Arteries are cardinal veins, posterior cardinal shown in red and veins in blue veins, and subcardinal veins. regardless of the oxygenation of the blood they carry.)

Right internal Superior Posterior cardinal and external vena cava vein jugular veins

Inferior vena Inferior vena cava cava

Right common iliac vein Interconnections form among these This process continues, veins, and a combination of fusion ultimately producing the and disintegration produces more- superior and inferior direct, larger-diameter connections venae cavae. to the right atrium.

Dorsal aorta Aortic arches

Yolk sac

• The Development of the Cardiovascular System • Week 4 (continued) • Umbilical arteries carry blood to the placenta • Umbilical veins carry blood to the liver • Week 12 • Right umbilical vein disintegrates • Ductus venosus allows some blood to bypass the liver • Veins draining the digestive tract fuse to form the hepatic portal vein

Dorsal aorta Aortic arches

We will follow the development of three major vessel complexes: the aortic arch, the venae cavae, and the hepatic portal and umbilical systems. (Arteries are shown in red and veins in blue regardless of the oxygenation of the blood they carry.)

Yolk sac

4 WEEKS

THE HEPATIC PORTAL AND UMBILICAL VESSELS

Heart Liver Heart

Liver Ductus venosus

Umbilical Hepatic portal Left umbilical vein veins vein Digestive tract Right umbilical Umbilical vein arteries 4 WEEKS 12 WEEKS Paired umbilical arteries deliver blood By week 12, the right umbilical vein disintegrates, and to the placenta. At 4 weeks, paired the blood from the placenta travels along a single umbilical veins return blood to umbilical vein. The ductus venosus allows some capillary networks in the liver. Veins venous blood to bypass the liver. The veins draining the running along the length of the digestive digestive tract have fused, forming the hepatic portal vein. tract have extensive interconnections.

Aortic arches We will follow the development of three major vessel complexes: the aortic arch, the venae cavae, and the hepatic portal and umbilical systems. (Arteries are shown in red and veins in blue regardless of the oxygenation of the blood they carry.)

Yolk sac

4 WEEKS

Lung Ductus arteriosus Pulmonary Foramen artery ovale Descending Pulmonary aorta vein Inferior Descending aorta vena cava Hepatic portal vein Liver Umbilical vein Umbilical arteries FULL TERM NEWBORN Shortly before birth, blood returning from At birth, pressures drop in the pleural cavities as the the placenta travels through the liver in chest expands and the infant takes its first breath. the ductus venosus to reach the inferior The pulmonary vessels dilate, and blood flow to the vena cava. Much of the blood delivered by lungs increases. Pressure falls in the right atrium, and the venae cavae bypasses the lungs by the higher left atrial pressures close the valve that traveling through the foramen ovale and guards the foramen ovale. Smooth muscles contract the ductus arteriosus. the ductus arteriosus, which ultimately converts to the ligamentum arteriosum, a fibrous strand.

• The Development of the Lymphoid System • Week 7 • The thymus gland forms from a pharyngeal pouch • The cells lose their attachment and form just inferior to the thyroid gland area • Week 8 • Numerous lymphatic sacs form throughout the body

Jugular lymph sac

The development of the lymphatic vessels is closely tied to the formation of blood vessels. Paired jugular lymph sacs form from the fusion of small, Primordial endothelium-lined pockets in the lymph sacs mesoderm of the neck. By week 7, these sacs become connected to Median the venous system. lymph sac Primordial lymph sacs form parallel with veins of the trunk, and a large median lymph sac marks the future 7 WEEKS location of the cisterna chyli. © 2012 Pearson Education, Inc. Embryology Summary 28.17 The Development of the Lymphoid System (Part 2 of 4) The Development of the Lymphoid System

Parathyroid The thymus forms from cells of the third pharyngeal pouch. These cells lose their connection with the epithelium and Third pharyngeal pouch divide repeatedly. As the embryo changes shape, the thymic lobes are brought together near the midline of the chest. Pharynx At birth, the thymus is relatively large, filling much of the Thyroid anterior mediastinum. 6 WEEKS

Pharynx Larynx

Thymus Thyroid

7 WEEKS

Larynx

Parathyroid Thyroid Esophagus Thymus Trachea

8 WEEKS

Right lymphatic duct As growth continues, the isolated lymphatic sacs fuse, forming the Thoracic duct thoracic duct and right lymphatic duct. As the limb buds enlarge, lymphatic vessels grow into the area along with developing arteries and veins. Cisterna chyli

Lymphatic sac Small blood vessels grow into areas where lymphocytes cluster within developing lymphatic sacs. Connective tissue Lymphocyte cluster capsules form, and the internal organization of a lymph node gradually appears. Lymph vessel

Capsule

• The Development of the Respiratory System • Week 3 • A pulmonary groove forms in the pharynx area • Week 4 • This groove forms deeper and then forms two blind pouches at the end of the groove • The groove forms a tube, which becomes the trachea, and the two blind pouches become the lungs • Week 9 • The diaphragm forms

• The Development of the Respiratory System (continued) • 3 Months • The lung buds continue to branch numerous times • 6 Months • There are a million branches • All the bronchioles are formed • 6–9 Months • Alveoli begin to form

THE LUNGS

Pharyngeal pouches

Pulmonary groove Lung buds Heart

Yolk sac 4 WEEKS 3 WEEKS By week 4, the groove has become A shallow pulmonary groove a blind pocket that extends caudally, appears in the midventral floor anterior to the esophagus. This tube of the pharynx after roughly will become the trachea. At its tip, 1 the tube branches, forming a pair of 3 /2 weeks of development. This groove, which lies near the level lung buds. of the last pharyngeal arch, gradually deepens. © 2012 Pearson Education, Inc. Embryology Summary 28.18 The Development of the Respiratory System, Part I (Part 2 of 2) The Development of the Respiratory System, Part I

The lung buds continue to elongate and branch repeatedly.

Bronchioles

3 MONTHS By the end of the sixth fetal month, there are around a million terminal Alveoli branches, and the conducting passageways are Over the next three months, each complete to the of the bronchioles gives rise to level of the several hundred alveoli. This bronchioles. process continues for a variable period after birth. © 2012 Pearson Education, Inc. Embryology Summary 28.19 The Development of the Respiratory System, Part II (Part 1 of 3)

The Development of the Respiratory System, Part II

THE PLEURAL CAVITIES

Digestive tube

Lung buds

Elongation of the tube carries Heart it into the mediastinum, and as the branching proceeds, the lung buds project into the ventral cavity dorsal to the developing heart. 4 WEEKS

The Development of the Respiratory System, Part II

Esophagus

Developing lung

Pleuropericardial membrane The pericardial sac begins forming in Heart week 6 as a thin pleuropericardial membrane forms between the heart 6 WEEKS and the developing lungs.

Pleural cavity

Lung

Heart Heart Pericardial Pericardium cavity Left lung 8 WEEKS Diaphragm By week 8, the pericardial sac is complete and the pericardial cavity is isolated from Liver the rest of the ventral body cavity. The 9 WEEKS diaphragm then completes its formation, attaching to the pericardial sac and tissues By week 9, the diaphragm of the mediastinum. This attachment completes its formation, separates the abdominopelvic cavity from forming a transverse sheet the pleural cavities. superior to the liver.

• The Development of the Digestive System • Week 3 • Yolk sac is formed • The layer between the yolk sac and the amniotic sac develops a hindgut and a foregut • The hindgut and the foregut form the digestive tube • Week 4 • Lateral to the digestive tube are cavities called the coelomic cavities • The digestive tube is held in position within the midline of the coelomic cavities by the dorsal mesentery and ventral mesentery

• The Development of the Digestive System • Week 4 (continued) • The pancreas is formed within the dorsal mesentery area • The liver is formed within the ventral mesentery area • Part of the ventral mesentery becomes the falciform ligament of the liver • The remaining mesenteric tissue becomes the greater omentum and lesser omentum

• The Development of the Digestive System (continued) • Week 6 • The intestines elongate and coil • The yolk sac and the body stalk fuse to form the umbilical cord

The Development of the Digestive System, Part I

Foregut Amniotic Somite Endoderm cavity Amniotic Hindgut cavity Developing Hindgut coelom Yolk Mesoderm sac Yolk Yolk stalk 3 WEEKS sac In sectional view, the embryonic gut is a simple endodermal tube By week 3, endodermal As the embryo forms on the surrounded by mesoderm. cells have migrated embryonic shield, two pockets Cavities appearing within around the inside of the of endoderm are created: the the mesoderm create the blastocyst, completing foregut and hindgut. A broad coelom (ventral body cavity). a pouch known as the connection between these yolk sac. pockets and the yolk sac remains within the yolk stalk.

The Development of the Digestive System, Part I

Neural tube

Notochord

Dorsal mesentery Digestive tube Coelomic cavity 4 WEEKS Yolk stalk Ventral mesentery Body stalk The digestive tube remains suspended in the coelom by a dorsal mesentery and a ventral mesentery. The ventral mesentery disintegrates everywhere except where major vessels or visceral organs have grown into it. It remains intact along the path of the umbilical arteries and where the umbilical vein and liver develop.

The Development of the Digestive System, Part I

Pancreas Neural tube Greater omentum Aorta

Lesser omentum Liver Pancreas

Liver

As the embryo enlarges, the stomach and liver rotate toward the right, creating two Pancreas The pancreas and liver begin pockets. The mesenteries as epithelial pockets that grow that form these pockets are Liver Coelom away from the digestive tract the greater omentum and and into the dorsal and ventral the lesser omentum. Stomach mesenteries, respectively. Falciform ligament

The Development of the Digestive System, Part II

Liver Stomach See sectional view at 4 weeks in Part I. The intestines begin to elongate, and 6 WEEKS with the breakdown of the ventral Pancreas mesentery, they push outward into Allantois the umbilical stalk. Further elongation Cloaca and coiling occur outside the body Umbilical stalk of the embryo. The hindgut extends into the tail, where it forms a large chamber, the cloaca. A tubular extension of the cloaca, the allantois (a-LAN-to-is; allantos, sausage), projects away from the body and into the body stalk. Fusion of the yolk stalk and body stalk will create the umbilical stalk, also known as the umbilical cord.

Entrance to trachea

Esophagus

Liver 8 WEEKS

Pancreas A partition grows across the cloaca, dividing it into a posterior rectum and an Small anterior urogenital sinus intestine that retains a connection to the allantois.

Urogenital sinus

Rectum

Heart 10 WEEKS By week 10, the intestines Stomach have begun moving back into the coelomic cavity, Gallbladder although they continue to grow longer. Small intestine

Umbilical cord

Urinary bladder

• The Development of the Urinary System • Week 3.5 • The kidneys begin as seven pairs of tubules called pronephric tubes • Week 6 • Branches of the aorta extend toward the pronephric tubes • Week 8 • The allantois becomes the urinary bladder • Week 12 • Kidneys begin producing filtrate even though there aren’t any waste products in it

The kidneys develop in stages along the axis of the urogenital ridge, a thickened area beneath Pronephros the dorso-lateral wall of the coelomic cavity. Mesonephros

Metanephros Cloaca

Urogenital ridge Kidney development proceeds along the cranial/caudal axis of this ridge, beginning with the formation of the pronephros, continuing along the mesonephros, and ending with the development of the metanephros. © 2012 Pearson Education, Inc. Embryology Summary 28.22 The Development of the Urinary System, Part I (Part 2 of 3) The Development of the Urinary System, Part I

Neural tube The pronephros consists of a Notochord series of tubules (generally 7 pairs) that appears within the Somite nephrotome, the narrow band Pronephric of mesoderm between the tubule somites and the lateral plate. Pronephric duct

Lateral plate mesoderm Nephrotome

The pronephric tubules are very small and nonfunctional, and they disintegrate almost at once. The only significant contribution of the pronephros is the formation of a pair of pronephric ducts that grow caudally until they connect to the cloaca.

1 3 /2 WEEKS

The Development of the Urinary System, Part I

Pronephros Developing aorta

Mesonephric duct

Mesonephros Mesonephric Mesonephric tubule duct Metanephros After approximately 4 weeks of development, Nephrotomal mesoderm of the the mesoderm midway along the urogenital ridge metanephros forms a dense mass begins organizing into the mesonephros. On without a trace of segmental either side of the midline, approximately 70 organization. This will become the tubules develop within these segments. These functional adult kidney. tubules grow toward the adjacent pronephric duct and fuse with it. From this moment on, the duct is called the mesonephric duct. 4 WEEKS

A ureteric bud, or metanephric diverticulum, forms in the wall of each mesonephric duct, and this blind tube elongates and branches within the adjacent metanephros. Tubules developing within the metanephros then connect to the terminal branch of the ureteric bud.

Glomerulus

Mesonephric Mesonephros duct

Allantois Renal Mesonephric corpuscle duct Cloaca In each segment, a branch of the aorta grows toward the nephrotome, and the tubules form Metanephros large nephrons with enormous glomeruli. Like the pronephros, the mesonephros does not Ureteric bud persist, and when the last segments of the mesonephros are forming, the first are already Most of the metabolic wastes produced by beginning to degenerate. the developing embryo are passed across the placenta to enter the maternal circulation. The small amount of urine produced by the kidneys accumulates within the cloaca and the allantois, an endoderm-lined sac that extends into the umbilical stalk.

6 WEEKS

Mesonephric duct

Degenerating mesonephros

Developing metanephros

Urinary bladder

Urogenital sinus

Rectum

8 WEEKS Near the end of the second developmental month, the cloaca is subdivided into a dorsal rectum and a ventral urogenital sinus. The proximal portions of the allantois persist as the urinary bladder, and the connection between the bladder and an opening on the body surface will form the urethra. © 2012 Pearson Education, Inc. Embryology Summary 28.23 The Development of the Urinary System, Part II (Part 3 of 3) The Development of the Urinary System, Part II

Nephron

Collecting tubule Collecting duct

Collecting Major calyx system

Metanephros Ureteric bud Ureter The ureteric bud branches 12 WEEKS within the metanephros, The kidneys begin producing filtrate by the creating the calyces and third developmental month. The filtrate does the collecting system. The not contain waste products, as they are nephrons, which form excreted at the placenta for removal and within the mesoderm of elimination by the maternal kidneys. The the metanephros, tap into sterile filtrate mixes with the amniotic fluid the collecting tubules. and is swallowed by the fetus and reabsorbed across the lining of the digestive tract. © 2012 Pearson Education, Inc. Embryology Summary

• The Development of the Reproductive System • Weeks 3–6 • Endodermal cells of the yolk sac migrate to the genital ridges in the abdominal cavity • Each ridge consists of columns of cells called primary sex cords; these will further develop into gonads

• The Development of the Reproductive System • Weeks 3–6 (continued) • If the gonad-developing cells are exposed to androgens, a male will develop, regardless of the genetic sex of the embryo • If they are not exposed to androgens, a female will develop, regardless of the genetic sex of the embryo • The male embryo will begin producing testosterone after week 6, which then directs the development of the external genitalia

The Development of the Reproductive System

SEXUALLY INDIFFERENT STAGES (WEEKS 3–6) DEVELOPMENT OF THE GONADS

Aorta

Gut Primary sex cords Genital ridge Mesonephric Yolk sac Allantois duct Paramesonephric (Müllerian) duct 3 WEEKS Each ridge has a thick epithelium continuous with columns of During the third week, endodermal cells cells, the primary sex cords, that extend into the center migrate from the wall of the yolk sac (medulla) of the ridge. Anterior to each mesonephric duct, a near the allantois to the dorsal wall of duct forms that has no connection to the kidneys. This is the the abdominal cavity. These primordial paramesonephric (Müllerian) duct; it extends along the germ cells enter the genital ridges genital ridge and continues toward the cloaca. At this sexually that parallel the mesonephros. indifferent stage, male embryos cannot be distinguished from female embryos.

The Development of the Reproductive System SEXUALLY INDIFFERENT STAGES (WEEKS 3–6)

DEVELOPMENT OF DUCTS AND Paramesonephric duct ACCESSORY ORGANS Mesonephric duct Gonad

Both sexes have mesonephric and Kidney paramesonephric ducts at this stage. Unless exposed to androgens, the Cloacal opening embryo—regardless of its genetic sex—will develop into a female. In a normal male embryo, cells in the core (medulla) of the genital ridge begin producing testosterone sometime after week 6. Testosterone triggers the changes in the duct system and external genitalia that are detailed on the following page.

The Development of the Reproductive System SEXUALLY INDIFFERENT STAGES (WEEKS 3–6) DEVELOPMENT OF EXTERNAL GENITALIA Urethral fold Genital tubercle Urogenital membrane Genital swelling Cloacal Cloacal fold membrane Anal fold 4 WEEKS 6 WEEKS After 4 weeks of development, there Two weeks later, the cloaca has been are mesenchymal swellings called subdivided, separating the cloacal cloacal folds around the cloacal membrane into a posterior anal membrane (the cloaca does not membrane, bounded by the anal folds, open to the exterior). The genital and an anterior urogenital membrane, tubercle forms the glans of the bounded by the urethral folds. A penis in males and the clitoris in prominent genital swelling forms females. lateral to each urethral fold.

• Development of the Male Reproductive System • Week 7 • The primary sex cords proliferate and eventually form the seminiferous tubules within the developing testes • Week 10 • The urethral folds begin to fuse to form the spongy urethra of the penis • This line of fusion can be seen in the adult as the raphe • 7 Months • The testes descend into the scrotum © 2012 Pearson Education, Inc. Embryology Summary 28.25 The Development of the Reproductive System: Development of the Male Reproductive System (Part 1 of 3)

The Development of the Reproductive System

DEVELOPMENT OF THE MALE REPRODUCTIVE SYSTEM DEVELOPMENT OF THE TESTES

Degenerating Tunica mesonephric albuginea Rete testis tubule

Testis Testis cords cords (seminiferous tubules) 7 WEEKS 12 WEEKS In the male, the primary sex cords Connections form between the arching proliferate and the germ cells testis cords and the adjacent mesonephric migrate into the sex cords. The nephrons. Although these nephrons resulting testis cords will form later degenerate, the seminiferous tubules the seminiferous tubules. remain connected to the mesonephric duct.

The Development of the Reproductive System

DEVELOPMENT OF THE MALE REPRODUCTIVE SYSTEM

DEVELOPMENT OF MALE DUCTS AND ACCESSORY ORGANS

Rete testis Seminal Paramesonephric gland Testis cord duct degenerates Ductus Developing Paramesonephric Prostate deferens testis duct

Testis cords Mesonephros Mesonephric duct (becomes ductus Testis Mesonephric deferens) Epididymis duct Urogenital sinus 4 MONTHS 7 MONTHS A view of the testis and ducts After four months of development, Definitive organization after the of the left side as seen in the testis cords are connected to testis has descended into the frontal section. Note the the remnants of the mesonephric scrotum (see Figure 27.2, p. 719). location and orientation of tubules by the rete testis. The Note the relationships between the mesonephros relative to paramesonephric (Müllerian) duct the definitive sex organs and the developing testis. has degenerated. the embryonic structures.

The Development of the Reproductive System DEVELOPMENT OF THE MALE REPRODUCTIVE SYSTEM

DEVELOPMENT OF MALE EXTERNAL GENITALIA External urethral orifice Urethral Spongy urethra Glans of folds penis Urethral folds Line of Scrotal fusion swelling Scrotum Anus

10 WEEKS BIRTH At 10 weeks, the genital tubercle has In the newborn male, the line enlarged, the tips of the urethral folds are of fusion between the urethral moving together to form the spongy folds is quite evident. urethra (see sectional views), and paired scrotal swellings have developed from the genital swellings.

• Development of the Female Reproductive System • Weeks 7–12 • Primary sex cords degenerate • Primordial germ cells migrate to the genital ridge • The urethral folds do not fuse; they become the labia minora • Week 10 • The ovaries and uterus develop

The Development of the Reproductive System DEVELOPMENT OF THE FEMALE REPRODUCTIVE SYSTEM DEVELOPMENT OF THE OVARIES

Primordial germ cells Uterine tube Mesonephric duct

Degenerating Primary primary sex cords Cortex sex cords In the female embryo, the primary 7 WEEKS sex cords degenerate and the 12 WEEKS primordial germ cells migrate into the outer region (cortex) of the genital ridge.

The Development of the Reproductive System

DEVELOPMENT OF THE FEMALE REPRODUCTIVE SYSTEM

DEVELOPMENT OF FEMALE DUCTS AND ACCESSORY ORGANS Ovary Peritoneal Mesonephric Degenerating opening of uterine tube tubule mesonephric remnants tubules Ovary Cortex Mesonephros Ovarian ligament of ovary

Paramesonephric Uterus (Müllerian) duct Uterune tube (from paramesonephric Vagina Uterus duct) Urogenital sinus 7 WEEKS 10 WEEKS BIRTH The mesonephric tubules and duct degenerate; the paramesonephric (Müllerian) duct develops a broad opening into the peritoneal cavity. Note the fusion of the ducts and the separation of the common chamber, which will form the uterus, from the urogenital sinus.

The Development of the Reproductive System DEVELOPMENT OF THE FEMALE REPRODUCTIVE SYSTEM

COMPARISON OF MALE AND DEVELOPMENT OF FEMALE EXTERNAL GENITALIA FEMALE EXTERNAL GENITALIA Clitoris Males Females Genital tubercle Penis Clitoris Labia minora Corpora cavernosa Erectile tissue Genital swelling Corpus spongiosum Vestibular bulbs Urethra Proximal shaft of penis Labia minora Urethral fold Labia majora Spongy urethra Vestibule Urogenital Opening Bulbo-urethral glands Greater vestibular glands membrane to vagina Scrotum Labia majora Hymen Anus 7 WEEKS BIRTH In the female, the urethral folds do not fuse; they develop into the labia minora. The genital swellings will form the labia majora. The genital tubercle develops into the clitoris. The urethra opens to the exterior immediately posterior to the clitoris. The hymen remains as an elaboration of the urogenital membrane.