Introduction to Human 1 Development DEVELOPMENTAL PERIODS, 1 Embryology in the Middle Ages, 4 Stages of Embryonic Development, 1 The Renaissance, 5 Postnatal Period, 1 GENETICS AND HUMAN DEVELOPMENT, 6 Infancy, 1 MOLECULAR BIOLOGY OF HUMAN Childhood, 1 DEVELOPMENT, 7 Puberty, 1 DESCRIPTIVE TERMS IN EMBRYOLOGY, 7 Adulthood, 2 CLINICALLY ORIENTED PROBLEMS, 9 SIGNIFICANCE OF EMBRYOLOGY, 2 HISTORICAL GLEANINGS, 4 Ancient Views of Human Embryology, 4 Human development is a continuous process that begins weeks), when embryonic and early fetal development is when an oocyte (ovum) from a female is fertilized by a sperm occurring. (spermatozoon) from a male to form a single-celled zygote (Fig. 1.1). Cell division, cell migration, programmed cell POSTNATAL PERIOD death (apoptosis), differentiation, growth, and cell rearrange- ment transform the fertilized oocyte, a highly specialized, This is the period occurring after birth. Explanations of totipotent cell, the zygote, into a multicellular human being. frequently used postnatal developmental terms and periods Most developmental changes occur during the embryonic follow. and fetal periods; however, important changes occur during later periods of development: the neonatal period (first 4 INFANCY weeks), infancy (first year), childhood (2 years to puberty), and adolescence (11 to 19 years). Infancy is the period of extrauterine life, roughly the first year after birth. An infant age 1 month or younger is called a neonate (newborn). The transition from intrauterine to DEVELOPMENTAL PERIODS extrauterine existence requires many critical changes, especially in the cardiovascular and respiratory systems. If It is customary to divide human development into prenatal neonates survive the first crucial hours after birth, their (before birth) and postnatal (after birth) periods. The devel- chances of living are usually good. The body grows rapidly opment of a human from a zygote to birth is divided into two during infancy; total length increases by approximately one main periods, embryonic and fetal. The main changes that half, and weight is usually tripled. By 1 year of age, most occur prenatally are illustrated in the Timetable of Human infants have six to eight teeth. Prenatal Development (see Fig. 1.1). Examination of the timetable reveals that the most visible advances occur during CHILDHOOD the third to eighth weeks—the embryonic period. During the fetal period, differentiation and growth of tissues and This is the period between infancy and puberty. The primary organs occur, and the rate of body growth increases. (deciduous) teeth continue to appear and are later replaced by the secondary (permanent) teeth. During early childhood, STAGES OF EMBRYONIC DEVELOPMENT there is active ossification (formation of bone), but as the child becomes older, the rate of body growth slows down. Early development is described in stages because of the Just before puberty, however, growth accelerates—the pre- variable period it takes for embryos to develop certain pubertal growth spurt. morphologic characteristics. Stage 1 begins at fertilization, and embryonic development ends at stage 23, which occurs PUBERTY on day 56 (see Fig. 1.1). A trimester is a period of 3 months, one third of the 9-month period of gestation. The most critical Puberty is the period when humans become functionally stages of development occur during the first trimester (13 capable of procreation (reproduction). In females, the first 1 2 THE DEVELOPING HUMAN signs of puberty may be after age 8; in males, puberty com- with abnormal development (birth defects). This branch of monly begins at age 9. embryology is concerned with various genetic and/or envi- ronmental factors that disturb normal development and ADULTHOOD produce birth defects (see Chapter 20). Clinically oriented embryology: Attainment of full growth and maturity is generally reached between the ages of 18 and 21 years. Ossification and growth • Bridges the gap between prenatal development and are virtually completed during early adulthood (21 to 25 obstetrics, perinatal medicine, pediatrics, and clinical years). Brain development continues into early adulthood, anatomy including changes in gray-matter volume. • Develops knowledge concerning the beginnings of life and the changes occurring during prenatal development • Builds an understanding of the causes of variations in SIGNIFICANCE OF EMBRYOLOGY human structure • Illuminates clinically oriented anatomy and explains how Clinically oriented embryology refers to the study of embryos; normal and abnormal relations develop the term generally means the prenatal development of • Supports the research and application of stem cells for embryos, fetuses, and neonates (infants aged 1 month and the treatment of certain chronic diseases younger). Developmental anatomy refers to the structural changes of a human from fertilization to adulthood; it includes The knowledge that physicians have of normal development embryology, fetology, and postnatal development. Teratology and the causes of birth defects is necessary for giving the is the division of embryology and pathology that deals embryo and fetus the best possible chance of developing TIMETABLE OF HUMAN PRENATAL DEVELOPMENT 1 TO 10 WEEKS DAYS Primary follicles Oocyte EARLY DEVELOPMENT OF OVARIAN FOLLICLE −2 MENSTRUAL PHASE PROLIFERATIVE PHASE Day 1 of last normal menstrual cycle Antrum Mature Oocyte follicle Ovulation −1 COMPLETION OF DEVELOPMENT OF FOLLICLE Oocyte Oocyte Ovary CONTINUATION OF PROLIFERATIVE PHASE OF MENSTRUAL CYCLE AGE 1Stage 1 2Stage 2 begins 34Stage 3 begins 56Stage 4 7Stage 5 begins (weeks) Trophoblast Zona pellucida Implantation begins 1 Fertilization Zygote divides MorulaEarly blastocyst Late blastocyst Embryoblast SECRETORY PHASE OF MENSTRUAL CYCLE 8 9 10 Cytotrophoblast 11 Maternal blood 12 Extraembryonic 13 Stage 6 begins 14 Lacunar Connecting stalk Lacunae appear in Amnion Eroded mesoderm syncytiotrophoblast gland network Primary villi Amnion Amniotic cavity 2 Embryonic disc Bilaminar embryonic Primary umbilical disc vesicle Closing plug Embryonic disc Coelom Prechordal plate Fig. 1.1 Early stages of development. Development of an ovarian follicle containing an oocyte, ovulation, and the phases of the menstrual cycle are illustrated. Human development begins at fertilization, approximately 14 days after the onset of the last normal menstrual period. Cleavage of the zygote in the uterine tube, implantation of the blastocyst in the endometrium (lining) of the uterus, and early development of the embryo are also shown. The alternative term for the umbilical vesicle is the yolk sac; this is an inappropriate term because the human vesicle does not contain yolk. CHAPTER 1 — INTRODUCTION TO HUMAN DEVELOPMENT 3 15 16 Stage 7 begins 17 Trilaminar embryo 18 Stage 8 begins 19 20 Stage 9 begins 21 Neural First missed groove menstrual period Amnion Neural plate Neural plate Brain First pairs Neural of somites Primitive groove Neural groove 3 streak Primitive Somite Somite Cut edge streak Arrows indicate of amnion Primitive node migration of Migration of cells from Thyroid gland begins Primitive streak mesenchymal cells primitive streak Length: 1.5 mm Primitive streak to develop 22 Stage 10 begins 23 24 Stage 11 begins 25 26 Stage 12 begins 27 Site of otic pit 28 Stage 13 begins Rostral neuropore Otic (ear) pit 2 pairs of Fore- pharyngeal brain Heart arches Primordia Upper begins of eye limb bud Pharyngeal 4 to beat and ear Heart bulge arches Primitive present Circulatory Rostral Caudal neuropore System Indicates neuropore closes 3 pairs of CRL = crown− Neural folds fusing pharyngeal arches actual size rump length CRL: 5.0 mm 29 30 31 32 Stage 14 begins 33 Stage 15 begins 34 Cerebral vesicles 35 Eye distinct Developing eye Eye Upper limb bud 5 Nasal Hand pit Heart plate Cord Lower Foot Lens pits, optic cups, limb plate CRL: 5.5 mm nasal pits forming Primordial mouth bud CRL: 7.0 mm present CRL: 8.5 mm 36 37 Stage 16 begins 38 39 40 External acoustic 41 Stage 17 begins 42 Large head meatus Ear Eye Digital Ear rays Eye 6 Eye Digital rays Foot- Oral and nasal plate Upper lip and Foot plate cavities confluent nose formed CRL: 9.5 mm CRL: 10.5 mm Ventral view CRL: 12.5 mm AGE (weeks) 43 Actual size 44 Stage 18 begins 45 46 47 Genital tubercle 48 Stage 19 begins 49 Actual size Amniotic sac Wall of uterus Head large but chin Eyelid Urogenital poorly formed. Uterine membrane Grooves between cavity External ear 7 digital rays Anal indicate fingers. membrane Wrist, Eyelids fingers CRL: 13.0 mm forming Smooth and fused CRL: 18 mm chorion 50 Stage 20 begins551 52 Stage 21 begins 3554 Stage 22 begins 556 Stage 23 Genital Ear Upper limbs Eye tubercle Eye Ear longer and bent External genitalia Urethral at elbows. groove Wrist Nose have begun 8 to differentiate. Fingers distinct Fingers Knee but webbed. Anus Elbow Toes Large forehead and Toes CRL: 30 mm 57 58 59 Placenta 60 Genitalia 61 62 Genitalia 63 Phallus Phallus Eye Ear Urogenital Urogenital Beginning fold fold of Wrist 9 fetal Labioscrotal Knee Labioscrotal fold period fold Perineum Perineum Toes Elbow CRL: 45 mm CRL: 50 mm 64 65 66 67 68 69 70 Clitoris Glans of penis Face has a Labium Genitalia have more developed minus or Urethral profile. 10 Urogenital characteristics groove groove but still not Note growth fully formed. of chin Labium Scrotum compared majus to day 44. Ears still positioned lower. CRL: 61 mm Fig. 1.1, cont’d 4 THE DEVELOPING HUMAN normally. Much of the modern practice of obstetrics involves the Hindus, called Garbha Upanishad, describes ancient views applied embryology. Embryologic topics of special interest concerning the embryo. It states: to obstetricians are oocyte and sperm transport, ovulation, fertilization, implantation, fetal-maternal relations, fetal From the conjugation of blood and semen [seed], the embryo circulation, critical periods of development, and causes of comes into existence. During the period favorable for conception, birth defects. after the sexual intercourse, [it] becomes a Kalada [one-day-old In addition to caring for the mother, physicians guard the embryo]. After remaining seven nights, it becomes a vesicle.