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Home , Buccopharyngeal membrane, Embryonic disc, Neurenteric canal, Primitive node, Primitive pit

BILAMINAR DISC, TRILAMINAR DISC & THEIR DERIVATIVES

Dr. Sangeeta Kotrannavar Assistant Professor Dept. of USM- KLE –IMP, Belagavi.

Learning Objectives • Define and describe , , , , notochordal process, prochordal plate, cloacal plate, , , and • Describe the formation of the bilaminar • Illustrate and identify the layers of bilaminar embryonic disc • Describe the formation of the primitive streak • Illustrate and identify the parts of the primitive streak • Describe the formation of the extraembryonic mesoderm • Describe the formation of the primary and secondary yolk sacs • Describe the formation of the • Describe the formation of the and the cavities • State the origin of the three embryonic germ layers that make up the trilaminar disc • Define • Describe the notochord formation • Describe the process • Describe the formation of cells • List the derivatives of the different germ layers Human Development

1st , , , & implantation week 2nd differentiation into & week Bilaminar embryonic disc Amniotic cavity & chorionic cavity Primary & secondary umbilical vesicle Somatic & splanchnic extraembryonic mesoderm 3rd Trilaminar disc, Notochord, week Intraembryonic mesoderm, Primitive blood vessels Secondary and tertiary Folding of Development during first week

Structures and events in fertilization Cleavage and the formation of the morula and blastocyst Quick Review Implantation

• Starts at 6-7th day of fertilization • Trophoblast secrets proteolytic enzyme • Interstitial implantation • Trophoblast stick to upper uterine segment • Trophoblast proliferate at embryonic pole & forms – Syncytiotrophoblast – Cytotrophoblast

Look for changes taking place in each step Development during second week

Trophoblast

Embryoblast

• Blastocyst - Cells of the , now called the embryoblast, are at embryonic pole, will develop into the embryo proper

• and those of the outer cell mass, or trophoblast, will develop into

Development of bilaminar embryonic disc

• Cells of embryoblast differentiate into 2 layers around 8 days after fertilization • Upper layer is epiblast (columnar cells) adjacent to the amniotic cavity • Lower layer is hypoblast smaller (cuboidal) adjacent to the blastocyst cavity • Form a 2 layered flat embryonic disc

Amniotic cavity

Day-8

• Fluid begins to collect between epiblast cells to form amniotic cavity • The inner side of cavity is lined by epiblast cells called amnioblasts • Amnioblasts secrete amniotic fluid • At first it is small, then becomes larger. Lies on dorsal aspect of embryonic disc.

Amniotic fluid • Volume 12 weeks – 50 ml 20 weeks- 400 ml 37 weeks- 800ml-1 liter Amniocentesis – study of fetal karyotype for anomalies Embryo at 8 weeks Functions : • Is vital for healthy fetal development. • It contains important nutrients, hormones, and antibodies and it helps protect the baby from bumps and injury.

Abnormalities • Polyhydramnios >2000 ml – Abnormal digestive system or CNS , esophageal atresia, Anencephaly • Oligohydramnios <500 ml – Abnormal urinary system, poor development of kidney, Urethra atresia

Exocoelomic cavity, or primitive

Day-9 • Mean while, at abembryonic pole, hypoblast cells form a thin membrane, the exocoelomic (Heuser’s) membrane, • That lines the inner surface of the cytotrophoblast. • This membrane, together with the hypoblast, forms the lining of the exocoelomic cavity, or primitive yolk sac. Extraembryonic mesoderm

• Cells from the hypoblast migrate & give rise to a layer of loosely arranged tissue called extraembryonic mesoderm around the amnion & primary yolk sac.

• Isolated cavities appear in this extraembryonic mesoderm which soon coalesce to form the extraembryonic or chorionic coelom. Extraembryonic mesoderm –division

• As the extraembryonic cavity develops, mesoderm splits into 2 layers. • The part lining the inside of the trophoblast & the outside of the amniotic cavity is called the somatopleuric extraembryonic mesoderm. • The part lining the outside of the yolk sac is called the splanchnopleuric extraembryonic mesoderm. Secondary yolk sac

• As the extraembryonic coelom forms, the primary yolk sac decreases in size & a small Secondary Yolk Sac is formed. • A small remnant called exocoelomic vesicle gradually disintegrates.

Functions – • Primary yolk sac allows transport of fluid from trophoblast to embryo • Blood vessels first form in its wall. • Primordial germ cells arise from its caudal end. • It forms the dorsal wall of gastro intestinal tract. Changes in trophoblast

Outer layer of blastocyst made up of a single layer of cells, trophoblast.

• It differentiates into 2 layers. • Cytotrophoblast - Cell margins are well defined, nuclei are round & cytoplasm is lightly stained. • Synciotrophoblast - cells are dark basophilic, with dark multi nucleated cells & cell margins are not well defined

Changes in trophoblast • Lacunar stage—10-11 days • Syncytiotrophoblast multiplies, spaces develop in it called lacunae.

• Thus forms Utero plancetal circulation stage—11-12 days • The syncytiotrophoblast invade the maternal blood vessels in endometrium, blood starts filling & flows through lacunar network.

Chorion

• The extra embryonic mesoderm, cyto & syncytiotrophoblast (from outside in) is called chorion. • Extra embryonic coelom is now named chorionic cavity & surrounds amniotic cavity & yolk sac. • Extra-embryonic coelom does not extend that part connects the embryo to trophoblast by a connecting stalk that becomes future

Chorionic villi Day 13

Syncytio trophoblast secretes Human chorionic gonadotropin (HCG). This maintains corpus luteum of pregnancy till placenta develops.

• The finger-like processes arise from chorion called chorionic villi. • They develop into primary, secondary & tertiary villi. • At the stage of formation of bilaminar disc, only primary chorionic villi are seen at the end of 2nd week. • Primary villus showing a core of cytotrophoblastic cells covered by a layer of syncytium.

Chorionic villi

• Secondary villus - with a core of mesoderm covered by a single layer of mesoderm & cytotrophoblastic cells, which in turn is covered by syncytium. • Tertiary villus – formed at end of the third week, small blood vessels, appear in mesoderm forming the villous capillary system. • Tertiary and secondary stem villi give the trophoblast a characteristic radial appearance. • Intervillous spaces filled with maternal blood CHORIONIC VILLI SBA

Embryo is formed by

1. Trophoblast cell 2. Inner mass cells 3. Cytotrophoblst cells 4. Synciotrophoblast cells https://www.youtube.com/watch?v=9JLQDmrj7fI https://www.youtube.com/watch?v=bIdJOiXpp9g Development during third week

• Formation of 3 germ layers

• Events Precordal plate Primitive streak Intra embryonic mesoderm Definite Endoderm and Ectoderm Notochord and Neural tube Segmentation of intra embryonic mesoderm Folding of embryonic disc

Gastrulation

• Process by which the bilaminar disc is converted into trilaminar embryonic disc is called as Gastrulation. • Starts on 14th day after fertilization • Begins with formation of primitive streak: thickened linear band on epiblast • Embryo is known as gastrula. Prochordal plate

• some cells of hypoblast (primitive endoderm) cells become columnar at one end of bilaminar embryonic disc. This area is called as Prochordal plate • It marks the head (anterior) end of the embryo Primitive streak

• a faint groove appears on dorsal aspect in the midline of epiblast is called as Primitive streak • It results from proliferation and migration of cells of epiblast • Elongates by addition of cells from caudal end • Primitive streak is at caudal end & helps in identifying craniocaudal (AP) axis of embryo

Axis of embryo

• Prochordal plate – head end of the embryo • Primitive streak – caudal region

Formation of Intra-embryonic mesoderm • Epiblast cells on each side of primitive streak begin to proliferate & they migrate through the primitive streak between epiblast & hypoblast to form intra-embryonic mesoderm

Electron micrograph showing epiblast cell and primitive node Formation of Intra-embryonic endo & ectoderm

• Some of the migrating epiblast cells displace the hypoblast to form the embryonic endoderm • After formation of endoderm & intraembryonic mesoderm, the epiblast forms the embryonic ectoderm. • cells passed throughout disc except at prochordal plate &

Prochordal plate & Cloacal membrane

Intraembryonic mesoderm spreads throughout the disc except in 1. Prochordal plate - is present near cranial end of germ disc. Later remains as Buccopharyngeal membrane. 2. Cloacal membrane - is a small circular area at caudal end of embryo. It is future anus.

• In these places ectoderm and endoderm come in contact

Tri-laminar embryonic disc -summary • Embryonic disc is flat  circular (Bilaminar) oval shape (trilaminar)

The ectoderm, intraembryonic mesoderm & endoderm all are therefore derived from epiblast SEQ Write about formation of tri laminar embryonic disc

• Process of formation of intra embryonic mesoderm is Gastrulation • Some cells of hypoblast, become columnar at one end of bi laminar embryonic disc. This area is called as Prochordal plate • Epiblast cells also proliferate at opposite end . This is primitive streak. • The cells of primitive streak divide very rapidly and spread between epiblast and hypoblast. Now this layer is intra embryonic mesoderm. • The cells of primitive streak also replace the hypoblast and form endoderm • Now remaining epiblast becomes ectoderm. Fate of primitive streak • Until 4th week, forms intraembryonic mesoderm, after slows down & diminishes in size • Insignificant structure in sacrococcygeal region • Degenerates

• Remnants of primitive streak persist as Sacrococcygeal teratoma

SBA

Epiblast cells give rise to

1. Definite ectoderm 2. Definite endoderm 3. Intraembryonic mesoderm 4. Primitive streak 5. All 6. None of the above

NOTOCHORD

• Cranial end of primitive streak proliferates to form primitive node or Hensen’s node & surrounded by & narrow appears • Cells in primitive node multiply & migrate cranially in midline, forming a median cellular cord, the notochordal process. It grows cranially between the ectoderm & endoderm until it reaches the prochordal plate • This process soon acquires a lumen, the notochordal canal. NOTOCHORD… • The floor of the canal & underlying endoderm break down • The roof of canal persists as notochordal plate • A transient opening communicates the yolk sac with amniotic cavity called neurenteric canal • Notochordal plate cells proliferate & detach from endoderm • Notochordal plate will then curved and form a solid rod called definitive notochord

https://www.youtube.com/watch?v=3AOoikTEfeo

Notochord – fate & importance

• It induces ectoderm to differentiate into to form the . • In later life notochordal position is occupied by vertebral column. • It remains as nucleus pulposus of each intervertebral disc Allantoenteric diverticulum : At the same time the posterior wall of the yolk sac forms a small diverticulum that extends into the connecting stalk & is called the allantoentric diverticulum or Allantois. This appears at about 16th day of development. Take part in formation of urinary bladder, blood vessels in allantois become umbilical vessels Neurulation /Neural tube

• Processes involved in the formation of the neural tube. • It appears rostral to the primitive node and dorsal to the notochord in midline overlies the notochordal process • Neural tube will form the (cranial part) brain & the spinal cord ( caudal part). • As the notochord elongates, the neural plate broadens and eventually extends from primitive node caudally to prochordal plate cranially. • Completed by the end of the 4th week.

Neurulation Neurulation….. • The notochord induces the overlying ectoderm to form the neuroectoderm.

• It thickens and forms plate • On the 18th day the plate invaginates in the longitudinal plane to form a neural groove. • The groove has neural folds on either side. • Then neural folds move towards each other & begin to fuse. • Thus they convert the plate into a tube – neural tube

Neurulation….. Anterior Neuropore

Neural tube

Posterior neuropore 23 days • Neural tube has 2 openings: – Cranial neuropore closes at day 25 (18- to 20- stage) – Caudal neuropore closes at day 28 (25-somite stage) • Neurulation is then complete • Central nervous system is represented by a closed tubular structure

Neurulation – Neural Crest cells

The cells at the junction of the & neuroectoderm become specialized & form Neural Crest Cells. Derivatives of germ layers DERIVATIVES OF ECTODERM Structures that maintain contact with outside world Derivatives of endoderm

Lines inner aspect of organs

• Epithelial lining of gut / GIT. • Epithelial lining of respiratory tract • Parenchyma of thyroid, parathyroid glands, liver & pancreas • Reticular stroma of tonsils &thymus • Epithelial lining of urinary bladder, urethra • Epithelial lining of auditory tube, tympanic cavity.

- It splits into two layers because of formation of intra embryonic coelom & forms Pleural, Peritoneal & Pericardial cavity

Summary………… Formation of Notochord summary….. OSCE- Identify structures

Notochordal plate Notochordal canal

Neurenteric canal Definitive Notochord Formation of neural tube summary….. Model SEQ

• Define and describe hypoblast, & epiblast • Describe the formation of the bilaminar embryonic disc • Describe the formation of the primitive streak • Describe the formation of the primary and secondary yolk sacs • Describe the formation of the amnion and the chorion cavities • State the origin of the three embryonic germ layers that make up the trilaminar disc • Define gastrulation • Describe the notochord formation • Describe the neurulation process • Describe the formation of neural crest cells • List the derivatives of the different germ layers

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