BILAMINAR DISC, TRILAMINAR DISC & THEIR DERIVATIVES Dr. Sangeeta Kotrannavar Assistant Professor Dept. of Anatomy USM- KLE –IMP, Belagavi. Learning Objectives • Define and describe hypoblast, epiblast, primitive streak, primitive node, notochordal process, prochordal plate, cloacal plate, notochord, ectoderm, mesoderm and endoderm • Describe the formation of the bilaminar embryonic disc • 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 connecting stalk • 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 Human Development 1st Fertilisation, Cleavage, Morula, Blastocyst & implantation week 2nd Trophoblast differentiation into syncytiotrophoblast & week cytotrophoblast Bilaminar embryonic disc Amniotic cavity & chorionic cavity Primary & secondary umbilical vesicle Somatic & splanchnic extraembryonic mesoderm 3rd Trilaminar disc, Notochord, Neural tube week Intraembryonic mesoderm, Coelom Allantois Primitive blood vessels Secondary and tertiary chorionic villi Folding of embryo 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 inner cell mass, 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 placenta 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 yolk sac 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 umbilical cord 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 germ layer 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 & cloacal membrane 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