Trilaminar germ disc By Dr Manah Chandra Changmai MBBS MS
Gastrulation Formation of embryonic mesoderm and endoderm Primitive streak on the surface of the epiblast The cephalic end of the streak,known as primitive node Primitive node surrounds the a small primitive pit
Day 14 -15 raised groove appears on dorsal surface of the epiblast – cells migrate inward at the streak, displacing underlying hypoblast to become the endoderm Day 16 – ingressing epiblast cells form the mesoderm (a new layer between the epiblast and endoderm) Epiblast cells that remain on the embryo’s dorsal surface make up the ectoderm Ectoderm & endoderm = epithelia Mesoderm = mesenchyme tissue (mesen=middle; chyme=fluid) Primitive streak
Cells invaginating the primitive pit Move straight forward in cephalic direction upto prochordal plate. Form a tube like process known as notochordal or head process. By the 18 th day,floor of the notochordal process fuses with underlying endoderm Lumen of the notochordal process disappears completely Notocordal process forms a narrow plate of cells Notochordal cells proliferate and forms solid cord,known as definitive notochord. Formation of notochord
Formation of cloacal membrane Cloacal membrane is formed caudal to primitive streak, consists of tightly adherent ectoderm and endoderm. At 16 th day a diverticulum is formed at the posterior wall of yolk sac which extends into connecting stalk. This diverticulum is called allantoenteric diverticulum or allantois. In humans it remains rudimentary.
Embryonic disc,initially flat and almost round Later becomes elongated with broad cephalic and narrow caudal end. At the end of 4 th week primitive streak shows regressive changes and soon disappears. Sometimes remnants of primitive streak persists persists and at birth cause tumours of sacrococcygeal region. Changes in embryonic disc
Further development of tropoblast After 3 rd week mesodermal cells penetrate the core of primary villi forming secondary villi. At the end of 3 rd week mesodermal cells in the core of villi differentiate into blood cells forming tertiary villus.
Cytotropoblastic cell penetrate into the syncytium until they reach the maternal endometrium Establish contact with similar neighbouring villous stems forming thin outer cytotropoblastic shell. Villi that extend from chorionic plate form decidua basalis called the stem or anchoring villi. Chorionic cavity becomes larger by 19 th or 20 th day Embryo is attached to tropoblastic shell by connecting stalk Further development of tropoblast .