Trilaminar germ disc

By

Dr Manah Chandra Changmai MBBS MS

Gastrulation

Formation of embryonic mesoderm and endoderm

Primitive streak on the surface of the epiblastThe cephalic end of the streak,known as primitive nodePrimitive node surrounds the a small primitive pit

Day 14 -15 raised groove appearson dorsal surface of the epiblast –cells migrate inward at the streak,displacing underlying hypoblast tobecome the endoderm

Day 16 – ingressing epiblast cellsform the mesoderm (a new layerbetween the epiblast andendoderm)

Epiblast cells that remain on theembryo’s dorsal surface make upthe ectoderm

Ectoderm & endoderm = epitheliaMesoderm = 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 18th day,floor of the notochordal process fuses withunderlying endoderm

Lumen of the notochordalprocess disappears completely

Notocordal process forms anarrow plate of cells

Notochordal cells proliferateand forms solid cord,known asdefinitive notochord.

Formation of notochord

Formation of cloacal membrane

Cloacal membrane is formed caudal to primitive streak,consists of tightly adherent ectoderm and endoderm.

At 16th day a diverticulum is formed at the posterior wallof yolk sac which extends into connectingstalk.

This diverticulum is called allantoenteric diverticulum orallantois.

In humans it remains rudimentary.

Embryonic disc,initially flat and almost round

Later becomes elongated with broad cephalicand narrow caudal end.

At the end of 4th week primitive streak showsregressive changes and soon disappears.

Sometimes remnants of primitive streak persistspersists and at birth cause tumours of sacrococcygealregion.

Changes in embryonic disc

Further development of tropoblast

After 3rd week mesodermal cellspenetrate the core of primary villiforming secondary villi.

At the end of 3rd week mesodermal cellsin the core of villi differentiate into bloodcells forming tertiary villus.

Tertiary villi

Cytotropoblastic cell penetrate into the syncytium until they reach thematernal endometrium

Establish contact with similar neighbouringvillous stems forming thin outer cytotropoblasticshell.

Villi that extend from chorionic plate form deciduabasalis called the stem or anchoring villi.

Chorionic cavity becomes larger by 19th or 20th

day

Embryo is attached to tropoblastic shell by connecting stalk

Further development of tropoblast.

The end