Embryonic folding

embryonic folding

overview

The trilaminar embryo at approximately day 21

Using standard embryological convention:

Day 23

The extraembryonic mesoderm forms a mushroom shape (forming what appears to be a cap and the beginning of a stalk)
The ectoderm-lined floor of the amniotic cavity curls under at its edges the endoderm-lined roof of the yolk sac, also tucks under.
Accordingly, so does the intraembryonic mesoderm.

Day 26

The extraembryonic mesoderm further curls and the stalk (the vitelline duct) narrows.
The curling of the amniotic cavity accentuates and so does the yolk sac and the intraembryonic mesoderm.

The trilaminar germ disc structures at Days 21, 23, and 26

Day 21

from top to bottom

The trilaminar germ disc comprises ectoderm, mesoderm (which is intraembryonic), and endoderm.
Within the mesoderm, lies the the ectoderm-derived neural tube, notochord, bilateral somites, and neural crest.
There is folding of the neural tube.

the neural folds abut centrally, first, but remain open at their ends anteriorly and posteriorly.
the neural groove lies deep within the neural tube.
the neural crests form at the tips of the neural folds.

Day 23

The neural tube is now folded a long distance along its center but remains open at the anterior and posterior neuropores.
We see the somites, centrally, where the neural folds abut; they generate bumps that appear on the surface of the overlying neural tube.

Day 26

The somites (the paraxial mesoderm) differentiate into the central musculoskeletal elements – (from medial to lateral): sclerotome (which forms bone), myotome (which forms muscle), and dermatome (which forms skin).

embryonic folding with the development of the key structures of the trilaminar germ disc

The trilaminar embryo at approximately day 21

Using standard embryological convention:

The ectoderm in blue (within the amniotic cavity)
The endoderm in yellow (within the yolk sac)
Intraembryonic mesoderm lies in between them.
Extraembryonic mesoderm surrounds the embryo.
The connecting stalk connects the embryo to the uterus.
The embryo lies within the chorionic cavity, which, itself, is lined with extraembryonic mesoderm.
The allantois is the tip of the posterior endoderm that extends into the connecting stalk – a hindgut diverticulum.

Day 23

The extraembryonic mesoderm forms a mushroom shape (forming what appears to be a cap and the beginning of a stalk)
The ectoderm-lined floor of the amniotic cavity curls under at its edges the endoderm-lined roof of the yolk sac, also tucks under, accordingly, so does the intraembryonic mesoderm.
The connecting stalk is tucked under the endo- and ectodermal folds.
The neural tube is now folded a long distance along its center but remains open at the anterior and posterior neuropores.

Day 26

The extraembryonic mesoderm further curls and the stalk (the vitelline duct) narrows.
The curling of the amniotic cavity accentuates and so does the yolk sac and the intraembryonic mesoderm.
There is further folding of the connecting stalk and outpouching of the endodermal allantois.
There's further the growth of the neural tube, which is fully closed (anterior and posterior neuropores have closed).
The gut structures endoderm forms are visible: from anterior to posterior – the foregut, midgut (which attaches to the yolk sac via the vitelline duct), and the hindgut.