Third Week of Development Flashcards
Describe the formation of the definitive endoderm.
In the amniotic cavity, a streak begins to form called the primitive streak, with the primitive groove forming on the surface. At the cranial end of the streak is a primitive node. A depression located posteriorly on the primitive node is the primitive pit.
Epiblast cells undergo epithelial to mesenchymal transition, then begin to migrate towards the primitive streak. They move into the groove and invade the hypoblast layer (invagination). Then they displace the hypoblast cells to replace it forming a new layer called the definitive endoderm.
What are the functions of the notochord?
Defined the primordial longitudinal axis.
Provides signals needed for the development of muscoskeletal structures and the CNS.
Contributes to the invertebral discs.
A primary inductor in the early embryo.
How is the notochordal process formed?
The cells of the epiblast migrating through the cranial part of the primitive pit will form and elongate the notochordal proess. This moves towards the cranial end, towards the oropharyngeal membrane.
The notochordal process is a hollow tube that lies between the endoderm and ectoderm.
The primitive pit extends into the notochordal process to form a notochordal canal.
How is the neural plate formed?
As the notochord forms, it induces the overlying ectoderm to form a thickened epithelial layer of an elongated neural plate.
Describe the formation of the mesoderm and ectoderm.
The epiblast cells remaining in its original layer become know as the ectoderm.
Epiblast cells remaining in between the endoderm and ectoderm layer differentiates into the mesoderm.
How is the notochord formed after the notochordal process takes on the form of a cellular tube?
The floor of the notochordal process fuses with the hypoblast layer underneath. This layer granually degenerate to result in communication between the amniotic sac and the yolk sac.
The floor eventually disappears, leaving a grooves notochordal plate.
Starting from the cranial end, notochordal cells proliferate and the notochordal plate infold to form the notochord.
Describe the structures surrounding the notochord at the end of its formation.
The proximal part of the notochordal canal remains as the neurenteric canal, forming communication between the yolk sac and amniotic sac. This will be obliterated with full formation of the notochord.
The notochord is detached from the endoderm, which again forms a continious layer underneath.
How does the neural crest form?
As the neural folds fuse, some cells along the inner margins undergo epithelial to mesenchymal transition, migrating away and forming the neural crest as neural crest cells.
This is a flattened irregular mass between the neural tube and the ectoderm.
They then separate into right and left parts shifting to the dorsalateral aspects of the neural tube.
How does the neural plate fold into the neural tube?
On approx day 18, the neural plate invaginates along its median axis to form a neural groove, with neural folds on each side.
The neural fold then begin to move together and fuse, forming the neural tube. This separates from the surface ectoderm as the neural folds meet.
The folding begins from the middle of the embryo.
What does the neuroectoderm of the neural plate give rise to?
The brain and spinal cord most importantly, as well as others like the retina.
Describe the formation of the primitive circulation.
-
What is the allantois?
-
What does the mesoderm initially separate into? Describe these separations.
Paraxial, intermediate and lateral.
The paraxial mesoderm is closest to the notochord. They continue as a thick, longitudinal column of cells.
The intermediate mesoderm is laterally continiuos with this. It then thins into lateral mesoderm, which is continious with the extraembryonic mesoderm covering the amniotic sac and yolk sac.
Which part of the mesoderm forms the somites?
Paraxial mesoderm
What structures do somites differentiate into? What do they give rise to?
The somites give rise to the axial skeleton, voluntary muscle, body walls and limbs as well as the dermis.
Somites differentiate into 2 structures, the sclerotome and the dermomyotome. The dermomyotome will form the dermis and muscles, while the sclerotome will contribute to the formation of the vertebrae.
