Please note that this video is not meant to provide a detailed insight into the third
week of development it's merely a quick review that's why we highly recommend
that you watch our whole detailed series on this topic before you watch this review
so that you can catch up with us perfectly. As you know that at the end the end of 2nd
week of development, we were left with a two layered (bilaminar) germ disk present between
two cavities I.e., amniotic cavity and yolk sac cavity. Whole this structure is covered
on the outside by extraembryonic mesoderm, cyto and syncytiotrophoblast, and this structure is
implanted into the endometrium of the uterus. During third week of development this bilaminar
germ disk is converted into three layered (trilaminar) embryonic disk
by the process known as gastrulation. The process starts when cells in the caudal
part of epiblast secrete chemical substances that cause proliferation and migration
of the peripheral cells towards the midline, hence producing a streak, called the primitive streak.
This streak is delineated cranially by an ovoid swelling called Hensen’s node or Primitive Node.
The proliferated cells start pinching down thus leaving a depression or groove behind which is
called primitive groove. They, then, displace the lower layer (i.e hypoblast layer), thus
taking its place. Some more cells come to lie in between the two layers thus producing
a third layer in between the two, hence forming a definitive three-layered germ disk.
The embryo becomes a trilaminar structure except at two places viz. Oropharyngeal membrane
and cloacal membrane. The endodermal part of oropharyngeal membrane is called prechordal plate.
The first few of these ingressing cells move forward till the oropharyngeal membrane.
Here, they displace the original prechordal plate laterally and these new population of cells now
forms the definitive prechordal plate. Cells at Caudal end of prechordal plate proliferate and
form a new layer dorsal to it called prechordal mesoderm. As it happens, the oropharyngeal
membrane is now restricted to cranial end only. Further cells ingressing from the primitive node
towards the prechordal plate make notochordal process. The central cells of this process then
die out and a canal called notochordal canal is formed. Notochordal canal then intercalates
into the underlying endoderm. The floor of this canal is lost and a temporary connection called
Neurenteric canal between the amniotic and yolk sac cavity is formed. As the floor is lost,
the canal is now converted into an arc like structure called notochordal plate. Later,
the notochordal plate rolls upward and grow to form a flexible solid rod called notochord.
Notochord induces the overlying ectoderm to thicken and form the neural plate.
This thickened neural plate then folds inwards joining the folds and leaving some cells called
Neural Crest Cells separated from it. This meeting of folds results in formation of neural tube,
which the precursor of future brain and spinal cord and this process is called neurulation.
The mesoderm is differentiated into different types. At the mid axis of the embryo is the
axial mesoderm also called notochord. The part of mesoderm that runs just parallel
and lateral to this axial mesoderm is paraxial mesoderm. The most lateral one is lateral plate
mesoderm and in between lateral plate, and paraxial mesoderm is intermediate mesoderm.
Cells that ingress from the Henson’s node form the prechordal plate and the notochord.
Cells that ingress from the cranial end of primitive streak form paraxial mesoderm. Cells
from midstreak form the intermediate mesoderm. Cells from caudal part of streak forms lateral
plate and the cells from the caudal end of streak contributes towards the extraembryonic mesoderm.
Now, let us revise the functions of these parts of mesoderm.
Paraxial mesoderm is divided first into partially segmented blocks called somitomeres. Later,
except first 7 Somitomeres, all other somitomeres are transected into completely isolated segmented
bodies called somites. These somites are further differentiated into dermotomes;
that form skin, myotomes; that form muscles and the sclerotomes; that form your axial skeleton.
The Intermediate mesoderm is the precursor of urogenital system.
For the Lateral Plate Mesoderm, cells within the center of lateral plate mesoderm die
and form a cavity called intraembryonic celom thus splitting the lateral plate
into somatic and splanchnic layers. In the third week, allantois also forms.
It is a sausage like diverticulum that arises from the yolk sac into the connecting stalk.
It functions in the excretion of waste materials in the early embryo and forms the
part of urinary bladder as the embryo develops. In this week blood vessels are also formed.
For this purpose, cells of the yolk sac wall induce the overlying extraembryonic
mesoderm to form the blood islands that are aggregates of hemangioblasts.
The central cells of each blood island die and a cavity forms. Some cells that remain alive
within this cavity are the embryonic precursors of the blood cells. The peripheral cells flatten
out to form endothelial cells. They orient themselves in such a way that they coalesce
with the neighboring islands to form long vascular cords. The mesenchymal cells around them form the
surrounding connective tissue and smooth muscle. This process is called vasculogenesis. It is an
exclusive embryologic process. A type of the endothelial cells,
called the tip cell sprout towards VEGF and form a totally new vessel from the existing vessel.
This process is called sprouting angiogenesis. In another type of angiogenesis called splitting
Angiogenesis or non-sprouting angiogenesis, a vessel literally splits into two. Angiogenesis
is an embryologic as well as a postnatal process. Another noticeable thing at the end of second week
was that the embryo was covered with primary villi that are solid finger like processes
of cytotrophoblast. In third week, a mesenchymal extension from the underlying extraembryonic
mesoderm develops in the core of these primary villi transforming them into secondary villi. This
mesenchyme then forms the blood vasculature of these villi that are now termed as tertiary villi.