Let´s return now to the last part of the mesoderm, the lateral plate mesoderm.
So recall that we´ve got the notochord right in the center of the mesoderm
setting up the different areas, the paraxial mesoderm which becomes somites,
the intermediate mesoderm which becomes the gonads and kidneys
and most laterally we have the lateral plate mesoderm.
The first thing that happens is on day 19 or so in this lateral plate mesoderm
is that we have the formation of little spaces, coelomic spaces, and with other spaces
that we´ve seen before the same process will happen here.
They´re going to be enlarged and fused and as they do so
they form a space called intraembryonic coelom or the space within the embryo.
Now, this space is gonna get larger and larger and as it does so,
it splits the lateral plate mesoderm into an upper portion
that´s in contact with the amniotic cavity
and the ectoderm that´s called the somatic layer of the lateral plate mesoderm
and the lower layer that´s in contact with the endoderm
that´s going to be the visceral layer of lateral plate mesoderm.
Now, as that intraembryonic coelom gets larger and larger,
the somatic layer closely associates with the ectoderm
and actually gets a new name, the somatopleure.
So when you talk about somatopleure that´s referring to both the ectoderm
and the somatic layer of lateral plate mesoderm together.
Likewise, the portion of lateral plate mesoderm, the visceral layer,
that´s in contact with the endoderm when we put those two together
we collectively refer to it as the splanchnopleure.
So splanchno is a prefix that means organs or visceral area
and that´s going to be exactly what happens with this layer,
it´s gonna form some of our organs.
Now, as those fully separate,
the somatic layer of lateral plate mesoderm grows outward
and then folds ventrally and wraps around the splanchnopleure.
So somatopleure enlarges and begins to wrap around the splanchnopleure
starting roughly on the 22nd day of development,
but by day 24 it´s moved even further around
and the splanchnopleure has pinched off
a small region of the definitive yolk sac.
And just as the primitive yolk sac was separated
into the definitive yolk sac and extra coelomic spaces,
the gut tube is forming here
and pushing the rest of the yolk sac further away.
And by day 26, the definitive yolk sac is more or less
been pushed outside of the developing body
all together and what´s left of it inside the body is going to form the gut tube.
And those lateral plate mesoderm somatic layer and ectoderm,
once again the somatopleure,
has completely wrapped around the embryo and seals itself shut on the midline
and this creates the actual body cavity
and this is what allows us to move from a plate of cells,
a little three layered cellular structure,
into an actual tube containing our organs
and the intraembryonic coelom is now completely within the developing embryo
and is gonna form our body cavities.
Thank you very much,
and we´ll return and discuss the formation of endoderm derived structures.