Hello, and welcome to this lecture on the
development of the gut, the second of two
lectures on gut development. In this one,
we’ll be looking at some structures which
grow from the gut or associated closely with
it. So we’ll be looking at the liver, the
gall bladder, the pancreas, the spleen and
we’ll also mention some abnormalities and
variations that can occur around your development.
To understand the gut, of course, we have
to return to folding of the embryo. So in
this image, we see the amniotic cavity in
the upper part of the image, and the yolk
sac down below that. As the embryo sweeps
round, rises up into the amniotic cavity,
so it does at the fore and hind end of the
embryo to begin to create the fore and hindgut.
The midgut remains open to the yolk sac.
Here, we’ve emphasized these in green. So we can
see the formation of the fore and hindgut
as the folding of the embryo takes place.
As the embryo is developing into a C-shape,
so the gut also will begin to develop a curve
running from the mouth down to the anus.
At these early stages, both the mouth and the anus
were closed by membranes, and these membranes
will later disappear. From the foregut, developed
the lungs, and that is explored in another lecture
But in this case, we’re looking particularly at
the liver. So this is going
to develop as a bud from the foregut. Let’s
look at the relationships between the tissues
in transverse section, because this will be
important to understanding the relationship
of the liver with the mesoderm. Here in this
transverse section, we see the amniotic cavity
above and the yolk sac below. As the amniotic
cavity sweeps round the body of the embryo,
so it begins to pinch off part of the yolk
sac, lined of course with endoderm.
It’s also important to understand this in transverse
view because the relationship of the gut and
the liver with the mesoderm is particularly
important. So, in this image, we can see,
first of all, transverse cross-section with
the amniotic cavity above and the yolk sac below.
As the amniotic cavity sweeps round, it begins
to create the definitive intraembryonic
coelom, and the gut is pinched off from the
yolk sac and comes to lie suspended from a
dorsal mesentery and attached by a ventral
mesentery. The yolk sac itself was reduced
to a stalk, and this is in the midgut region
of the embryo. In the image marked D, we can
now see the gut suspended in its dorsal mesentery
above and attached by its ventral mesentery
to the anterior wall of the abdomen on the
inside. It is surrounded by the intraembryonic
coelom, which will later be the abdominal
cavity. It is from the gut that the liver
will begin to grow, and it grows eventually
as down the way from the original gut.