we have a positive feedback system, which
is the opposite. Well, let’s look at the
pituitary gland. It’s often called a hypophysis.
Here is, on the left-hand side, a diagram
representing the structure of the pituitary gland.
And on the right-hand side is a histological
section through the pituitary gland. And it’s
important that we go through a number of
different components. The pituitary gland is really
a very small pea-shaped gland. In males, it’s
about half a gram in white. In females that
have had two or more children, there can be
about one and a half grams. So there’s a
difference in size. The pituitary gland sits
in a depression called the sella turcica in
the sphenoid bone at the base of the brain,
at the base of the skull. And as you can see
in both these sections, first the section
of the image or histological section through
the pituitary gland on the right, and the
diagram on the left, the pituitary gland is
connected to the brain. It’s connected to
the hypothalamus through a structure we call
the infundibulum. And that’s a very important
connection because the hypothalamus influences
the secretory nature and products coming out
the pituitary gland. Well, the other thing you
see in this diagram is that the pituitary
gland has two different components, two different
major components. On the right-hand side on
the histological section, the far left component
of that section is a lighter purple color
compared to the rather more enlarged component
on the right-hand side of this section.
And this coloration defines or distinguishes the two
parts of the pituitary gland. The lighter
stained area is the posterior part of the
pituitary or the posterior pituitary gland.
We call it often the pars nervosa or the nervous
part or the neurohypophysis because another
name for the pituitary gland is the hypophysis.
On the right hand side, the darkest stained
region represents the anterior pituitary gland,
the bit towards the anterior of the body.
It’s often called the adenohypophysis, adenoma
glandula. It’s often called the pars
distalis as well. Now, both these components
of the pituitary gland are derived from two
separate embryological locations. And that’s why
they should be really considered two separate
glands. The pars nervosa, the nervous component,
is derived from a downgrowth from the third
ventricle of the brain. So it’s derived
from neuroectoderm. The anterior pituitary
is derived from an out-pocket of the oropharynx or Rathke's
pouch. So it’s derived from oral ectoderm. So these two
differences are the reasons why histologically,
they are very different when we look at them
using a microscope. Now, the diagram on the
left-hand side also shows a number of different
structures associated with the pituitary gland.
There is part of the pars distalis or the
anterior lobe of the pituitary, that extends
up and wraps around the infundibulum, the
part of the stalk or the connection to the hypothalamus.
There's another part . They are called the pars intermedia.
This is of little relevance to us,
so I’m not going to cover it
in any detail. Have a look at the hypothalamus,
that yellow stained component. In the hypothalamus,
you can see two nuclei labelled. These nuclei are called
the paraventricular nucleus, and also the supraoptic nucleus.
They are going to be very very important structures
that I’m going to refer to in the next couple of slides.
This diagram, again,
appears to be rather complicated, but there is
only a couple of very simple stories that I want to emphasize.
First of all, have a look in the hypothalamus.
Again, look at the paraventricular nucleus
and the supraoptic nucleus.
A nucleus in the brain is a collection of nerve cell bodies.
And they’re shown here on this diagram. Notice though,
that the cell bodies project their axons down the
infundibulum or the pituitary stalk
into the location of the posterior pituitary,
Now these neurons are neurosecretory. They don’t transmit
an impulse. What happens is that oxytocin and ADH,
two hormones, are produced by these two nuclei,
by the cells in these two nuclei. Oxytocin is produced by
mostly a paraventricular nucleus.
ADH is produced mostly by the supraoptic nucleus.
And those secretory products travel down the
axons and are stored in axon terminals
in the neurohypophysis. And we’ll see that in a later slide.
So the secretion of oxytocin and ADH is from neurosecretory cells
directly into fenestrated blood capillaries indicated
there in the diagram. The blood supply
to this region comes from the inferior hypophyseal artery.
So that’s the major component of
the paraventricular nuclei and the supraoptic nuclei
providing secretory products into the posterior pituitary.
There’s also another nuclei,
group of nuclei that you see in the hypothalamus as well.
We’ve got a very long name to it,
hypothalamohypophysiotropic nucleus or nuclei.
They contain neurons
that secrete releasing factors. And those releasing factors
get taken up by capillary bed I’ll mention in a moment,
up in the hypothalamus. And those releasing factors
when they get into the capillary bed,
can affect the secretory products from the anterior pituitary.
And I’ll go through that in a moment.
If you look at the adenohypophysis,
the anterior pituitary, you’ll notice that
in the diagram, there are three cell types
represented, basophils, acidophils,
and then cells that don’t take up any stain. And this is because
in the anterior pituitary, the typical glandular
epithelial component of the pituitary gland,
there are different cell types
based on their staining characteristics.
We have cells collectively called chromophils.
These are the ones that have an affinity for color,
and they are the basophils or the acidophils.
Then we have the chromophobes
which make up a large proportion of the cells that
don’t take up the stain. And we’re not quite sure
whether this have any functional significance or
whether or not they happened to be
the chromophils that are just going through a
process of resynthesizing their secretory products.
But I’ll show you these cells
in more details in a moment.
But just imagine now that the
anterior pituitary is full of these
different secretory cells that secrete different products we’ll see
in a moment. And those cells secrete into the vascular system.
So it’s important now that I explain
the vascular supply to the
pituitary gland. If we go back
to the hypothalamus region,
you’ll see just at the top, a vessel called the
superior hypophysial artery. Now this artery
branches and it provides the hypothalamus with a
network of capillaries. It’s called the primary capillary plexus
or network. And it’s in just the upper part of the
infundibulum, that stalk that connects the pituitary gland
to the hypothalamus
That’s a very important primary plexus of capillaries
because those secretory products,
the releasing factors I mentioned earlier from those
hypothalamohypophysiotropic nuclei, are secreted
into that primary capillary plexus. From that plexus,
the blood then flows down through portal veins,
along the infundibulum, and then they all
supply the anterior pituitary. They don’t go the
posterior pituitary. So this is a situation where the releasing factors
that are secreted from those nuclei in the hypothalamus,
I’m not going to try and say that word again, they pass down
through the capillaries and can affect
the secretion of the cells in the
anterior pituitary. These releasing factors can
either stimulate cell secretion
from these cells, and they’re very specific what cells
they actually stimulate, or they can inhibit
secretion from these cells.
So there is this control of the secretory products
coming out of the anterior pituitary
by neurons in the hypothalamus.
Posterior pituitary, as I said before, does not receive these
portal vessels, and therefore, the secretion products coming from the
neurosecretory cells in the posterior pituitary
are unaffected by these releasing factors.
This hypothalamic-hypophysial portal system
is an extremely important concept to understand
and it’s an extremely important piece of histology and microanatomy
influencing the function of the anterior pituitary gland.
Well, here is a little summary of, first of all, on the
right-hand side, the components of the pituitary gland, and on
the left-hand side, those components in
diagrammatic form. And I think it’s important for
you to fully understand that there are these
components in the hypothalamus, neurons in the hypothalamus,
understand where they project their axons
and what the function of these projections
and their products are in the control
of the other glands that we’ll see
from these secretory products, and understand also
the concept of epithelial cells in the anterior pituitary
staining differently and influenced via
releasing factors coming down that
hypophysial or hypothalamic-hypophysial
portal system. Let’s have a look