00:00
Let’s now look at the medulla.
Here, the preparation has
used a certain type of stain. And the medulla
appears or the cells in the medulla appear
to be stained with this brownie pigment. They’re
called chromaffin cells because they react
with the chromic component of the stain or
the dye used. And these chromaffin cells make
out the bulk of the histological component
or tissue within the adrenal medulla.
00:35
These chromaffin cells are actually postsynaptic
neurons that have lost the plot. They’ve
gone in a different direction. That’s why
I’ve put a question mark up against their
name in the heading on this slide. The adrenal
medulla was sort of destined to be an autonomic
ganglion, a parasympathetic autonomic ganglion.
Parasympathetic autonomic ganglia are located
either within the organ or very next to the
organ. And they contain preganglionic fibres
coming from this central nervous system that
then synapse with postganglionic fibres in
the ganglion themselves and then they travel
very short distances to the target organs
and cells that they are destined to control or
influence. What happened during the development
of the adrenal medulla is that these cells, these
chromaffin cells decided to become neurosecretory.
01:38
They didn’t develop external processes and become
true postganglionic neurons, true postganglionic
sympathetic neurons. And I've mentioned earlier the
influence of the blood supply drain the
cortex on cells in the medulla. Well, one
idea is that the glucocorticoids that are
secreted by the adrenal cortex by the zona
fasciculata, they percolate down through the
capillary networks in the adrenal cortex into
medullary region, and they influence these
chromaffin cells. They are thought to maintain
these chromaffin cells as being neurosecretory
and inhibit them going through that final
differentiation of developing an axon and
becoming a postganglionic neuron. If you look
in the section, you can see often axons, these
tiny pink round circles you may make out in
the lower left-hand side of this particular
slide. These axons are preganglionic fibers
coming in and influencing the secretion of
the chromaffin cells or the postganglionic
cells that become neurosecretory. And again
on the diagram, the diagram points out
labels these chromaffin cells and indicates
the sort of secretory products that they give.
They secrete adrenaline and noradrenaline.
03:21
If you look very, very carefully in the adrenal
gland, in the adrenal medulla, and here’s
another stained section, there’s a differently
stained component or section of the adrenal medulla.
03:35
If you look very, very carefully, you sometimes
see ganglion cells. These are cells that didn’t
turn into neurosecretory cells. They are postganglionic
cells and they then project their axons to
the cortex, then they can influence the cortex,
the secretions of cells in the cortex.
04:01
Now, one thing I just want to mention before we
move on is that, have a look on the diagram
and have a look down the very bottom and you’ll
see an image of the adrenomedullary vein.
04:14
This carries all the blood from both the cortex,
remember, and the medulla. This is a very
special type of vein. That’s an atypical
blood vessel. It’s atypical because sometimes,
the tunica media is actually absent, and the
chromaffin cells can sit up right against
the wall of the tunica intima, the internal
lining of the vein, and they can secrete their
components, adrenaline and noradrenaline,
directly into the lumen of the blood vessel,
to hasten the rate at which we can transfer
adrenaline and noradrenaline throughout the
body. The other very important structural
change or difference in this particular vessel
is that the smooth muscle of the media is
lined in a longitudinal direction and that
allows the vein to actually contract, to shorten,
and therefore, that acts as sort of a sponge
and squeezes the products along the vein a
lot quicker and add into the blood stream.
05:27
So it’s a specialized vessel, having a specialized
function.