Let’s now look at the thymus.
Section here shows what the typical thymus
looks like. It has got a capsule and
thymic lobules. The thymic lobule has within it a
dark staining cortex and a lighter staining
medulla. And one feature of the thymus, one way
in which you can identify the thymus from
other organs is the fact that it contains
thymic or Hassall’s corpuscles which I’ll
show you in high magnification. The thymus
is quite a complex organ, but quite easy to
describe and identify histologically, as I
just pointed out to you. On the left-hand
side is a diagram illustrating the compartments
of the thymus, the cortex and the medulla.
What essentially happens is that the cortex
contains T cells undergoing education.
And they undergo education in various units or
subdivisions of the cortex. And then they
pass into the medulla where they undergo their
final education before then moving out to
the body. I sort of used the analogy that’s
like a school. The cortex is like grade school
or primary school where you’re receiving
a certain education, and then you graduate
and go to high school. Here in the cortex,
T cells undergo a certain amount of training.
They go to the medulla then and then graduate
as fully mature immunocompetent T cells. But one difference
here is that only 2% ever graduate and move out
of the thymus medulla. The rest are apoptosed
or digested by macrophages. They don’t complete
their education. They recognize maybe self-antigens etc.
So the body gets rid of them. On
the diagram on the left, you can see rather
a complex array of cells. There are six major
cells that live in the thymus cortex and the
thymus medulla. And they’re called epithelioreticular
cells. They’re derived from the epithelium,
and as I said before, they don’t secrete
reticular fibres, they’re not reticular
cells. The type I are the ones that are on
the surface of the cortex. These are the ones
that actually separate the trained or the
educating thymus T cells from connective tissue.
These cells form a barrier around all the
connective tissue, the capsule, the trabeculae,
and even the connective tissue around the
blood. So they had jobs to separate these
thymus cells or T cells undergoing education.
Type II cells are in the network of the cortex.
They compartmentalize the cortex into areas
such as you can see shown on the diagram.
And I’m not going to go into all the details
of the physiology of the thymus here,
I’m just pointing out some of the histological
or structural features. The type II cells
are also involved in training, educating the
T cells. Type III cells are also involved
in education of the T cells, and they form
a barrier between the cortex and the medulla,
a very strong barrier to stop cells from passing
from the cortex into the medulla that haven’t
gone through the proper training program.
The type IV cells are in the medullary side.
And what they do is they form a seal again
on the medullary side so you have two cells,
the type III and the type IV, making this
barrier from the cortex into the medulla.
Notice that the medullaries in most of that
section you see on the right-hand side, lighter
staining, lighter staining because there're
far less cells in there because as I said,
only 2% of cells ever make it out from the
thymus medulla. The rest die. And then finally,
get the type VI. These are called the Hassall’s
corpuscles. They’re the ones that are probably
very aged epithelioreticular cells, and they
form these spiral coils very easy to identify
in the medulla, and therefore, they're characteristic
of the thymus, and as I said earlier, enables
you to identify thymus compared to other organs.
And finally, we need to say something about the
blood-thymus barrier. We don’t want antigens
and other components of the blood, self-antigens,
interfering with the education of these T cells.
So, these type II and type I epithelioreticular
cells form a barrier, a wrap around these
capillaries. And along with the endothelial
cells and the basal lamina and macrophages,
prevent the contamination of the educated
T cells or T cells undergoing education from
being exposed to those antigens.
Macrophages live within the thymus, the cortex,
and the medulla. And they are busy wrapping up or
phagocytosing cells that don’t pass the grade.
So again, in a review, make sure you’re
aware of the structure of the cortex and the
thymus medulla, and the role or function of
these special epithelioreticular cells.