00:00
First, I will describe
the thyroid gland. It’s a bilobed gland. It’s
in the neck region. You can see labelled
here at the top of the left-hand section through
the neck region, a lobe of the thyroid.
00:18
It’s separated by the trachea, so one lobe is on
either side, joined by a very thin isthmus
of thyroid secretory units that I won’t describe
in this particular lecture. Trachea
is a roughly circular structure. If you look
carefully, it has got the cartilage rings
that support it. And then the other structure
on the far left of this particular section
is the esophagus. On the right-hand side,
you can see details of the thyroid gland at
a higher magnification. If you look very carefully
at this particular section on the right-hand
side, you can see that it’s made up of structures
or functional units of the thyroid we call
thyroid follicles. And look very carefully into
this section. These thyroid follicles are
of very different size. Their diameters range
from about 0.2 millimeters to about 1 millimeter.
01:22
And this probably reflects their real size,
not necessarily the different state of activity
of each follicle. We’ll see this again when
we look at the endocrine component of the
pancreas. When you look at the follicle, the
thyroid follicle under higher magnification,
you can see on the left-hand picture more
details of the structure of this follicle.
01:51
It has cells in it called follicular cells.
These follicular cells are epithelial cells,
they're endocrine cells. They form the lining
or the wall of each follicle. And within the
follicle itself, you can see colloid. This
is a gel-like component. It’s actually
stored product that consists of thyroglobulin.
These follicular cells secrete that thyroglobulin
and store it in the lumen of each follicle.
This is an example of the only cell in the
body, the only endocrine cell that actually
synthesizes its hormonal product and stores
it extracellularly outside the cell. These
are cuboidal type epithelial cells. But have
a look at the very edge of the cells or inside
some of the cells you see, and you can see
some little tiny dot specs. The edge of the cell
is seem to be a little bit sort of unusual
or uneven. This represents structures that
I’ll show you and talk about in a later
slide. This slide shows a diagram of two cells,
two follicular cells in the thyroid follicle.
03:14
The green colored component of each of the
cells in the diagram at the top represents
the colloid. You saw it stained pink, actually,
in the previous slides. Remember that colloid
stores thyroglobulin. When that thyroglobulin is
synthesized on the rough or granular endoplasmic
reticulum inside the cell, and then it’s
glycosylated as it passes through the endoplasmic
reticulum, and also the Golgi apparatus shown
in blue. That Golgi apparatus then packages
the thyroglobulin into vesicles, which move
to the surface of the cell and are released
into the space of lumen as colloid by
a process called exocytosis. These follicular
cells also have microvilli. So that surface
that I explained to you earlier in the slide,
on the light microscope picture when I said
they had little specs, little uneven boundaries
close to the lumen, that represents all these
exocytotic activity and the little vesicle
stored at the apex of the cell on their way
to being deposited into the colloid.
04:38
On the right-hand side, you can see that the cell
is representing the absorption, the endocytosis
of the colloid. The microvillus projections
wrap around segments of the colloid. And then
by endocytosis, that colloid is then brought
inside the cell, where it fuses with lysosomes
again at the apex of the cell. And again,
this sort of activity is reflected in the
uneven surface that you see when you look
at the surface of the cells I pointed out
in the light microscope pictures. That endocytosed
thyroglobulin then reacts with the lysosomal
enzymes and other enzymes in the cell, and
then the thyroid hormones, T3, and T4, are
released on the basal aspect of the cell away
from the colloid or the luminal surface into
blood capillaries that passed by. In this
slide, again, it brings the two previous groups
of slides together. On the left-hand side,
you see the light microscope picture taken
through thyroid follicles. You see the follicular
cells labeled and the colloid thyroglobulin.
06:07
And look carefully again at the surface of these
cells, and you can often see little specs
or unevenness that reflect what you see in
the diagram on the right-hand side. In the
middle slide, in the middle diagram, I explained
how the thyroglobulin is produced and deposited
in the colloid, and then how it’s reabsorbed
by endocytosis from that colloid into the
cell to then be broken down and transformed
into the two thyroid hormones, T3, and T4.
06:43
Another step in the process that I want to explain
at this point is that iodide is absorbed
into the cell and highly concentrated. And
that iodide is then oxidized and passed into
the colloid as iodine. And that forms a complex
with the thyroglobulin, a very important complex
which when it’s reabsorbed back into this
cell, as I’ve explained before, contributes
to the formation of the thyroid hormones. So
two very important functions of this cell,
one, to produce the thyroglobulin, and one
to concentrate and oxidize iodide that helps
form this iodinated thyroglobulin complex
inside the colloid. Another very important
feature of this slide is if you again look
at the light microscope picture, you see a
darker pink component running really between
the walls of two thyroid follicles. That dark
pink component happens to be red blood cells,
moving through in capillaries. And as I mentioned
before, the thyroid follicular cells, they
produce the hormone product and secrete them
basally, on the basal aspect of their cells
into these capillaries. And it’s important
to point out here, and you certainly cannot see
it, but these capillaries are very fenestrated.
08:19
And this is a typical type of capillary you
see in endocrine glands. Fenestrations allow
very easy movement of fluid from the interstitium
which contains initially the hormonal product.
08:32
It enables that fluid to pass in to the blood,
and therefore, be carried to the rest of the
body. There is another type of cell in a thyroid
follicle, squeezed inside the epithelium within
the basal lamina or basement membrane around
the follicular epithelium. And this is called
the C-cell or the parafollicular cells. They
have a rather foamy vesicle dominated cytoplasm.
09:05
They secrete calcitonin which is responsible
for lowering calcium levels in the blood. By
means I will describe in another lecture on
the bone, on bone growth and remodeling.