Download Liver, Pancreas, and Gallbladder Anatomy

Endocrine
Anatomy-Histology Correlate
By: Michael Lu, Class of ‘07
- The endocrine system maintains homeostasis and long-term control using
chemical signals, or hormones. These hormones are chemicals released into
the bloodstream and arrive at their appropriate target cells with specific
receptors. In contrast, the nervous system coordinates rapid and precise
responses to stimuli using action potentials that travel along neuron axons.
- In addition to the endocrine route of hormone delivery, there are also the
paracrine pathway (secreted locally to affect neighboring cells) and the
neuroendocrine pathway (hormones that are released by neurons to reach
the target cells).
- The endocrine system involves
the pituitary gland, thyroid
gland, parathyroids, and adrenal
glands, which will be discussed
here. Other components not
discussed here are the
pancreas, ovaries, testes, and
enteroendocrine cells.
- We will start at the pituitary
gland, also known as the
hypophysis. Note its location in
the head, inferior to the
hypothalamus and just above
the nasal sinuses.
- Note the different components
of the pituitary gland. You can
use this picture as a reference
when we discuss each part in
more detail.
- The anterior pituitary, or the
adenohypophysis, is shown here. The
cells are arranged in irregular,
anastomosing cords or cell clusters that
receive rich, abundant blood supply. The
vascular bed may not be obvious.
- The pars distalis (another name for the
adenohypophysis) contains many different
hormone-secreting cells. The type of
staining does not allow us to differentiate
the hormone content. They are as follows:
- A) acidophils – stained by eosin, giving it
a lighter red or orange color.
- somatotropes – secrete growth
hormone (GH)
- mammotropes – secrete prolactin
- B) basophils – stained by both eosin and
hematoxylin, giving it a blue or purple hue.
- gonadotropes – secrete follicle
stimulating hormone (FSH) and
luteinizing hormone (LH)
- thyrotropes – secrete thyroid
stimulating hormone (TSH)
- corticotropes – secrete
adrenocorticotropin (ACTH)
- C) chromophobe – poorly stained
cytoplasm with solitary nucleus.
- The pars intermedia is not prominent in humans
(shown to the left). It is located between the anterior
and posterior pituitaries, often characterized by cystic
spaces filled with colloid or sloughed cells.
- The cysts are remnants of the lumen of the embryonic
Rathke’s pouch, an ectodermal outpocketing of the oral
cavity that gave rise to both the pars distalis and
intermedia.
- There may be a thin layer of basophilic cells
“infiltrating” the posterior pituitary, but they probably
have little functional importance.
- On the right, the pars tuberalis is shown. It is an
upward extension of the pars distalis and essentially
has the same histological appearance.
- Note how the pars tuberalis wraps around the neural
or infundibular stalk, which contains sinusoidal
capillaries.
- The pars tuberalis contains the portal vessels that
comprise the hypothalamic-hypophyseal portal
blood vessel system. Factors released from the
hypothalamus via neuron axons in the median
eminence are carried by this portal blood system to
influence the cells in the anterior pituitary.
- The posterior pituitary, also called the
neurohypophysis or pars nervosa, is shown here. It
resembles lightly stained nervous tissue, like brain
tissue.
- The large arrows (left panel) indicate Herring bodies,
which are accumulations of neuro-secretory material
within the axon terminals.
- The neuron cell bodies are located in the
hypothalamus. The nerve fibers extend down the
infundibular stalk to the pars nervosa. There are no cell
bodies except for pituicytes (glia cells).
- The nerve fibers carry oxytocin and antidiuretic
hormone (ADH) and release them into nearby
capillaries upon stimulation.
- As a review, note the general appearance of the
different parts of the pituitary gland or hypophysis.
- The anterior pituitary contains irregular cords of
acidophils and basophils with a rich vascular bed.
- The posterior pituitary looks like nervous tissue.
- The intermediate region, which may not be easily
seen in human slides, is located in the middle and
contains portal vessels.
- The next endocrine gland we will look at
are the adrenal, or suprarenal, glands. As
the name implies, the adrenal glands are
found right above the kidneys. Both the
kidney and adrenal gland are completely
surrounded by renal fascia.
- Note the arterial blood supply of each
adrenal gland:
- the superior suprarenal arteries
branch off the inferior phrenic artery.
- the middle suprarenal artery
comes directly off the abdominal
aorta.
- the inferior suprarenal artery is a
branch of the renal artery.
- Note the venous drainage, especially
paying attention to the differences:
- the left central suprarenal vein drains
into the left renal vein, which then drains
into the inferior vena cava (which is on the
right side of the body. The left renal vein
also receives the left gonadal vein and
maybe left inferior phrenic vein.)
- the right central suprarenal vein drains
directly into the inferior vena cava, as do the
other veins on the right side.
- The glands receive input from the greater
thoracic splanchnic nerves (not shown).
- Note the capsule, cortex, and medulla of the adrenal gland. There are much more
distinguishing characteristics in the histological slides.
- The adrenal medulla (bottom left) is the source of norepinephrine (noradrenalin)
and epinephrine (adrenalin). These secretory cells are also called chromaffin cells
because the secretory granules. The cells are often more basophilic than the cells of
the cortex. The organization of the tissue and nuclei are also more distorted. Neuron
cell bodies of sympathetic ganglia are also commonly found in the medulla. Another
identifiable characteristic of the adrenal medulla (not shown here) is the large central
vein.
- The adrenal cortex can be divided into 3 zones, each one named after the
organization of the cells within that zone – the zona glomerulosa, the zona
fasciculata, and the zona reticularis.
- The zona fasciculata (shown
below) is a relatively broad zone of
cells that are arranged in straight
cords, which run at right angles to
the gland surface.
- The cells have a “frothy”
appearance due to the extraction of
lipid droplets during slide
preparation.
- Cells in the zona fasciculata
produce cortisol. It is dependent on
the hormone ACTH.
- The zona glomerulosa (shown
above) is found outermost in the
cortex.
- Note the distinct appearance of
cells arranged in rounded or arched
clusters, which look similar to the
glomeruli of the kidneys.
- The cells in the zona glomerulosa
produce and secrete aldosterone,
which regulates salt and water
retention in the kidneys.
- Note the fibrous capsule on one
side, and the zona fasciculata closer
to the medulla.
- In the innermost cortex we can find
the zona reticularis (shown above).
- It stains more deeply than the
other cortical regions and is
arranged as anastomosing reticular
or net-like cords.
- The cells of the zona reticularis
produce many other androgens and
steroid hormones of the endocrine
system.
- The thyroid gland is situated in the neck right below the thyroid
cartilage, at the lower part of the larynx and upper part of the
trachea. To the naked eye, it has left and right lobes connected by
the isthmus. There may be a pyramidal lobe extending superiorly.
- It is supplied by the superior and inferior thyroid arteries. The
superior thyroid artery comes off the external carotid artery and
gives off the superior laryngeal artery. The inferior thyroid artery
branches off the thyrocervical trunk from the subclavian artery.
- The thyroid gland is drained by the superior, middle, and
inferior thyroid veins. The superior and middle thyroid veins drain
into the internal jugular vein, while the inferior thyroid vein drains
into the brachiocephalic vein.
- This figure is also helpful to review the distribution of nerves.
- The thyroid contains many follicles of
variable diameter filled with colloid. The
follicular epithelium is lined by cuboidal
cells. The appearance of thyroid follicles
varies with activity, shown next.
- Shown below is a hypoactive
thyroid gland, which is often the
case when there is no stimulation by
the pituitary via TSH.
- Due to inactivity, the cuboidal cells
are quiescent and flattened.
- The colloid completely fills the
lumen with no evident “scalloping”.
- Larger cells that are among the
follicles, but not part of the follicular
epithelium, are not dependent upon
TSH. Known as C-cells, they
secrete calcitonin (will be discussed
later).
- The normal thyroid follicles,
shown above, have high cuboidal
epithelial cells.
- The colloid, or stored thyroglobulin,
fills the lumen. When stimulated by
pituitary TSH, the colloid is ingested
and broken down to release thyroid
hormones.
- The “scalloping” or bubbly
appearance of the colloid near the
edge of the colloid is characteristic
of the follicular cells ingesting the
thyroglobulin.
- When the thyroid is hyperactive,
such as in hyperthyroidism, the
cuboidal cells of the follicles
become very high. The lumen is
very small with small amounts of
colloid and evident scalloping. This
can occur when the oxidation of
iodide is blocked. Iodide is required
for the normal production and
release of thyroid hormones, and
lack of thyroid hormones in the
blood stimulates increased TSH
release from the pituitary.
- The parathyroid glands are located on
the posterior surface of the right and left
lobes of the thyroid gland. Note: The
parathyroid glands may not appear as such
distinct nodules to the naked eye.
- Once again, we can review the arterial and
nerve distribution of the larynx and upper
trachea.
- Note the common carotid artery branching
into the internal and external carotid
arteries. The internal carotid does not have
any branches. The external carotid gives off
the superior thyroid artery, from which the
superior laryngeal artery branches and
enters the thyrohyoid membrane. The
inferior thyroid artery comes off the
thyrocervical trunk from the subclavian
artery.
- The vagus nerve is the major nerve supply
of this region. As it descends in the neck, it
gives off the superior laryngeal nerve. The
internal branch pierces the thyrohyoid
membrane together with the superior
laryngeal artery. The external branch
descends outside the muscles to innervate
the larynx. The vagus nerve continues and
gives off the recurrent laryngeal nerve,
which loops under the subclavian artery on
the right and uner the aortic arch on the left,
going back up to the larynx.
- The secretory cell of the parathyroid gland is the chief
or principal cell. They produce and secrete parathyroid
hormone (PTH) which increases the activity of
osteoclasts in bones, effectively increasing serum
calcium levels.
- The much larger cells found in isolated groups are
oxyphil cells. They contain more extensive,
eosinophilic cytoplasm and smaller, darker nuclei. They
are thought to be “aged” chief cells. Although they are
functionally unimportant, they help in the identification
of the parathyroid glands.
- The parathyroid gland, like other endocrine glands,
contain many capillaries (red arrows).
- Parafollicular cells, or C-cells, found in the thyroid
(arrows in the bottom right) secrete calcitonin which has
the opposite effect from that of PTH and decreases
serum calcium levels.