OBJECTIVES:
1. Be able to identify the parts of the pituitary gland, and describe their
embryological origin.
2. Name the cell types that produce the various hormones of the anterior
pituitary, and know whether they are acidophils or basophils.
Describe the role of hypothalamic neurosecretion in the function of the posterior
pituitary, and in the regulation of the anterior pituitary, and recognize the
histological structures involved in these functions.
I. PITUITARY GLAND
These slides show mostly pars distalis, but also have a small area of
neurohypophysis (either pars nervosa or infundibular stalk) on one side (see slide
orientation diagrams) and are stained in alternate sets with H&E or with Masson
trichrome. The two classes of anterior pituitary cells (acidophils and basophils) are
most easily distinguished with Masson trichrome staining, but you should also see
how they look with routine H&E staining.
Study the pars distalis (anterior lobe) in the slide stained with Masson trichrome
[example]. The cells are arranged in irregular clusters or cords and are
distinguishable by their staining as either acidophils, basophils, or chromophobes.
The acidophils stain red or orange-red, while the basophils stain various shades of
blue or blue-gray. Remember that the acidophils include two different cell types,
somatotropes (growth hormone) and mammotropes (prolactin), while the basophils
include gonadotropes (FSH and LH), thyrotropes (TSH) and corticotropes (ACTH).
ACTH is actually a cleavage product of pro-opiomelanocortin (POMC), which is made
by corticotropes and processed primarily into ACTH in these cells. To a lesser extent,
corticotropes also produce other signaling factors derived from POMC such as
lipotropins (involved in lipid metabolism), endorphins (endogenous opioids that
reduce pain perception), and melanocyte stimulating hormone (MSH). Occasional
cells in the anterior pituitary show no distinctive staining and are called
“chromophobes”. You will only be required to distinguish acidophils from basophils.
Your best strategy is first to identify acidophils, which are more distinctively stained,
and then the remaining cells are almost entirely basophils. The cells are not
uniformly distributed throughout the pars distalis, but instead there are areas where
acidophils predominate, other areas where basophils are more numerous, while still
other regions may show a more even mixture of acidophils and basophils. What
would be the most pronounced histological difference between the pituitaries of
castrated and non-castrated males (assuming no hormone replacement)? (EN1)
Note the abundant sinusoidal capillaries (often filled with red blood cells) [example]
that lie between the cell cords or clusters. You can appreciate how readily the
hormones secreted from the cells can reach the blood. Since collagen stains bright
blue with the Masson trichrome method, you can see the delicate connective tissue
partitions between cords and around blood vessels.
In the routine H&E-stained sections, you can also identify acidophils and basophils,
although the difference is not as obvious as it is with Masson trichrome staining.
Here again, you should first identify acidophils, which stain various shades of
reddish pink, and then the remaining cells are almost entirely basophils, which vary
generally from bluish/grayish-pink to blue。
Although the pars nervosa can be found on the human pituitary slides in our
collection, the monkey pituitary specimens (H&E and trichrome-stained) contain
a significant portion of the pars nervosa (posterior lobe) [example] and are probably
better for studying this tissue. The pars nervosa looks like brain tissue, which it is.
It is an extension of the brain, composed primarily of nerve fibers (axons) which
originate from nerve cell bodies in the hypothalamus and pass to the pars nervosa
by way of the hypothalamo-hypophyseal tract and the infundibular stalk. These
nerve fibers carry oxytocin and antidiuretic hormone (ADH, vasopressin) to nerve
endings, from which they are released into nearby capillaries upon neural
stimulation from the hypothalamus. There is not much to see in the posterior lobe in
these histological sections. Since there are no neuron cell bodies in this structure,
most of the prominent nuclei belong to pituicytes [example], which are the
characteristic glial cells of the pars nervosa. You will also see the nuclei of blood
vessel endothelial cells [example], and fibroblasts which are in the connective tissue
around these vessels.
The pars intermedia is very poorly developed in the human pituitary, but is
prominent in the pituitaries of most other mammals. For example, in the monkey
pituitary, you will see the pars intermedia [example] as a prominent layer several
cells thick, lying between the pars distalis and pars nervosa. In some places you
may also see a long cleft between the pars distalis and the pars intermedia, which is
a substantial remnant of the lumen of the embryonic Rathke’s pouch, an ectodermal
outpocketing of the oral cavity which gave rise to both the pars distalis and the pars
intermedia. Cells of the pars intermedia also produce POMC (pro-opiomelanocortin),
which in these cells is processed primarily into endorphins and MSH (melanocyte
stimulating hormone).
In contrast to its substantial presence in other mammals, the pars intermedia of the
human pituitary is usually represented merely by a thin layer of basophilic cells that
can be seen in both the human trichrome [example] and H&E-stained human
sections [example] lying against the pars nervosa, and is probably of little
functional importance. Between the pars intermedia and pars distalis are occasional
fluid-filled cysts (again visible in both trichrome [example] and H&E-stained
[example] sections), which are the only vestiges of the lumen of Rathke’s pouch
(see R pg 690, 21.4 for summary diagram of pituitary development). Although most
of the human axial sections in your sets do not show the human pars intermedia
very well, the sagittal sections in some of the sets show some indication of it.
Electron Micrograph Wall Charts
#109 ANTERIOR PITUITARY (survey view) [WinLab] [Mac] [WinHome]
Here you see pituitary cells of various size clustered between capillaries. The main
cell types are somatotropes (GH), mammotropes (prolactin), gonadotropes (FSH
and LH), thyrotropes (TSH), and corticotropes (ACTH). You will not be required to
identify these cell types in electron micrographs, but should note that the cells differ
in size, shape, and in the number, size and distribution of their secretory granules
(small black structures in the cytoplasm). When the content of a secretory granule
is released from the cells, the hormones diffuse to nearby capillaries. The
endothelium of these capillaries is very thin, and, as with most endocrine organs,
contains fenestrations (not seen clearly in this micrograph).
#110 ANTERIOR PITUITARY CELLS [WinLab] [Mac] [WinHome]
This electron micrograph shows two of the pituitary cell types in more detail. A
gonadotrope occupies most of the upper left side of the figure, while another
gonadotrope is at the upper right. There is a somatotrope extending across the
bottom of the figure, and cytoplasm of another somatotrope is at middle right.
Hormones are synthesized on the rough endoplasmic reticulum (RER) of the two cell
types. The RER in somatotropes (at right) has the usual appearance, while the
gonadotrope RER is distended with recently-synthesized hormone. The hormone
subsequently passes through the Golgi complex and is then formed into secretory
granules. Since FSH and LH are glycoproteins, terminal sugars are added to the
oligosaccharide chains as the hormone passes through the Golgi stack. When the
cell is stimulated by the appropriate releasing hormone from the hypothalamus, the
content of granules is released from the cell by exocytosis (see also W, pg 331,
17.3).
#111 POSTERIOR PITUITARY (survey view) [WinLab] [Mac] [WinHome]
The nerve fibers (axons) that comprise the posterior lobe carry small secretory
granules containing oxytocin and antidiuretic hormone (ADH, vasopressin), as well
as their carrier proteins (neurophysins). The granules accumulate in nerve endings
that can be seen in this figure (for example in the area below the capillary in the
center; also note the process labeled “nerve ending”). When appropriate neural
stimulation arrives from the hypothalamus, the content of granules in the endings is
released and the hormones pass to nearby capillaries and then out to the body.
Large accumulations of the granules, probably no longer functional, are called
“Hering bodies”. Most of the large nuclei seen here belong to pituicytes, the glial
cells in this portion of the brain. Other nuclei belong to capillary endothelial cells and
fibroblasts.
OBJECTIVES:
1. Explain how structures seen in the thyroid gland, at both the light and
electron microscope levels, are involved in the production of thyroglobulin,
its storage, and its subsequent breakdown to yield thyroid hormones.
Recognize the parathyroid gland in histological section, and within the gland identify
the chief cells (source of parathyroid hormone) and oxyphil cells.
II. THYROID GLAND
Examine slide 217 at low magnification, then at higher magnifications. Note that the
thyroid gland is made up of functional units called follicles [example], which in three
dimensions are approximately spherical, their walls being composed of a simple
cuboidal epithelium, surrounding a lumen that contains colloid. Note that the
follicles vary in size and that the height of the follicular epithelial cells may also vary.
The colloid is composed primarily of thyroglobulin, a glycoprotein synthesized by
the follicular epithelium. Under stimulation from pituitary TSH, the thyroid cells
break down the thyroglobulin to release thyroid hormones (T3 and T4), which pass
into nearby capillaries.
Occasional parafollicular cells (C-cells), source of the hormone calcitonin, are also
present between the follicles and in the follicular epithelium [example] [CAVEAT].
However, they are difficult to distinguish in routine histological slides of human
thyroid, and you are NOT expected to recognize them based on light microscopy
alone (but you should know that they are the source of calcitonin which is packaged
into secretory granules that makes these cells readily identifiable by electron
microscopy --see W pg. 335, 17.11; R pg. 702, 21.15).
There are three versions of slide 218 that show a rodent thyroid at three different
levels of functional activity: (1) normal [example], (2) hypoactivity due to
hypophysectomy [example], and (3) hyperactivity [example] due to treatment
with the drug thiouracil. Compare the tissue shown in each slide --the variation is
not overwhelming since the experiments were performed conservatively, but you
should be able to see some differences in epithelial cell height and in the size of the
follicular lumens.
The hypophysectomized sample is a rather extreme example of “pituitary failure”
(the “failure” in this instance is due to the fact that the pituitary was surgically
removed), but it is illustrative nonetheless: there is no stimulation by TSH, so the
follicular epithelial cells become reduced in height, and the colloid in the lumen is
abundant since it is not being resorbed to make thyroid hormones. Also of note is
that the C-cells [example] are more obvious as these cells are fully functional and
NOT dependent on TSH.
In contrast, in the hyperactive follicles of thiouracil-treated animals the epithelium
is columnar, and the follicular lumen is much reduced in size. The reason for this
hyperactivity is that thiouracil blocks the oxidation of iodide, with the result
that functional thyroid hormones can no longer be produced. The lack of
thyroid hormones in the blood stream leads to stimulation of the pituitary to
produce large quantities of TSH, causing the thyroid follicular cells to hypertrophy
and resorb colloid very actively from the lumen, reducing its size. The frantic effort
of these cells is futile, however, since the oxidized iodine necessary to make
functional thyroid hormones is unavailable. What would be the appearance of the
thyroid of a person with Graves disease? (EN3)
III. PARATHYROID GLAND
Sections of parathyroid gland can be seen on slides 217, 220, and 221. In slide
217, parathyroid tissue will be found on one side of the much larger mass of thyroid
tissue. To find the parathyroid tissue on slides 217, scan around the periphery of the
thyroid tissue at low magnification. (Note: slide 217 in a few of the glass slide sets
lacks parathyroid.)
The parenchyma of the gland [example] is made up of two identifiable cell types:
the predominant chief (or principal) cells (source of parathyroid hormone) and
occasional oxyphil cells. Observe the arrangement of chief cells in the parathyroid
as seen on slides 217, 220, and 221. The chief cells are arranged as
interconnecting cords or clusters, with blood vessels and connective tissue forming
the partitions between the cell cords. The capillaries in slide 221 may be more
easily seen because erythrocytes have been retained within the lumens. The
individual chief cells, seen well in slide 220, have relatively little cytoplasm, which
may be almost unstained or lightly basophilic.
The lightly stained cells are thought to be quiescent while the more basophilic cells
are believed to be more actively involved in the synthesis and secretion of
parathyroid hormone. What hormone is secreted by the parafollicular cells of the
thyroid and what hormone produces the opposite physiological effect? (EN4)
In either slide 217, 220, or 221 try to find oxyphil cells. Oxyphil cells are much less
numerous than chief cells, and can be differentiated from them by the following
criteria: (1) larger than chief cells, with more extensive, eosinophilic cytoplasm, (2)
nuclei smaller and darker staining, (3) usually occur in isolated groups. Not every
specimen in the glass slide sets contain readily identifiable oxyphil cells, but they
can be found on each of the virtual slides: 217 [example], 220 [example], and 221
[example].
OBJECTIVES
1. Recognize the zones of the adrenal cortex that produce aldosterone and
cortisol, and explain how the blood supply is arranged for efficient uptake of
the hormones.
2. Recognize the adrenal medulla in histological section, and explain the
functional similarity of its cells to those of the sympathetic nervous system.
IV. ADRENAL (Suprarenal) GLAND
At low magnification on the human adrenal gland (slide 230), note that the gland is
enclosed by a connective tissue capsule and has two principal regions - a cortex and
a medulla. The cortex [example] occupies the greatest area on your slide. In many
regions of slide 230 you will see only cortex, because some parts of the human
adrenal lack medulla. The cortex is made up of three regions or zones: the zona
glomerulosa, the zona fasciculata and the zona reticularis. The zona fasciculata
[example] is probably the easiest layer to spot as it is a broad zone of cells arranged
in straight cords, one or two cells thick, which run at right angles to the surface of
the gland. The cells of the fasciculata are lightly stained and have a frothy
appearance, due to the extraction of lipid droplets from the cell cytoplasm during
tissue processing. Interior to the fasciculata is the zona reticularis [example], which
stains more deeply than the other two regions of the cortex. The cells of the zona
reticularis are arranged as anastomosing (reticular or net-like) cords. The zona
glomerulosa [example] is found outermost in the cortex and consists of cells
arranged in rounded or arched clusters although in the human adrenal gland, the
zona glomerulosa may not be present around the entire periphery of the cortex. In
other species, however, this zone exists as a complete layer around the entire
periphery of the cortex as shown in slide 231, which of the monkey adrenal gland.
Continuing inward on slide 231, you should be able to recognize the zona
fasciculata [example], zona reticularis [example], and, finally, medulla [example].
Notice that throughout the cortex of both the human and monkey adrenal glands
are numerous capillaries, with somewhat expanded lumens.
Return to the medulla of slide 230 (human adrenal section) [example]. The
medullary cells, source of norepinephrine (noradrenalin) and epinephrine
(adrenalin), are often more basophilic than the cells of the cortex. What is the
difference between the origin of cells of the cortex and the origin of medullary cells
in the adrenal gland? (EN5) The cells of the medulla are considered to be modified
postganglionic sympathetic neurons (derived from neural crest cells). These
secretory cells are also called chromaffin cells, because their secretory granules
(containing norepinephrine or epinephrine) stain brown with potassium dichromate.
Note the branches of the central (or medullary) vein [see example] in the medulla,
and review the blood circulation of the adrenal. In the medulla, how can you tell
which cells secrete epinephrine and which secrete norepinephrine? (EN6)