Plates 10-1 thru 10-5, pp. 214-223
Plate 15-2 (pancreas), pp. 324-325

Ch 20, Endocrine System, pp. 348-370

Overview:

The goal of this lab is to examine and describe the cellular organization of the major organs of the endocrine system. Below is a short summary of the nomenclature, key structural features, and main hormones of each organ.

Slide Descriptions

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.

A. Pars distalis (aka anterior pituitary or adenohypophysis)

226_BM Pituitary gland Masson cross [WebScope] [ImageScope] [orientation image]
226human_xc_HE Pituitary gland H&E cross [WebScope] [ImageScope]
226human_sag_HE Pituitary gland H&E sagittal [WebScope] [ImageScope] [orientation image]
227 Pituitary gland Masson sagittal [WebScope] [ImageScope] [orientation image]

Study the pars distalis (anterior lobe) in slide 226_BM 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). Depleted or immature secretory cells of the anterior pituitary show no distinctive staining and are called “chromophobes”.

The cell types 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. As you identify these cell types, keep in mind that most histological surveys find that 50% of the overall cells in the anterior pituitary are chromophobes, 40% are acidophils, and only 10% are observed to be basophils.

What would be the most pronounced histological difference between the pituitaries of castrated and non-castrated males (assuming no hormone replacement)? 

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 basophils, which vary generally from bluish/grayish-pink to blue. 

What would happen to the various endocrine glands in the absence of hormone production by the anterior pituitary (for example hypophysectomy)?

B. Pars nervosa (aka posterior pituitary or neurohypophysis)

227monkey_xc_triC Pituitary gland monkey Masson cross [WebScope] [ImageScope] [orientation image]
227monkey_xc_HE Pituitary gland monkey H&E cross [WebScope] [ImageScope] [orientation image]
227 Pituitary gland human Masson sagittal [WebScope] [ImageScope]

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) 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 , which are the characteristic glial cells of the pars nervosa. You will also see the nuclei of blood vessel endothelial cells, and fibroblasts which are in the connective tissue around these vessels.

C. Pars intermedia

227 Pituitary gland monkey Masson cross [WebScope] [ImageScope]
227 Pituitary gland monkey H&E cross [WebScope] [ImageScope]
226 Pituitary gland H&E sagittal [WebScope] [ImageScope]
227 Pituitary gland Masson sagittal [WebScope] [ImageScope]

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 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 and H&E-stained human sections 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. 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.

D. Infundibular stalk and hypophyseal portal vessels

229B Infundibular stalk, and hypophyseal portal vessels H&E [WebScope] [ImageScope] [orientation image]
229 Infundibular stalk, hypophyseal portal vessels H&E [WebScope]  [ImageScope]

This slide shows the hypophyseal portal blood vessels [example] that carry releasing hormones from the hypothalamus to regulate the release of hormones from the pars distalis of the pituitary. Look for these prominent blood vessels in portions of the pars distalis that extend up around the infundibular stalk. In many of these slides the sinusoidal capillaries [example] lying between cords of pituitary cells throughout the anterior pituitary are shown particularly well. Scan over the slide, reviewing other features you saw in slides 226 and 227 (e.g. acidophils vs. basophils; neurohypophysis vs. adenohypophysis).

II. THYROID GLAND

217 Thyroid gland, parathyroid H&E [WebScope] [ImageScope] [orientation image]
218-norm Thyroid gland rat Masson [WebScope] [ImageScope]
218-hypo Thyroid gland rat hypoactive Masson [WebScope] [ImageScope]
#218-1  Thyroid gland rat hyperactive Masson [WebScope] [ImageScope]

Examine slide 217 at low magnification, then at higher magnifications. Note that the thyroid gland is made up of functional units called follicles, 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 VERY difficult to distinguish in routine histological slides of human thyroid, so you should not worry about trying to find them (however you should recall their function in maintaining calcium homeostasis).

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]
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. Look first at the normal thyroid to get a baseline.

Now, look at the hypoactive thyroid gland caused by removal of the pituitary gland (hypophysectomy). After hypophysectomy 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 (in these hypoactive slides, 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?

III. PARATHYROID GLAND

217 Thyroid gland, parathyroid H&E [WebScope] [ImageScope] [orientation image]
220 Parathyroid H&E [WebScope] [ImageScope]
221 Parathyroid H&E [WebScope] [ImageScope]

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 slide 217, scan around the periphery of the thyroid tissue at low magnification.

The parenchyma of the gland 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?

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. The exact function of these cells is unknown, but they are unique to the parathyroid gland and therefore helpful in identifying it. Some examples may be seen by clicking on these links: slide 217, slide 220, and slide 221.

IV. ADRENAL (Suprarenal) GLAND

Slide 230 (adrenal gland, human, H&E) [WebScope] [ImageScope] [orientation image]
Slide 231 (adrenal gland, monkey, H&E) [WebScope] [ImageScope] [orientation image]

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 occupies the greatest area on your slide. In many regions of slide 230 you will see only cortex [example], 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.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 [example] in the medulla, and review the blood circulation of the adrenal.

V. ENDOCRINE PANCREAS

Webslide 0095_F: Pancreas, monkey. H&E, Plastic
[WebScope] [ImageScope]

Survey the slide at 10X, looking for groups of lightly stained cells, the islets of Langerhans. Roughly estimate the relative volume of acinar to islet tissue. Special stains are needed to distinguish the three principal cell types in islets. Note the large capillaries passing among the islet cells. Some artifactual dilatations are seen in the islet cells