Endocrine System

Resources

Pituitary Gland

AtlasWheater’s, Pituitary Gland
TextRoss and Pawlina, Overview of Endocrine Glands Ross and Pawlina, Pituitary Gland and Hypothalamus

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.
  3. 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.

A. Pars distalis (aka anterior pituitary or adenohypophysis)

Slide 226 Pituitary gland Masson cross Webscope Imagescope orientation
Slide 226 Pituitary gland H&E cross Webscope Imagescope
Slide 226 Pituitary gland H&E sagittal Webscope Imagescope orientation
Slide 227 Pituitary gland Masson sagittal Webscope Imagescope  orientation

Study the pars distalis (anterior lobe) in the slide stained with Masson trichrome #226 pars distalis  Webscope   ImageScope . 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)?

 answerFirst consider the products of the cells in the anterior pituitary. Acidophils include somatotropes (produce growth hormone) and mammotropes (produce prolactin). Basophils are either thyrotropes (produce TSH), gonadotropes (produce LH and FSH) or corticotropes (produce ACTH). Basophilic cells are all regulated by feedback inhibition, where the products of the target organs they stimulate can act directly on the anterior pituitary to downregulate the production of the initial hormone (LH, FSH, TSH, and ACTH). In castration, the cells in the testis that are normally stimulated by LH and FSH are gone and can’t produce hormones to downregulate their production. Instead, the absence of negative feedback causes an increase in LH and FSH produced by the basophilic gonadotropes of the anterior pituitary. These cells subsequently hypertrophy in a vain attempt to induce some type of response in the testis (which are no longer there). Histologically, this is seen as hypertrophy and increased numbers of basophils in the anterior pituitary. Now see if you can figure out what would happen to the pituitary if other glands were removed (for example the thyroid).

Note the abundant sinusoidal capillaries (often filled with red blood cells) #226 sinusoidal capillaries  Webscope   ImageScope 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. 

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

 answerThe anterior pituitary produces hormones that directly stimulate the mammary glands, the adrenal glands, the thyroid, and the ovaries or testis. Removal of GH, PRL, ACTH, TSH, FSH and LH from the system would have varying effects. Loss of ACTH would lead to the collapse of the adrenal cortex and a nearly complete loss of production of glucocorticoids and some weak androgens. The lack of prolactin would lead to decreased milk production in mammary glands. Without TSH, the follicular cells of the thyroid would shrink, leading to hypothyroidism. In the gonads, FSH and LH are essential in producing androgens and estrogens necessary for reproduction. The loss of these two hormones would prevent the maturation of eggs or sperm in the gonads as well as hinder the development of secondary sex characteristics.

B. Pars nervosa (aka posterior pituitary or neurohypophysis)

Slide 227 Pituitary gland monkey Masson cross Webscope Imagescope orientation
Slide 227 Pituitary gland monkey H&E cross Webscope Imagescope orientation
Slide 227 Pituitary gland 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

Slide 227 Pituitary gland monkey Masson cross Webscope Imagescope
Slide 227 Pituitary gland monkey H&E cross Webscope Imagescope
Slide 226 Pituitary gland H&E sagittal Webscope Imagescope
Slide 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 #227   Webscope   ImageScope and H&E-stained #226   Webscope   ImageScope 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

Slide 229B Infundibular stalk, and hypophyseal portal vessels H&E Webscope Imagescope orientation
Slide 229 Infundibular stalk, hypophyseal portal vessels H&E Webscope  Imagescope

This slide shows the hypophyseal portal blood vessels #229   Webscope   ImageScope 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 #229   Webscope   ImageScope 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).

 

Electron Micrographs

Thyroid & Parathyroid Glands

AtlasWheater’s, Thyroid & Parathyroid Glands
TextRoss and Pawlina, Thyroid & Parathyroid Glands

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.
  2. Recognize the parathyroid gland in histological section, and within the gland identify the chief cells (source of parathyroid hormone) and oxyphil cells.

I. THYROID GLAND

Slide 217 Thyroid gland, parathyroid H&E Webscope Imagescope orientation
Slide 218-norm Thyroid gland rat Masson Webscope Imagescope
Slide 218-hypo Thyroid gland rat hypoactive Masson Webscope Imagescope
Slide 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 #217   Webscope   ImageScope . 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).

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 #218-1   Webscope   ImageScope 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. 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 #218-1   Webscope   ImageScope 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?

 answerAs you will learn in the immune system sequence, Graves disease is a type II immune response that targets thyroid stimulating hormone (TSH) receptors on the thyroid. Even though the excessive T3 and T4 in the blood stream downregulate the production of TSH, antibodies continue to stimulate the receptors causing an increase in T3 and T4 production. Graves disease is essentially a form of hyperthyroidism and, as such, would show hypertrophic epithelial cells and profoundly scalloped colloid within the follicles.

II. PARATHYROID GLAND

Slide 217 Thyroid gland, parathyroid H&E Webscope Imagescope orientation
Slide 220 Parathyroid H&E Webscope Imagescope
Slide 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 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 slide 217 Webscope Imagescope 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?

 answerParafollicular cells are involved in the production of calcitonin, which lowers blood calcium levels by preventing bone resorption. The hormone with the opposite effect as this is parathyroid hormone (PTH, made by chief cells in the parathyroid), whose production is triggered by low levels of calcium in the blood and which acts to raise calcium levels by stimulating osteoclasts and the resorption of bone.

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   Webscope   ImageScope , #220  Webscope Imagescope , and #221 Webscope Imagescope

 

Electron Micrographs

Adrenal Gland

AtlasWheater’s, pgs. 338-41, Adrenal Gland
TextRoss and Pawlina, pgs. 706-15, Adrenal Glands

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.

I. ADRENAL (Suprarenal) GLAND

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

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. 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.

 

Electron Micrographs

Endocrine Practice Questions

Practice Question

Practice Questions 1 through 6 all refer to this trichrome-stained section of the pituitary gland.  To see high magnification view of region “A.

Match each of the target organs below with the appropriately labeled cell or region of the hypophysis. Each labeled cell or region may be used once, more than once, or not at all. For some target organs, there may be MORE THAN ONE correct answer, in which case you should choose all answers that apply. (Don’t worry, there won’t be a complicated matching question like this on the quiz or exam, but hopefully you’ll find this exercise useful in terms of matching target organs with the cell types observed in the pituitary gland).