VM8054 Veterinary Histology

Exercise 24

Endocrine Organs

Author: Dr. Thomas Caceci
Sensations sweet,
Felt in the blood, and felt along the heart.

--William Wordsworth (1770-1850)
Lines Composed a Few Miles Above Tintern Abbey (1798)
Objectives for This Exercise

Endocrine organs are glands whose products are secreted into the blood, to affect tissues distant from the site of secretion. These secretory products are hormones. Because endocrine organs secrete directly into the blood they haven't got the elaboarte duct system exocrine glands have. Histologically they tend to be quite uniform in structure: masses of cells with blood vessels running through them. It's been said that endocrine physiology is definitive proof that the Universe is not rationally ordered, and anyone who's spent time trying to unravel its subtleties recognizes the kernel of Truth in that wry remark. But some understanding of the structure of the organs of the system gives insight into their relationship to each other, and to the brain, which runs the whole show.

Most endocrine organs are "epithelial" and would of course therefore be categorized as glandular epithelium. Be aware, however, that not every organ that could be considered "endocrine" is necessarily "epithelial." The brain is quite rightly considered "endocrine" (for reasons that I hope will be clear shortly) but it's not an epithelial structure.

In this exercise we'll be concerned with the structure of the pituitary gland, the pineal body, the endocrine portions of the reproductive organs, the endocrine pancreas, the adrenal glands, and the thyroid and parathyroid glands.

Pituitary Gland

We'll start with the pituitary gland, widely referred to as the "Master gland" because its hormonal secretions have as their target organs other endocrine glands. The pituitary controls the hormonal output of many other glands, but it isn't really a true "Master" gland, as we'll see. Nevertheless it's important and its structure is rather elaborate. The pituitary gland is the point of convergence--and coordination--of the body's two great integrative mechanisms, the nervous and endocrine systems, and its nature and origins reflects this role.

The pituitary gland arises from two different embryonic rudiments: one is a depression in the floor of the forming brain and the other is an evagination of the dorsal surface of the forming gut. Consequently, in the finished state, this organ has two parts that are distinctly different in appearance and in nature.

Regions of the Pituitary: Adenohypophysis and Neurohypophysis

Let me begin the discussion of the structure of the pituitary gland by asking you to jettison any terms you may have used in the past, most specifically including the following: "anterior pituitary" and "posterior pituitary." I'm going to be using a more precise descriptive nomenclature, and I'm going to be a stickler about it. The terms we'll use for the different parts of the gland are the adenohypophysis and the neurohypophysis, which more accurately reflect the origins of this organ from, respectively, the gut and the neural tube.


The adenohypohysis is that portion of the gland that comes from the gut: it is epithelial in nature. The root word "adeno" implies its origin from epithelium, not neural tissue.

The mature adenohypophysis has several distinct parts: the cranial pars distalis, the pars intermedia, which is physically associated with the neural rudiment, and the pars tuberalis, which comes to form a sort of sleeve-like covering of the stalk from which the neural portion is dependent. In some species, there may be visible an opening or gap between the pars distalis and the pars intermedia. This--"Rathke's Pouch"--is the vestigial remnant of the lumen of the gut pouch. It has no functional significance and in some species it's obliterated.


The neurohypophysis is that part of the organ derived from the brain, and unlike the adenohypophysis, it retains its connection to the tissue of origin, via long stalk. The neurohypophysis has a lumen, and that lumen is continuous with the lumen of the brain's third ventricle. The lumen of the neurohypophysis, like the ventricles of the brain, is lined by ependymal cells. This part of the pituitary gland, in an anatomic sense, may be considered a part of the brain, and its histologic appearance confirms this. It consists mainly of nerve fibers with associated glial elements. It is a dependent part of the median eminence of the hypothalamus.

The cells of each pituitary region have distinct hormonal products associated with them. These are summarized elsewhere.

Microscopic Anatomy of the Pituitary

A representative section of the pituitary gland or hypophysis cerebri is seen in slide 155. This section is taken vertically through the gland, and the section on your slide may or may not have come from the approximate midline of the organ. If it happens that yours is from very far off center, you may not see some of the structures described below; in that case, try a slide from another box. Begin by holding the slide up to the light to determine this general outline of the organ; next scan it at low magnification.

To see the layout of the pituitary gland and the different sections, click here.

Pars Distalis

The most prominent area, consisting of a uniform mass of relatively basophilic cells, is the pars distalis. There may be a space or cleft next to it (this is the residual lumen of the gut rudiment) and on the opposite side of the cleft you should be able to see the narrow region of the pars intermedia.

To see the pars distalis, click here.

The cells of the pars distalis can be characterized as acidophilic and basophilic types, easily identified by their staining reactions. A third population that appears to be unstained is present; these are probably basophils that have discharged their hormones. The acidophils produce growth hormone and prolactin; the basophils produce thyroid stimulating hormone, adrenocorticotrophic hormone, and gonadotrophic hormones.

Pars Intermedia and Pars Tuberalis

The pars intermedia is pressed up close to the neurohypophysis. It too is an epithelial area, but its staining reaction is distinctly different from the pars distalis. There are normally no acidophilic cells obvious in it. The arrangement of the cords and sinuses is somewhat different too, and you should have no trouble telling this region from the pars distalis. However, the pars intermedia grades into the pars tuberalis without a definite boundary.

To see the pars intermedia, click here.

The pars intermedia secretes MSH, or "intermedin," which has some similarities to ACTH in structure and in activity. The pars intermedia is more extensive in the domestic animals than in humans. The cleft next to the pars intermedia (if you have one on your slide) is the remnant of the embryonic Rathke's pouch, the evagination of the gut tube that forms one rudiment of the organ. The pars tuberalis is not yet known to secrete any hormones.

Even though it may be similar to the pars intermedia, the pars tuberalis is quite clearly distinct from the adjacent pars nervosa; it forms a "cuff" or "sleeve" around the stalk of the pars nervosa itself.

Pars Nervosa

Now examine the pars nervosa. It has a long, narrow lumen in it. The pars nervosa arises as a ventral outpouching of the floor of the brain, and it retains its connection with the brain. The lining cells of the pars nervosa are the same sort of ependymal cells found lining the central nervous system. If you have a favorable section, you may be able to trace the connection between the space in the pars nervosa and the brain ventricle.

To see the pars nervosa, click here.

The pars nervosa is neural tissue. It consists of well vascularized fiber tracts. Most of these are axons coming from neurons whose cell bodies are located in the hypothalamus. These axons carry materials made in the brain down into the deep part of the pars nervosa. Unlike most other axons, they don't form synapses with other neurons, however.

When these axons reach the pars nervosa they end in contact with blood capillaries. Now, this is a little unusual: it implies direct release of neuron products into the blood, and that's exactly what happens in the pars nervosa.

The pars nervosa is a place of storage and release of hormones made in the brain. The two hormones associated with it are oxytocin and vasopressin. Instead of neurotransmitters, these hormones are made in the brain's supraoptic and paraventricular areas, then carried to the pars nervosa in the axons that end at the surface of capillaries. Upon appropriate stimulation, they release their products directly into the blood at these sites. Thus the brain is acting as an endocrine organ, and releasing its products at a dependent site.

Vascular Considerations and Their Relationship to Pituitary Function

The entire hypophysis is extensively vascularized. But the arrangement of its blood supply is special, and will bear some discussion here. If you will examine the adenohypophysis in particular, you'll see that there are irregularly shaped sinusoids between the cords of cells in the pars intermedia, the pars distalis, and the pars tuberalis. Sinusoids are essentially capillaries, and the blood flowing through these obviously will have direct contact with the adjacent cells of the cords in the adenohypophysis.

Portal Circulation and the Adenohypophysis

The pituitary receives blood from branches of the internal carotid artery. The bulk of this supply goes first to the median eminence of the brain, where it feeds an extensive capillary plexus. As with any capillary bed, the next step is to form small venules; but the veins in this case don't simply drain into the right side of the circulatory system directly. Instead, they drain into the blood sinusoids of the adenohypophysis. This arrangement--a capillary bed draining into a second capillary bed via a short set of venules--is a portal system, and it's vital to normal function. The key point to remember here is that anything put into the "upstream" end of the portal system in the neurohypophysis is immediately transported to the adenohypophysis.

The neurons of the brain, particularly those of the hypothalamus are secretory cells. Some of them secrete not neurotransmitters, but hormones, and these hormones have as their target organ the cells of the adenohypophysis. These neuronal releasing factors are chemical signals that trigger the release of other hormones from cells in the non-neural portions of the gland. The hormones of the pars distalis and intermedia are released into the blood in response to the releasing factors. These hormones in turn target other organs, including other endocrine organs.

It should be obvious from the above discussion that the pituitary is not in any sense a "Master Gland". The brain (through the agency of the pituitary gland) is what really controls the output of the endocrine system. In the case of oxytocin and vasopressin, the brain actually bypasses the pituitary entirely and dumps hormones directly into the blood. The pituitary is, at best, a sort of "Overseer Gland" whose job it is to transmit the orders of the true "Master Gland," the brain, to the rest of the organs.

Pineal Gland

After that rather exhausting tour of the pituitary, it's something of a relief to examine a much less complicated endocrine organ, the pineal gland, sometimes called the epiphysis cerebri. You'll see it on slide 618. Grossly, the pineal gland is a sort of cone shaped structure in the midline of the brain, projecting from the roof of the diencephalon. The name comes from the Latin pinus, a pine tree, from its vague resemblance to one of these.

Histologically speaking, the pineal gland is uninteresting; about 95% of the cells are the pinealocytes, the functional cells which manufacture the pineal hormone melatonin (5-methoxy-N-acetyl-tryptamine). These cells are large and lightly stained in H&E, with a rounded nucleus. The remaining cells of the organ (about 5%) are astrocytes, which cannot be seen on this H&E stained slide.

Brain Sand

The most obvious aspect of the histology of the pineal gland is the presence of the acervuli cerebri (from Latin, "acervus," a heap). This is the so called "brain sand", and in reality it consists of calcareous secretions that are easily seen with the microscope. These are not a degenerative change; they're present in almost all adult mammals and are perfectly normal. Their mineral composition makes them radio opaque and they have some clinical significance. The adult pineal is a radiological marker for the midline of the brain. Displacement is easily seen on radiographs, and is a possible indication of the growth of a tumor.

To see the pineal gland, click here.

To learn something about the functioning of this gland, click here.

Endocrine Tissues of the Reproductive System

Both the testis and the ovary are important endocrine organs, and the hormones they produce are important in normal sexual activity, in maturation of the adult body form, and in controlling estrus and pregnancy.

Interstital Cells

The testis is seen on slide 51. The bulk of this slide consists of seminiferous tubules, easily identified by their circular profile and characteristic cell types. (These will not be dealt with here, since you will cover them in detail in the reproductive system lab exercise.)

The interstitial areas are well vascularized, and between the clusters you should be able to make out capillaries containing erythrocytes. Near the capillaries you'll find small clusters of cells. These are the interstitial cells, the source of the male steroid hormone testosterone. Ultrastructural examination will show that the interstitial cells have the characteristic features associated with steroid synthesis, especially the presence of large quantities of smooth endoplasmic reticulum.

To view the interstitial cells of the testis, click here.

Ovary and Corpus Luteum

The ovary is a major endocrine organ, and the variations of hormones in the female reproductive cycle is of great importance. We'll deal with the ovarian secretory tissues in the exercise devoted to the female reproductive tract.

Endocrine Pancreas

In the exercise on the digestive system we looked at the pancreas in light of its exocrine function, the source of digestive enzymes. But this organ is also an endocrine gland. Examine slide 34; while the bulk of the pancreatic tissue is exocrine in nature, the endocrine pancreas is distributed through the mass of the organ as "islands" of lighter staining material. These pancreatic islets are easily seen at low power. At higher magnification several characteristics of the islets can be seen.

To visit the exciting and exotic Isles of Langerhans, see your travel agent, or click here.

Islets are more or less set off from the surrounding exocrine tissue by a thin "capsule" of CT. They're extensively vascularized, much more so than the surrounding exocrine tissue: an islet is essentially a capillary bed surrounded by secretory cells.

The cells of the islets are not easily distinguished from one another in H&E preparations, but there are 4 types known to exist. The principal ones are the A and B cells; the A cells make glucagon and the B cells make insulin. These two antagonistic hormones are vital to normal carbohydrate metabolism.

Adrenal Gland

Slide 1202 is a section of adrenal gland. The adrenal gland has a distinct cortex and medulla, each of different embryologic origin and different function.

Blood Supply

As you examine this slide, note the extensive vascularization of both parts of this organ. As in any endocrine organ, release of hormones to the blood must be rapid, and the great degree of vascularity permits all cells to have good access to the circulation. The cells of the cortex and the medulla have the familiar cord and sinus arrangement, which facilitates contact between the cells that make hormones and the blood flowing past them. The cells of the adrenal cortex all synthesize steroid hormones, and have a "foamy" appearance in some sections. This is due to the extensive lipid material in them, and to the numerous SER and Golgi profiles in their cytoplasm.

The adrenal cortex and the medulla have separate and distinct arterial supplies, and this is related to the different origins and functions of the two parts, discussed below. You may be able to see some of the medullary arteries which run straight through the cortex, bypassing it completely, and ramify into the sinusoids of the medulla. The cortex is supplied by cortical arteries which enter the organ via the capsule and discharge their blood into the sinusoids of the zona glomerulosa (see below). Venous outflow from the cortex is mixed with that of the medulla, and carries secretions from both parts.

For a general view of the adrenal gland, click here.

The Adrenal Cortex

Capsule and Zona Glomerulosa

The adrenal gland as a whole is surrounded by a distinct CT capsule, which sends a few septa into the parenchyma of this gland; generally, though, there is little internal lobulation. The layers of cells immediately beneath the capsule are organized into the zona glomerulosa, or "region of little globes." They form arcades or small arches of cells.

Zona Fasciculata

Deep to this layer is a second distinct region where the cells form somewhat more regular rows, radiating away from the center of the gland. This is the zona fasciculata, the name coming from the Latin word "fascis," a bundle of sticks. Like sticks bound together, these cords of cells run in roughly the same direction.

Zona Reticularis

The cells in that region of the cortex closest to the medulla lose this regular arrangement, and are organized into anastomosing cords to form a network-like zona reticularis. Separating the rows and cords of cells are irregular vascular channels, or sinusoids similar to those in the liver.

You can see the zones of the adrenal cortex in more detail if you click here.

Adrenal Medulla

The adrenal medulla comes from an entirely different embryonic rudiment than does the cortex. The cortex arises from a condensation of embryonic mesoderm, but the medulla, ultimately, is of neuroectodermal origin. As a result it has different properties and its secretions are triggered by different stimuli.

The corticomedullary junction is quite easy to see. The cells of the adrenal medulla are somewhat more basophilic than those of the cortex. With appropriate stains, these medullary cells show the so called "chromaffin reaction" characteristic of cells producing catecholamine compounds, which of course is what they are doing. The adrenal medulla makes the catecholamine hormones epinephrine and norepinephrine.

The adrenal medulla comes from an aggregation of neural crest cells. In the fully formed organ, innervation of the medulla by the autonomic nervous system is direct; there is no intervening ANS ganglion as in other target organs. In reality, the adrenal medulla can be though of as a sort of ganglion combined with its effector organ.

To see the adrenal medulla, click here.

Functional Implications of Adrenal Gland Structure

As with other organs, the adrenal gland illustrates convergence of the endocrine and nervous systems in the way it's made and the way it operates. The adrenal gland's dual blood supply has significant consequences.

The cortex, like any ordinary endocrine gland, responds to a pituitary hormone than directs the cells to put their product into the blood stream. The pituitary hormone ACTH is brought to the cortex via the cortical arteries, passes into the sinusoids, and stimulates release of steroid hormones from the cortical cells back into the sinusoids and out to the rest of the body.

The medulla however, does not need and does not have any such chemical messenger. It is stimulated by direct neural signals. Axons from the autonomic nervous system directly synapse with cells of the medulla, and if those axons discharge, the cells respond by releasing their products into the blood flowing through the sinusoids in the medulla.

The most familiar example of medullary action is the "flight or fight" reaction. Imagine you are walking on the Appalachian Trail, and a large bear pops out from behind a bush 10 feet from you. Your eyes perceive the image of the bear; your brain integrates the information and decodes the image BEAR! Immediately your sympathetic nervous system, in response to the information that a B-B-B-BEAR!! is standing in front of you sends a message to the adrenal medulla; the adrenal medulla releases epinephrine into the blood. This hormone--also called "adrenaline"--has a number of target organs. It causes your gut musculature to stop peristalsis; your pupils to dilate; your circulatory system to start sending plenty of oxygenated blood to your limbs. Thus, when you turn and run as fast as you can away from that B-B-B-B-B-BEAR!!! you are putting maximum effort into the escape. (Hope it helps!)

Epinephrine receptors are present on nearly all cells, and the adaptation of this material (which is in some places known to be a neurotransmitter) to this use makes it a sort of "general quarters alarm" for the entire organism.

Thyroid and Parathyroid Glands

The thyroid gland can be seen on slide 691. (Slide 691 also has a section of parathyroid gland on it.).

Thyroid Gland

The organization of the gland into follicles is easily seen. The follicles are hollow balls of cells, each with a wall composed of simple cuboidal epithelium. The amorphous material present inside the follicles is colloid, an inactive storage form of the gland's secretion. The thyroid is unique among endocrine glands in that it stores its secretory product extracellularly. Indeed, it's unusual for an endocrine gland to store products at all.

For an overall view of the thyroid gland, click here.

In the regions between the follicles, CT cells of the septa which divide the organ are present. Also present are the C-cells or clear cells, sometimes called parafollicular cells. These are oval shaped, larger, and more lightly stained than the follicular cells are. They are often hard to identify in H&E preparations, but you should be able to find a few. The C-cells do not produce colloid; they produce the hormone calcitonin instead.

To see C-cells click here.

Vascular Relationships of the Thyroid Follicles

As you would expect in any endocrine gland, the blood supply to the thyroid is very good. There are capillary beds in all of the interfollicular spaces (these can usually be identified by the presence of erythrocytes in the capillary lumen, and/or by coagulated, stained blood plasma).

Slide 691, which is stained with Masson's stain. The outer CT capsule of the organ and the degree of septation is obvious in this slide. The CT fibers are stained green, and examination under high power will reveal that wisps of CT are present in all of the interfollicular areas, providing support for the overall structure of the gland. You may be able to make out a few erythrocytes in the blood vessels running through the CT.

Parathyroid Gland

Slide 691 also demonstrates a parathyroid gland. This is one of four small oval bodies associated with the thyroid gland, but which are separate endocrine organs in and of themselves.

You will note that this structure has its own distinct capsule, and is not lobulated as is the thyroid gland. Like other endocrine organs it's well vascularized, and all of the functional cells of the parathyroid parenchyma are closely associated with a blood vessel. The most numerous parathyroid cells are the chief cells, with a round nucleus and weakly stained cytoplasm. Other types have been described but are much rarer and will not be considered here. The chief cells produce parathyroid hormone (or parathormone, as it's sometimes called, abbreviated PTH).

To see the parathyroid gland in situ, click here.

Functional Considerations

The detailed physiology of the thyroid and parathyroid glands are extremely important clinically, but will be dealt with in other courses. The thyroid gland's principal product, thyroxin, effects an increase in metabolic rate. In some animals under prolonged cold stress, such as Arctic wolves and sled dogs, thyroxin output is often elevated to increase the rate of body heat generation. Such dogs will sleep outdoors in weather as cold as -40°!

The calcitonin produced by the C-cells brings about a decrease in blood calcium levels, and an increase in the deposition of calcium in bone. This effect is antagonized by parathyroid hormone, which acts to release calcium from bone and increase its concentration in the blood.

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