Endocrine glands may be independent organs, or they may be part of an organ that performs nonendocrine functions. Organs that perform both endocrine and nonendocrine functions include the brain, stomach, pancreas, liver, and skin.
Hormones are carried by the blood to all organs of the body; only certain organs, however, can respond to a given hormone. These are called the target organs for the hormone. Hormones affect the metabolism of their target organs and in so doing help to regulate growth and de- velopment, total body metabolism, and reproduction.
 

A. OVARY
The ovary is an endocrine gland as well as the producer of female gametes (ova). The ovum can be thought of as an exocrine secretion because it enters a duct (the uterine tube) after leaving the ovary. The hormones of the ovary, estrogen and progesterone, are secreted into the blood of the circulatory system.

    The ovarian follicles are brought to maturity under the influence of the gonadotropic hormones secreted by the anterior pituitary. In every cycle, one of the follicles eventually ruptures through the surface of the ovary to release its ovum. The empty follicle is then converted into a new endocrine structure called the corpus luteum. If fertilization does not occur, the corpus luteum regresses, and the cycle is ready to begin again.

    The microscopic appearance of the ovary is thus continuously changing as the cycle progresses. A single slide of the ovary will reveal many follicles at different stages of maturation. To simplify matters, only the structure of a mature follicle (called a graafian follicle) just prior to ovulation will be considered in detail.

    Examine the slide of an ovarian follicle taken under low- power, and try to locate a circular-to- elliptical structure that encloses a space filled with fluid and scattered cells. Identify the following parts of the follicle:

1. Ovum. The ovum (egg cell) is the largest cell in the follicle and, at this stage of development, it
is called a secondary oocyte.

2. Granulosa cells. Granulosa cells are the numerous small cells found within the follicle.

3. Antrum. The antrum is the central fluid-filled cavity of the follicle.

4. Cumulus oophorus. Cumulus oophorus  means "egg-bearing hill."  This is the mound of granulosa cells that supports the ovum.

5. Corona radiata. The corona radiata is the layer of granulosa cells that surrounds the ovum. The ovum continues to be surrounded by its corona radiata after ovulation and this layer of cells presents the first barrier to sperm penetration during fertilization.
 

B. Testis
The testis produces both the male gametes (sperm) and the male sex hormone (testosterone). Sperm are produced within the seminiferous tubules and travel through these tubules to the epididymis, where the sperm are passed into a single tubule that becomes the ductus (vas) deferens. The ductus deferens picks up fluid from the seminal vesicles and the prostate and passes its contents, now called semen, to the ejaculatory duct.

    The seminiferous tubules are highly convoluted and tightly packed within the testis. The small spaces, or
interstices, between adjacent convolutions of the tubules are filled with connective tissue known as interstitial tissue. Leydig cells are endocrine cells that produce the androgens (male sex steroid hormones) within this connective tissue.  The major androgen secreted by the Leydig  cells of sexually mature males is testosterone.

    Because the seminiferous tubules are highly convoluted, the chances of seeing a longitudinal section of a tubule are rernote. Most of the tubules will be cut more or less in cross section, so that they will appear circular or oblong.

    Examine this slide of the testis.  Because there are only one to four seminiferous tubules per lobule of the testis, most of the tubules you will see will be sections through the same tubule that has been cut at different places.  Observe the following structures:

1 .Spermatogenic cells.  Spermatogenic cells form the germinal epithelium that composes the walls of the tubules. These cells divide by meiosis to produce the sperm.  The cells in the outer wall are diploid (forty-six chromosomes), whereas the cells in the inner wall have completed meiotic division and are haploid (twenty-three chromosomes).  Chromosomes can be seen at various stages of meiosis within the tubular epithelium.  Mature spermatozoa can be seen within the tubular lumen.

2. Interstitial Leydig cells. he interstitial cells can be seen between adjacent convolutions of the tubules.
 

C. ISLETS OF LANGERHANS
The pancreas has both an exocrine and an endocrine function. The exocrine secretion (pancreatic juice) is produced by pancreatic cells called acini that are arranged in clusters around a central duct. The exocrine secretion drains into interlobular ducts located in bands of connective tissue.  From here the secretion drains into the pancreatic duct and passes into the duodenum.

    The endocrine secretions of the pancreas, insulin and glucagon, are produced by scattered groups of cells called the islets of Langerhans. These hormones do not enter the interlobular ducts, but rather leave the pancreas via the vascular system.
Examine the slide of the pancreas and attempt to identify the following structures:
1. Pancreatic acini. The pancreatic acini are dark-staining clusters of cells that form most of the body of the pancreas
2. lnterlobular ducts. The interlobular ducts may be mistaken for veins because of their large size, thin walls, and flattened, irregular shape. Unlike veins, however, their walls are composed of only a single layer of columnar epithelial cells, and no red blood cells will be seen in the lumina.
3. Islets of Langerhans.  The islets will appear as light patches, circular in shape, against the dark background of the acini. The alpha cells (which secrete glucagon) can easily be distinguished from the beta cells (which secrete insulin). The alpha cells are smaller and contain pink-staining granules, whereas the beta cells are larger and stain blue.
 

D. ADRENAL GLAND
The adrenal gland is actually two different glands located in the same organ.  In lower organisms, these glands are separated; in higher organisms (including humans), they are closely associated as the adrenal cortex (outer part) and the adrenal medulla (inner part).
The adrenal cortex secretes steroid hormones. These include hormones that regulate salt balance, the mineral- ocorticoids, and hormones that regulate glucose homeo- stasis, the glucocorticoids. The adrenal medulla secretes two hormones, epinephrine and norepinephrine, that act
together with sympathetic nerve stimulation to enhance the response of the cardiovascular system to increased physical demand. The cells of the adrenal medulla are de- rived from the same embryonic tissue (the neural crest) as postganglionic sympathetic neurons, whereas the adrenal cortex is derived from a different embryonic tissue (me- soderm). Therefore, these two regions of the adrenal gland are different both physiologically and histologically.
 
 
 

Procedure
Before observing the slide of the adrenal gland under the microscope, hold it up to the light and note the clear distinction between the adrenal cortex and ad- renal medulla. Using the low-power objective, focus on the edge of the gland. As you scan from this point inward, identify the following structures (fig. 4.5):
1 .Capsule. The capsule is a thin, tough layer of connective tissue that surrounds the gland.
2. Zona glomerulosa. The zona glomerulosa is
the outer layer of the adrenal cortex; its cells
are tightly packed in an irregular arrangement. The z. glomerulosa secretes the mineralocorticoids (mainly ?Idosterone and d,goxjlcorticoste,rone). The secretion of these hormones is largely under the control of angiotensin II.
3. Zona fasciculata. The zona fasciculata is located under the z. glomerulosa; this layer is the thickest part of the adrenal cortex. The cells in this layer are arranged in columns. They secrete the glucocorticoids when stimulated by adrenocorticotropic hormone (ACTH) secreted by the anterior pituitary. The most important glucocorticoids are 17jldrocortisone(cortlsol)
and cortlcostorone.
4. Zona reticularis. The epithelial cells in the
zona reticularis form interconnections (anastomoses) with one another and stain a darker color than those of the z. fasciculata.
The z. reticularis is the innermost layer of the adrenal cortex and is also involved in the secretion of the glucocorticoids.
5. Adrenal medulla. The adrenal medulla is the central region of the gland and stains a lighter color than the surrounding z. reticularis. It is composed of tightly packed clusters of C17romc?,(fln Co//S.
 

E. Thyroid
Like the ovary, the functional units of the thyroid are arranged in the form of follicles.  The thyroid follicles are composed of a single layer of epithelial cells surrounding a homogenous fluid, the colloid.

    The secretions of the thyroid follicles are triiodothyronine (T₃) and tetraiodothyronine (T₄ or thyroxine).  These hormones are passed from the epithelial cells of the follicle into the adjacent capillaries and are important regulators of growth and metabolism.

    The thyroid gland also contains parafollicular cells that secrete the hormone calcitonin (also called thyrocalcitonin).   This hormone is believed to play a relatively minor role in the regulation of blood calcium concentrations.

    This microscope section shows the follicular structure of the thyroid gland. The walls of the follicles are composed of a single layer of cuboidal- or columnar- shaped thyroid follicle cells with circular nuclei. The follicles are filled with eosinophylic thyroid
colloid, the storage-form of thyroid hormone. The flattened nuclei outside of the follicles belong, primarily, to capillary
endothelial cells.

    This higher magnification image of the plastic section shows the thyroid gland. The walls of the thyroid follicles are
composed of a single layer of cuboidal cells with circular nuclei. The follicles are filled with colloid, the storage-form of thyroid
hormone. Note numerous, darkly stained nuclei of endothelial cells in the connective tissue between the follicles.
 

F. PITUITARY GLAND
The pituitary, or hypophysis, like the adrenal gland, is composed of two distinct embryonic units. The anterior pituitary, also known as the adenohypophysis (adeno means "glandular"), is derived in the embryo from a dorsal outpouching (Rathke's pouch) of oral epithelium. Often referred to as the master gland, the anterior pituitary secretes the trophic hormones that control other glands. These hormones include adrenocorticotropic hormone (ACTH), thyroid-stimulating hormone (TSH), and others. The posterior pituitary (also known as the neurohypophysis) is derived in the embryo from a ventral out-pouching of the floor of the brain and secretes only two hormones:  vasopressin (or antidiuretic hormone) and oxytocin.

    The secretion of both the anterior and the posterior pituitary is controlled by a part of the brain known as the hypothalamus. Since the posterior pituitary is derived from a downgrowth of the brain, there is a direct neural connection between this gland and the hypothalamus.  Vasopressin and oxytocin are manufactured in the cell bodies of hypothalamic neurons and travel down the axons of these neurons to the posterior pituitary. The cells of this gland store these hormones until stimulated to release them by depolarization of the axons. The posterior pituitary is therefore simply a storage organ.

    Since the anterior pituitary is derived from oral epithelium, and not from brain tissue, there is no direct neural connection between the hypothalamus and the anterior pituitary. There is, however, a special vascular connection between these two organs. A capillary bed in the hypothalamus is connected to a capillary bed in the anterior pituitary by means of venules that run between them. This vascular connection is known as the hypothalamo-hypophyseal portal system.

    Unlike the posterior pituitary, the anterior pituitary manufactures its own hormones.  These hormones are released upon the arrival of specific chemical  messengers, called releasing hormones, secreted into the hypothalamo-hypophyseal portal system by the hypothalamus.  The anterior pituitary, then, is not actually the "master gland," since the release of its hormones is in turn controlled by hormones secreted by the hypothalamus, and both hypothalamus and anterior pituitary are controlled by negative feedback inhibition from the target glands.

    This medium magnification image of the pituitary gland shows the pars nervosa (to the left) composed of elongated cells
called pituicytes, the pars intermedia containing colloid-filled vesicle (arrow) and the pars anterior (to the right).
You can immediately distinguish the anterior pituitary from the posterior pituitary by observing the distribution of capillaries. After you have distinguished the anterior and posterior pituitary, examine this higher power image of the pars anterior.  Look for the following structures:
(a) Sinusoids. Sinusoids are modified capillaries that lack an endothelial wall.
They can easily be identified by the presence of red blood cells.
(b) Chromophils. Chromophils derive their name from the fact that the cytoplasm of these cells readily takes up stain. They are divided into two general categories on the basis of their staining properties. Acodphils (arrow) contain red-staining granules; basophils (next to acidiphil on the right) contain blue-staining granules. These two categories of cells produce different hormones.
(c) Chromophobes (next to acidophil on the left).  Chromophobes have big nuclei and their cytoplasm does not pick up stain, and hence these cells appear quite dull next to the chromophils. It is believed that the chromophobes are not involved in hormone production.

    Here is an additional high magnification image of the anterior pituitary.  Basophils cytoplasm stain redish (next to the chromophobe on the lower left), basophils (next to the chromophobe on the lower right) stain purple, and chromophobes (arrow) have big nuclei and their cytoplasm remain unstained. Note numerous blood vessels present in this region of the pituitary gland.

    Examine the image of the posterior pituitary.  You should be able to see the following:
(a) Nerve fibers. Nerve fibers are the axons of hypothalamic neurons and compose most of the mass of the gland.
(b) Pituicytes. Pituicytes are randomly distributed among the nerve fibers and lack the bright color of the anterior pituitary cells.
The pars nervosa is composed of elongated cells with a long processes called pituicytes, and the unmyelinated nerve fibers. Note Herring body (arrow) which is an expanded region of the nerve fiber containing neurosecretion.