Endocrine Anatomy-Histology Correlate

1. EndocrineAnatomy-Histology Correlate By: Michael Lu, Class of ‘07

2. The endocrine system maintains homeostasis and long-term control using chemical signals, or hormones. These hormones are chemicals released into the bloodstream and arrive at their appropriate target cells with specific receptors. In contrast, the nervous system coordinates rapid and precise responses to stimuli using action potentials that travel along neuron axons. • In addition to the endocrine route of hormone delivery, there are also the paracrine pathway (secreted locally to affect neighboring cells) and the neuroendocrine pathway (hormones that are released by neurons to reach the target cells). • The endocrine system involves the pituitary gland, thyroid gland, parathyroids, and adrenal glands, which will be discussed here. Other components not discussed here are the pancreas, ovaries, testes, and enteroendocrine cells. • We will start at the pituitary gland, also known as the hypophysis. Note its location in the head, inferior to the hypothalamus and just above the nasal sinuses. • Note the different components of the pituitary gland. You can use this picture as a reference when we discuss each part in more detail.

3. The anterior pituitary, or the adenohypophysis, is shown here. The cells are arranged in irregular, anastomosing cords or cell clusters that receive rich, abundant blood supply. The vascular bed may not be obvious. • The pars distalis (another name for the adenohypophysis) contains many different hormone-secreting cells. The type of staining does not allow us to differentiate the hormone content. They are as follows: • A) acidophils – stained by eosin, giving it a lighter red or orange color. • somatotropes – secrete growth hormone (GH) • mammotropes – secrete prolactin • B) basophils – stained by both eosin and hematoxylin, giving it a blue or purple hue. • gonadotropes – secrete follicle stimulating hormone (FSH) and luteinizing hormone (LH) • thyrotropes – secrete thyroid stimulating hormone (TSH) • corticotropes – secrete adrenocorticotropin (ACTH) • C) chromophobe – poorly stained cytoplasm with solitary nucleus.

4. The pars intermedia is not prominent in humans (shown to the left). It is located between the anterior and posterior pituitaries, often characterized by cystic spaces filled with colloid or sloughed cells. • The cysts are remnants of the lumen of the embryonic Rathke’s pouch, an ectodermal outpocketing of the oral cavity that gave rise to both the pars distalis and intermedia. • There may be a thin layer of basophilic cells “infiltrating” the posterior pituitary, but they probably have little functional importance. • On the right, the pars tuberalis is shown. It is an upward extension of the pars distalis and essentially has the same histological appearance. • Note how the pars tuberalis wraps around the neural or infundibular stalk, which contains sinusoidal capillaries. • The pars tuberalis contains the portal vessels that comprise the hypothalamic-hypophyseal portal blood vessel system. Factors released from the hypothalamus via neuron axons in the median eminence are carried by this portal blood system to influence the cells in the anterior pituitary.

5. The posterior pituitary, also called the neurohypophysis or pars nervosa, is shown here. It resembles lightly stained nervous tissue, like brain tissue. • The large arrows (left panel) indicate Herring bodies, which are accumulations of neuro-secretory material within the axon terminals. • The neuron cell bodies are located in the hypothalamus. The nerve fibers extend down the infundibular stalk to the pars nervosa. There are no cell bodies except for pituicytes (glia cells). • The nerve fibers carry oxytocin and antidiuretic hormone (ADH) and release them into nearby capillaries upon stimulation. • As a review, note the general appearance of the different parts of the pituitary gland or hypophysis. • The anterior pituitary contains irregular cords of acidophils and basophils with a rich vascular bed. • The posterior pituitary looks like nervous tissue. • The intermediate region, which may not be easily seen in human slides, is located in the middle and contains portal vessels.

6. The next endocrine gland we will look at are the adrenal, or suprarenal, glands. As the name implies, the adrenal glands are found right above the kidneys. Both the kidney and adrenal gland are completely surrounded by renal fascia. • Note the arterial blood supply of each adrenal gland: • the superior suprarenal arteries branch off the inferior phrenic artery. • the middle suprarenal artery comes directly off the abdominal aorta. • the inferior suprarenal artery is a branch of the renal artery. • Note the venous drainage, especially paying attention to the differences: • the left central suprarenal vein drains into the left renal vein, which then drains into the inferior vena cava (which is on the right side of the body. The left renal vein also receives the left gonadal vein and maybe left inferior phrenic vein.) • the right central suprarenal vein drains directly into the inferior vena cava, as do the other veins on the right side. • The glands receive input from the greater thoracic splanchnic nerves (not shown).

7. Note the capsule, cortex, and medulla of the adrenal gland. There are much more distinguishing characteristics in the histological slides. • The adrenal medulla (bottom left) is the source of norepinephrine (noradrenalin) and epinephrine (adrenalin). These secretory cells are also called chromaffin cells because the secretory granules. The cells are often more basophilic than the cells of the cortex. The organization of the tissue and nuclei are also more distorted. Neuron cell bodies of sympathetic ganglia are also commonly found in the medulla. Another identifiable characteristic of the adrenal medulla (not shown here) is the large central vein. • The adrenal cortex can be divided into 3 zones, each one named after the organization of the cells within that zone – the zona glomerulosa, the zona fasciculata, and the zona reticularis.

8. The zona fasciculata (shown below) is a relatively broad zone of cells that are arranged in straight cords, which run at right angles to the gland surface. • The cells have a “frothy” appearance due to the extraction of lipid droplets during slide preparation. • Cells in the zona fasciculata produce cortisol. It is dependent on the hormone ACTH. • The zona glomerulosa (shown above) is found outermost in the cortex. • Note the distinct appearance of cells arranged in rounded or arched clusters, which look similar to the glomeruli of the kidneys. • The cells in the zona glomerulosa produce and secrete aldosterone, which regulates salt and water retention in the kidneys. • Note the fibrous capsule on one side, and the zona fasciculata closer to the medulla. • In the innermost cortex we can find the zona reticularis (shown above). • It stains more deeply than the other cortical regions and is arranged as anastomosing reticular or net-like cords. • The cells of the zona reticularis produce many other androgens and steroid hormones of the endocrine system.

9. The thyroid gland is situated in the neck right below the thyroid cartilage, at the lower part of the larynx and upper part of the trachea. To the naked eye, it has left and right lobes connected by the isthmus. There may be a pyramidal lobe extending superiorly. • It is supplied by the superior and inferior thyroid arteries. The superior thyroid artery comes off the external carotid artery and gives off the superior laryngeal artery. The inferior thyroid artery branches off the thyrocervical trunk from the subclavian artery. • The thyroid gland is drained by the superior, middle, and inferior thyroid veins. The superior and middle thyroid veins drain into the internal jugular vein, while the inferior thyroid vein drains into the brachiocephalic vein. • This figure is also helpful to review the distribution of nerves. • The thyroid contains many follicles of variable diameter filled with colloid. The follicular epithelium is lined by cuboidal cells. The appearance of thyroid follicles varies with activity, shown next.

10. Shown below is a hypoactive thyroid gland, which is often the case when there is no stimulation by the pituitary via TSH. • Due to inactivity, the cuboidal cells are quiescent and flattened. • The colloid completely fills the lumen with no evident “scalloping”. • Larger cells that are among the follicles, but not part of the follicular epithelium, are not dependent upon TSH. Known as C-cells, they secrete calcitonin (will be discussed later). • The normal thyroid follicles, shown above, have high cuboidal epithelial cells. • The colloid, or stored thyroglobulin, fills the lumen. When stimulated by pituitary TSH, the colloid is ingested and broken down to release thyroid hormones. • The “scalloping” or bubbly appearance of the colloid near the edge of the colloid is characteristic of the follicular cells ingesting the thyroglobulin. • When the thyroid is hyperactive, such as in hyperthyroidism, the cuboidal cells of the follicles become very high. The lumen is very small with small amounts of colloid and evident scalloping. This can occur when the oxidation of iodide is blocked. Iodide is required for the normal production and release of thyroid hormones, and lack of thyroid hormones in the blood stimulates increased TSH release from the pituitary.

11. The parathyroid glands are located on the posterior surface of the right and left lobes of the thyroid gland. Note: The parathyroid glands may not appear as such distinct nodules to the naked eye. • Once again, we can review the arterial and nerve distribution of the larynx and upper trachea. • Note the common carotid artery branching into the internal and external carotid arteries. The internal carotid does not have any branches. The external carotid gives off the superior thyroid artery, from which the superior laryngeal artery branches and enters the thyrohyoid membrane. The inferior thyroid artery comes off the thyrocervical trunk from the subclavian artery. • The vagus nerve is the major nerve supply of this region. As it descends in the neck, it gives off the superior laryngeal nerve. The internal branch pierces the thyrohyoid membrane together with the superior laryngeal artery. The external branch descends outside the muscles to innervate the larynx. The vagus nerve continues and gives off the recurrent laryngeal nerve, which loops under the subclavian artery on the right and uner the aortic arch on the left, going back up to the larynx.

12. The secretory cell of the parathyroid gland is the chief or principal cell. They produce and secrete parathyroid hormone (PTH) which increases the activity of osteoclasts in bones, effectively increasing serum calcium levels. • The much larger cells found in isolated groups are oxyphil cells. They contain more extensive, eosinophilic cytoplasm and smaller, darker nuclei. They are thought to be “aged” chief cells. Although they are functionally unimportant, they help in the identification of the parathyroid glands. • The parathyroid gland, like other endocrine glands, contain many capillaries (red arrows). • Parafollicularcells, or C-cells, found in the thyroid (arrows in the bottom right) secrete calcitonin which has the opposite effect from that of PTH and decreases serum calcium levels.