Endocrine Systems Lab

Learning Objectives

  • Describe the organization and function of endocrine tissues, including the key endocrine organs as well as diffuse endocrine cells.
  • Distinguish the different types of pituitary cells using the light microscope and electron microscope.
  • Name and describe the different layers of the adrenal gland, as well as the blood supply to this gland.
  • Explain what is unique about the structure of the thyroid gland.
  • Contrast the structure of the thyroid with that of the parathyroid.
  • Identify islets of Langerhans within pancreatic tissue and explain the relative positions of alpha and beta cells.
  • Recognize the differences between normal and pathological specimens of thyroid, adrenal gland, parathyroid, and pancreas.

Pre-Lab Reading

Introduction

The term "endocrine" implies secretion into the internal milieu of a multicellular organism. In contrast to exocrine tissues, where the secretory products are discharged into the external space - the outer surface of the body, mucosal surfaces, duct systems - the endocrine organs and cells secrete their products into the vascular system. The blood vessels then serve as conduits for these secretions to travel to their target tissues.

Endocrine tissues function according to a basic model: individual cell types respond, via surface receptors, to different signals. They produce a very limited spectrum of secretory products, called hormones, that themselves function as signaling molecules for specific cell populations. These hormones are effective at exceedingly low concentrations. Because of this, relatively few cells are sufficient for the integrative function of large, multicellular organisms. Therefore, endocrine tissues are characteristically present in small quantities.

The focus of this laboratory will be on five endocrine tissues: the pituitary gland, the thyroid, the parathyroid, the adrenal gland, and the endocrine pancreas.

Pituitary Gland

The pituitary gland, or hypophysis, is a collection of different cell types that control the activity of other endocrine organs. It is governed by the hypothalamus, which sends both electrical and hormonal signals to the pituitary, and by feedback regulation (both positive and negative) through the secretions of its target glands. The pituitary gland sits within the sella turcica of the sphenoid bone and is traditionally divided into two sections that can be distinguished structurally, functionally, and embryologically.

The anterior pituitary is also known as the adenohypophysis or pars distalis and is derived from Rathke's pouch, a growth of the primitive oral cavity. It contains cells that, when viewed under the light microscope, appear as acidophils, basophils, or chromophobes.

  • Acidophils are typically eosin-stained and tend to be located in the center of each half of the pituitary. They secrete protein hormones like growth hormone and prolactin.
  • Basophils appear purple and are more prominent at the edges of the gland. They secrete glycoprotein hormones such as ACTH, TSH, FSH, and LH.
  • Chromophobes have clear nuclei and scant cytoplasm. They may be cells that are non-secretory or exhibit minimal hormone storage.

The posterior pituitary is also known as the neurohypophysis or pars nervosa and is derived from the hypothalamus. It is mostly composed of unmyelinated axonal processes and terminals of the supraoptic and paraventricular nuclei of the hypothalamus. The pituitary stalk connects the two glands. The posterior pituitary has characteristic Herring bodies, focal axonal swellings that are packed with secretory granules. Pituicytes are the glial cells of the pituitary gland; the nuclei that are visible in sections of posterior pituitary belong to these cells, as well as cells of the vasculature.

A third section of the pituitary, the intermediate lobe, is vestigial in man.

The pituitary gland has a unique vascular system. Cells from the hypothalamus secrete releasing hormone into a hypophysisal portal system that flows from the hypothalamus to the anterior pituitary. This system supplies the anterior pituitary and drains its products, which include the stimulating hormones produced there. It then enters the systematic circulation, which delivers those hormones to target organs throughout the body. The posterior pituitary has a separate system that carries away its products, as it does not receive hormonal stimulation from the hypothalamus.

Adrenal Gland

The adrenal gland has two distinct parts, the cortex and medulla, which differ in structure, function, and embryological origin.

The adrenal cortex is organized into cords that are two cells thick and are arranged radially around the medulla. The cortex is similar in embryology to the gonads; this can be easily recalled because both glands secrete steroid hormones synthesized from cholesterol. Three cortical zones can be distinguished on the basis of differences in the arrangement and cytology of the cells.

  • The zona glomerulosa is the thin outer layer of the adrenal cortex. Its cells are pale staining and organized in ovoid clusters that are separated by capillaries. The cells of the zona glomerulosa produce mineralocorticoid hormones like aldosterone, which regulates salt and water balance.
  • The zona fasciculata comprises the thick middle layer of the cortex. Its cells are extensively vacuolated because of the presence of lipid droplets. The cells of this region are organized into parallel cords separated by straight capillaries and produce glucocorticoids like cortisol, which has metabolic effects.
  • The zona reticularis is the innermost layer of the adrenal cortex. The border between it and the zona fasciculata is less distinct than that between the previous two zones. Cells in the zona reticularis stain deeply and are less vacuolated. This region produces androgens, which supplement sex hormones produced by the gonads.

The adrenal medulla is the innermost portion of the gland and shares an embryological origin with the sympathetic nervous system. Its cells possess abundant cytoplasmic granules that contain stored peptide hormones and catecholamines. These cells are frequently called chromaffin cells because they can be stained with chromium salts. Preganglionic sympathetic fibers traverse the adrenal cortex and synapse directly on chromaffin cells, where they release acetylcholine to stimulate the exocytosis of catecholamine granules during a sympathetic response.

It is important to understand the blood supply of the adrenal gland. The suprarenal arteries form a plexus beneath the capsule of the gland. These arteries give rise to two sets of downstream arteries:

  • The short cortical arteries supply the cortex by giving rise to an anastomosing network of capillary sinusoids. These sinusoids descend down through the three layers of cortex and drain into venules and the central vein of the medulla.
  • The long cortical arteries form a large capillary network around the secretory cells of the medulla and also drain into the central vein of the medulla.

Thyroid

The thyroid is located in the neck and is surrounded by a collagenous capsule continuous with the middle cervical fascia. It is unique because it stores large amounts of inactive hormone within extracellular compartments; most glands store small amounts of hormone intracellularly.

The thyroid consists of structural units called follicles, which are composed of secretory epithelial cells called principal cells that are adjoined by junctional complexes and surrounded by a basement membrane and reticular connective tissue. Follicles vary in size, but each displays a central lumen containing colloid. Colloid consists of the glycoprotein thyroglobulin, which is secreted by the principal cells and serves as a precursor to thyroid hormone. The height of the principal cells varies according to their level of secretory activity; in hypothyroidism, the cells are squamous or cuboidal, whereas in hyperthyroidism, they are columnar.

Parafollicular cells (C-cells) lie adjacent to the principal cells in the parenchyma of the organ and do not extend into the follicular lumen. They secrete the hormone calcitonin, which is involved in calcium regulation.

The thyroid is a highly vascular gland, and the follicular epithelium is intimately associated with the capillaries.

Parathyroid

The parathyroid glands are closely associated with the thyroid and are derived embryologically from the third and fourth branchial arches. They consist of closely packed groups of two cell types:

  • Chief (principal) cells, which have prominent central nuclei surrounded by pale cytoplasm. Chief cells produce parathyroid hormone (PTH), which is the most important regulator of calcium metabolism in humans.
  • Oxyphilic cells, which are large and fewer in number, have small, dark nuclei and an acidophilic cytoplasm with many mitochondria. The function of these cells is unknown, but they increase in abundance as a person ages.

Endocrine Pancreas

The endocrine portion of the pancreas is comprised of the islets of Langerhans. During development, the cells of the islets migrate away from the duct system and aggregate around capillaries. The islets contain three important cell types:

  • Alpha cells produce glucagon, which increases the plasma glucose concentration. They are primarily located around the periphery of the islets.
  • Beta cells produce insulin, which decreases plasma glucose by promoting uptake by liver, skeletal muscle, and adipose tissue. Beta cells are typically located in the center of the islets.
  • D-cells produce somatostain, which has broad effects on gastrointestinal function and inhibits insulin and glucagon secretion. Delta cells are scattered throughout the islets.

Pre-Lab Quiz

  1. Describe the blood supply of the adrenal cortex and medulla. How does the blood that the cortical cells receive differ in content from the blood that the medullary cells receive? Why might this be important in terms of influence on medullary secretion?
  2. Answer:
  3. What are three ways that PTH increases the level of serum calcium? Explain, in terms of calcium regulation, why a person can live without a thyroid but not without a parathyroid.
  4. Answer:
  5. Draw an islet of Langerhans in the space below. In your drawing, indicate the positions of alpha, beta, and D-cells, as well as some exocrine cells.
  6. Answer:
  7. Categorize each of the following as acidophilic, basophilic, or chromophobic. What hormone does each cell produce?
    • Thyrotroph
    • Corticotroph
    • Somatotrophs
    • Gonadotroph
    • Mammotroph
    Answer:

Slides

Please select whether to view the slides in study mode or quiz mode. In study mode, the images will contain labels and a description. In quiz mode, labels and description will be hidden.

  1. Pituitary Gland
  2. Anterior Pituitary
  3. Anterior Pituitary EM
  4. Posterior Pituitary
  5. Adrenal Gland
  6. Zona Glomerulosa
  7. Zona Fasciculata
  8. Zona Reticularis
  9. Adrenal Medulla
  10. Thyroid
  11. Parathryroid
  12. Endocrine Pancreas
  13. Islets of Langerhans

Virtual Microscope Slides

  1. Adrenal Gland
  2. Begin at the outer capsule of the adrenal gland and move inward. Identify the zona glomerulosa, the zona fasciculata, the zona reticularis and medulla.
  3. Thyroid
  4. Begin by locating the outer fibrous capsule of the thyroid. Identify several of the numerous blood vessels present in the capsule. Note how the capsule extends through fine collagenous septa to divide the gland into lobules. Observe the thyroid follicles at low and high power.
  5. Parathyroid
  6. Begin by distinguishing the parathyroid tissue from that of the thyroid. At 5x power, what structural differences do you notice in how the cells are assembled in each of these glands?
  7. Endocrine Pancreas
  8. Pick an islet and view the border between the endocrine and exocrine cells at 20x power. What important structural distinctions can be made between these cells?

Pathology

Please select whether to view the slides in study mode or quiz mode. In study mode, the images will contain labels and a description. In quiz mode, labels and description will be hidden.

  1. Grave's Disease
  2. Hashimoto's Thyroidosis
  3. Adrenal Cortical Adenoma

Quiz

  1. What region of the brain primarily controls this tissue?
  2. Answer: Hypothalamus - this is an image of the anterior pituitary. Note the mix of basophils and acidophils.
  3. This is an image of the adrenal gland which has been stained with a dye that labels lipids. Identify the heavily stained (dark) region and the hormone it produces.
  4. Answer: Region = zona fasciculata. Recall that the cells in the fasciculata have many lipid droplets and would be expected to stain dark with a lipid dye. Hormone = cortisol.
  5. Name the tissue.
  6. Answer: Parathyroid
  7. What is the name of the homogenously stained substance and of what is it composed?
  8. Answer: Colloid, thyroglobulin
  9. Name 3 secretory products of the cells in this organ.
  10. Answer: Insulin, glucagon, somatostatin
  11. What hormones might be located in the granules?
  12. Answer: Oxytocin, ADH
  13. Under the light microscope, it is possible to tell the relative activity level of the thyroid (hyperthyroidism, hypothyroidism, euthyroidism) based upon the appearance of the epithelial cells and the colloid. Draw or describe a thyroid follicle in each level of activity that indicates the changes in these two features.
  14. Pituitary adenomas frequently involve the proliferation of somatotrophs. What changes would you expect to find in a child with such an adenoma, and how would they differ from the changes that you would see if that same person developed their adenoma as an adult instead of during their childhood?
  15. Answer: In a child with a pituitary adenoma, you would expect to see gigantism. In an adult, you would see acromegaly. This is because the bones have ceased growing in the adult, so they cannot increase in length.