Endocrine Lab:

See Content of Chapter 17

Overview:

The endocrine system is a diffuse system, scattered throughout the body. It is composed of numerous organs that serve strictly an endocrine function (i.e., secretion of hormones) and many endocrine tissues or cells that are part of larger organs. All endocrine organs and tissues are glands that are ductless. Theysecrete these chemical messengers, hormones, into the bloodstream to travel to distant target tissues where each hormone exerts its actions.

Hormones are chemical signals the body uses to direct biological actions. Each hormone can only stimulate a response in cells that have receptors for that hormone. The location of the receptor, membrane bound or intracellular, is dictated by the chemical nature of the hormone. The body tightly regulates the overall receptor numbers at a target tissue; without receptors hormones have no ability to cause cells to change their function.

Many homeostatic mechanisms are tightly regulated by hormone actions. Blood glucose levels are an example of such a mechanism.

This lab has two major components: an anatomical overview and a physiology exercise. Remember that you will probably need to spend time in open lab to move from familiarity with the information to understanding the where and how our endocrine system work.

Anatomy Learning Objectives:

  1. Students will be able to identify the endocrine glands on the models, figures and cadaver described in this lab.
  2. Students should be able to name the main hormones secreted by each of the endocrine glands studied.
  3. Students should be able to differentiate the various histological structures on the endocrine slides listed below.

Endocrine Gland Anatomy:

Major and minor endocrine glands of the body are part of the way our bodies communicate to regulate and control growth, metabolism and sexual development and function. (Be sure that your laboratory period is not spent looking at the book and a torso model, much of this type of review can be completed in open lab.)

Exercise 1: Organ overview

Part 1: Working in a group of three to four students, gather around a torso model, refer to figure 17.1, and identify the following major endocrine organs.

1.Pituitary gland

2.Pineal gland

3.Pancreas

4.Adrenal gland

5.Thyroid gland

6.Parathyroid gland

7.Ovary

8.Testes

Once organ identification is complete, fill in the function of the hormones secreted by each gland on the endocrine chart that is part of the laboratory worksheet. In addition to identifying the function, list any tropic hormones that promote the release of the hormone.

Part 2: As our knowledge of the endocrine system expands, more and more organs appear to have some endocrine gland function. Some of the glands on the torso model would be considered minor glands. Identify these minor endocrine glands on the torso model, cadaver and figure 17.1:

1.Kidney – erythropoietin

2.Heart -- atrial natiuretic hormone

3.Liver– angiotensinogen (precursor to Angiotensin II)

4.Thymus -- thymosins

Again return to the endocrine chart and describe the function of the hormones listed for the minor endocrine tissues.

Part 3: Finally, a quick nervous system review of the hypothalamus: Open your book to page 515 and use figure 14.2 as a reference. Examine a model of the brain and identify the hypothalamus on its inferior surface. The hypothalamus is located in the lower central part of the brain. Find the hypothalamus on the diagram. It is shaped like a funnel that forms the floor and walls of the third ventricle of the brain (figures 14.2 and 14.12b).

With its intimate tie to the posterior pituitary via the nerve tracts and is connection to the anterior pituitary via the hypothalamohypophyseal portal system (figure 17.4 a and b, page 639), the hypothalamus is a major control center for many of the hormones in the body. It secretes tropic hormones that stimulate or suppress the release of hormones from the anterior pituitary, thereby playing a role in homeostasis.The hypothalamus plays an important role in the regulation of satiety, metabolism, sex drive and body temperature.

SPECIFIC ENDOCRINE GLANDS: For the remaining anatomical review exercises, you will need to find the individual structure of the endocrine glands and look at the histology of each.

Exercise 2: Pituitary Gland

Some of the brain models have a pituitary gland depicted, see the very pink bean-sized structure identified by the blue arrow is on. The pituitary gland (figure 17.4a and b, p. 639) is suspended from the floor of the hypothalamus by a stalk(i.e., infundibulum) and is housed in a depression in the sphenoid bone (i.e., sella turcica of the sphenoid bone). It is physically smaller than a marble, more like a kidney bean in shape and size. It is composed of two structures: the adenohypophysis (i.e., anterior pituitary) and the neurohypophysis (i.e., posterior pituitary). These two structures have independent origins developmentally, and have distinct physiological functions.

View figure 17.4a and note that neurons extend from two brain nuclei (i.e., paraventricular nucleus and suprapotic nucleus) in the hypothalamus to terminate in the posterior pituitary. This indicates that the secretions from the posterior pituitary are neurosecretions and are sometimes called neurohormones. The paraventricular nucleus is responsible for the synthesis of oxytocin (which promotes uretine contraction or milk ejection), while the supraoptic nucleus produces antidiuretic hormone (to counter dehydration and is involved in blood pressure regulation).

The anterior pituitary is composed of glandular tissue and is involved in the release of 6 different hormones directly into the bloodstream. Tropic hormones released by the hypothalamus into the bloodstream control the secretion of anterior pituitary hormones. Look at figure 17.4b and note the extensive capillary networks associated with the hypothalamus and anterior pituitary. If we follow the superior hypophyseal artery, it branches into a primary capillary bed right at the base of the hypothalamus. This capillary bed is drained by portal venules that terminate in a second capillary bed surrounding the anterior pituitary. This arrangement of vessels connecting two capillary beds in a series is called a portal system. And this specific one is called the hypothalamohypophyseal portal system.

Part of learning the anatomy of any system is to look at the tissues at a microscopic level. This examination reveals specific cellular structures and tissue organization you will be required to know. Remove slide 34 from the slide box and place on a piece of white paper. Notice with your naked eye the pituitary gland slide has two distinct parts. The darker stained region is the anterior pituitary. The lighter stained region is the posterior pituitary. Now, place the slide on the stage of the microscope and focus on low power. (Remember that you always want to start on low power.) Scan the slide and find the junction between the anterior pituitary and the posterior pituitary. Now focus on this junction on high power. Notice that the cells of the anterior pituitary are relatively cuboidal in shape and have relatively large nuclei. Remember the anterior pituitary is glandular in nature.

Now focus on the opposite side of the junction. This side has an almost fibrous appearance with numerous small dark nuclei interspersed. Recall that the posterior pituitary is an extension of the hypothalamus – brain tissue. It is composed of bundles of axons(the more fibrous appearance -- tracts) intermixed with glial cells (the darker, small nuclei)(figure 17.5, p. 636).

In the space provided on the worksheet, draw what you see in your field of view for the anterior pituitary and posterior pituitary. Then compare your drawing to that of your peers and also the website hyperlink for the pituitary. (This is not about how well you draw, rather it is the actual act of drawing that helps you remember the structures that you are observing at a cellular/tissue level.)

Go to the endocrine chart on the worksheet and fill in the functions of the hormones of the anterior (6) and posterior pituitary (2). Most of the hormones of the pituitary are controlled by negative feedback, with at least one notable exception, oxytocin. Use your text to review these hormones.

Exercise 3: Thyroid Gland

The thyroid gland is one of the easiest glands to palpate. Run your fingers down your trachea towards your manubrium (the broad upper part of the sternum). You should feel the cartilaginous supports of your trachea and you should feel the thyroid gland that spans this structure just caudal to the thyroid cartilage of the larynx. While our palpation is kind of crude in nature, practiced physicians learn to detect gland texture as well as size with just their fingers.

The gland itself is butterfly shaped where the two wings create the glands bulk as it wraps the trachea. The two lobes (i.e., the left and right lobes) are attached inferiorly by a bridge-like structure called the isthmus (figure 17.9a).

Remove slide #4 from the slide box, place the slide on the stage of the microscope and focus on low power. Quickly advance to high power. The most notable feature of the slide is the numerous functional units interspersed throughout the gland called thyroid follicles(figure 17.9b). The walls of the follicle are composed of simple cuboidal epithelial tissue. The lumen of the follicle is filled with a protein-rich colloid precursor of the thyroid hormones. The follicular cells encompass the follicle and secrete thyroxine (i.e., T4 or tetraiodothyronine) and triiodothyronine (i.e., T3). These hormones are collectively called the thyroid hormones.

In the space provided on your worksheet draw what you see in the field of view for the thyroid gland. Label the thyroid follicle with the colloid inside of it and the follicular cells.

There are also clusters of cells located between the follicles called parafollicular cells. These cells, also called C cells, are hard to distinguish on a normal slide, but you need to know that these cells exist and are the source of the hormone calcitonin which helps prevent hypercalcemia. Label the parafollicular cells in your diagram as well. Once complete, compare your diagram to the website illustration and to the diagrams of your lab partners.

Exercise 4: Pancreas

The pancreas is an elongate glandular structure (about the texture of cottage cheese – when looking at the cadaver). It is located below and behind the stomach (figure 17.12a). The bulk of the gland is exocrine tissue involved in the release of digestive enzymes into the small intestine through the pancreatic duct. Only a small portion of this gland is devoted to endocrine actions.

Remove slide # 20 from the slide box and focus on low power. Quickly step up to high power.The pancreas contains both exocrine and endocrine cells. Scan the slide and isolate a portion that shows the darker, exocrine tissue with interspersed with clusters of lighter stained cells of the endocrine functions derived in the pancreatic islets(figure 17.12b,c).

The exocrine tissue is composed of tightly packed serous acini that appear like rather puffy cell collections.The histological stain used is specific for the rough endoplasmic reticulum (i.e., RER), the organelle used to make enzymes and proteins in the cells.Because the serous acini secrete more digestive enzymes, they have more RER and are therefore stained darker.

The islet cells are less enlarged and are paler in color. A fine capsule that allows it to be distinguished as a unit surrounds the entire islet, but you will not be able to see this capsule specifically under the light microscope, instead you will only see that the endocrine cells are packed together. There are several cell types within each islet including the alpha cells which secrete glucagon, that increases blood glucose if needed (prevent hypoglycemia), the beta cells which secrete insulin that decreases blood glucose if needed (prevent hyperglycemia), and the delta cells that secrete somatostatin which helps regulate digestion as well as secretion of insulin or glucagon. But to identify these individual cells there would have to be additional staining. Go to the endocrine chart on the worksheet and fill in the hormones of the pancreatic islets.

Draw the histology of the pancreas as seen in the microscope field of view. Label the serous acini of the exocrine pancreas and the pancreatic islets. Then compare your diagram to that of the web picture and that of your classmates.

Exercise 5: Adrenal Gland

The adrenal glands are small, almost triangular shaped glands that are superior to the kidneys (figure 17.11a). The glands are about the size of a hazel nut. Like the pituitary, the adrenal gland develops from two distinct glands in the embryo, both with endocrine functions. Identify this gland on the torso model and from diagrams from your book.

Remove slide # 36 from the slide box and focus on low power. The entire gland is typically surrounded by a layer of adipose tissue. The capsule surrounds the outside of the gland proper. Focus on the cortex of the adrenal gland. The outer layer, called the cortex, is the larger of the layers consisting of about 80-90% of the entire structure, while the inner medulla about 10% of the entire structure. The cortex surrounds the deep medullary tissue and is a structurally diverse tissue that can be divided into three layers (from outside toward the medulla): zona glomerulosa, zona fasciculate and zona reticularis (figure 17.11b). You are not responsible for identifying the specific cortical layers histologically, but rather should know that the adrenal cortex is involved in secreting three classes of steroid hormones: androgens, minerocorticoids and glucocorticoids. The medulla is both an endocrine organ and a ganglion of the sympathetic nervous system. Chromaffin cells make up this ganglion and are responsible for secreting the catecholamine hormones (e.g., epinephrine, norepinephrine and dopamine). Note the highly vascular nature of the adrenal medulla and ponder how these blood vessels are needed to carry catecholamines to the body following secretion at these modified sympathetic nerve endings. Fill these hormones in on the endocrine chart of the worksheet.

Exercise 6: Gonads

The ovary is not only a reproductive organ responsible for oogenesis, but is also an endocrine organ releasing estrogens and progesterone (figure 28.1). The ovaries are small, almond-shaped organs. Identify the ovary on the torso model and from the figure in your text. The ovary has a cortex and medulla, like many of the other organs examined (figure 28.2). The primary hormones of the ovary are estrogens, progesterone and inhibin. Fill these in on the endocrine chart.

The ovary is surrounded by a fibrous capsule and the cortical stroma is where the ovarian follicles can be found. The follicles are a collection of epithelial cells and an oocyte surrounded by a capsule called the theca. The follicles can be at various stages of development, although most are primordial follicles. (This differs from the diagram in your book that is illustrating the various stages of development the follicles progress through indicated by the series of black arrows.) Figure 17.13a is a histological view of a mature ovarian follicule. At this stage of development the oocyte (collectively includes the egg and the egg nucleus) is obvious. Surrounding the oocyte are layers of granulosa cells that secrete estrogens. Surrounding the granulosa cells are the theca. The medulla is again the inner region of the organ that is composed of connective tissue and is also highly vascularized.

Remove slide #40 from the slide box and focus using low power. At this point you should be able to see the entire ovary section. Notice the cortex and the medulla. Now advance to 100X magnification. Focus on the cortex and look for a good example of a mature follicle. Isolate that follicle and examine it under 400x magnification. At this point you should be able to identify the cortical stroma and the isolated follicle as well as the oocyte, granulosa cells and theca. Draw what you see in the field of view of the microscope and label these structures.

Return to 100x magnification and find a cluster of darker cells slightly below the follicle layer. This cell cluster is a corpus luteum and is responsible for progesterone secretion that assists in maintaining pregnancy. Again draw what you see in the field of view of the microscope and label this structure. Compare your diagram with that of the study group across the table.

The testes, like the ovary, has two functions: spermatogenesis and sex hormone production. Testes are paired organs housed in the scrotum. They are about the size of walnuts gross structure (figure 27.9a,b,c). In terms of an endocrine organ, the testes must be examined histologically.