XVII. URINARY SYSTEM

A. Kidney

Slides 47, 76 (human kidney)

Use your atlas, any models, and your low power objective to identify the following structures:

1. Capsule composed of dense collagenous fibers (may not be present)

2. Cortex and Medulla

3. Renal pyramids (Malpighian pyramids) projecting into the calyx as papillae

4. Renal pelvis with calyxes

5. Glomeruli

6. Renal corpuscle

7. Bowman's capsule

Most of the following structures can be located on slides 46 and 76 at higher magnifications:

1. Vascular pole of Bowman's capsule. The afferent and efferent arterioles join the glomerulus at this point.

2. Urinary pole of Bowman's capsule. The proximal convoluted tubule begins here.

3. Visceral and parietal layers of Bowman's capsule. The visceral cells are podocytes; the parietal cells are squamous and uncomplicated.

4. Capsular space or the urinary space of Bowman's capsule.

5. Blood cells in capillaries of glomeruli.

6. Proximal convoluted tubules. These cells have brush borders. The boundaries between these cells are not visible with the light microscope because there are so many basal and lateral interdigitations.

7. Thin limb of Henle. These have very thin walls. Look for examples in the outer part of the medulla. They can be mistaken for capillaries, but they contain no blood cells. Capillaries usually have only one nucleus visible in a transverse section; the thin limbs of Henle usually have two or three. Do not spend a lot of time looking for these in sections.

8. Distal convoluted tubule. The cellular boundaries are not visible in the light microscope. The cells lack a brush border although they have some short microvilli. The cells usually stain rather lightly compared to the cells of the proximal tubule.

9. Collecting tubules. The collecting tubules near the glomeruli of nephrons are arched and eventually become straight and finally become the ducts of Bellini. They increase greatly in diameter in the inner part of the medulla as they enter the papilla. The cells along the collecting tubules increase in height from cuboidal to high columnar.

B. Ureter

Slide 77 (ureter)

Examine slide 77 of the ureter and identify the following: transitional epithelium, lamina propria, smooth muscle and adventitia. The epithelium is usually folded and the lumen has a stellate shape in cross section. Inner longitudinal smooth muscle fibers can usually be found. These are surrounded by circular fibers and an outer layer of longitudinal fibers.

C. Bladder

Slide 78 (bladder)

The epithelium of the bladder is of the transitional type. Beneath the epithelium is a thin lamina propria and some smooth muscle. These layers constitute the mucosa. There is no distinct submucosa according to some authors although the connective tissue near the muscularis is sometimes called a submucosa. The muscularis is well developed and is composed of inner longitudinal, middle circular and outer longitudinal fibers (as in the ureter). With both the ureter and blabber, you should be able to tell what tissue type and where you are in the body.

The outermost layer is the serosa, composed of mesothelium and a small amount of connective tissue. The epithelial cells at the surface are highly specialized to prevent water from entering the lumen and thus diluting the urine. These cells contain flattened vesicles derived from the Golgi bodies. These vesicles develop thickened plaques on their inner surfaces. These vesicles fuse with the plasmalemma and form thick membranes.

XVIII. ENDOCRINE GLANDS

The endocrine glands are the ductless glands. The epithelial cells of endocrine glands are not oriented around ducts as are the cells of exocrine glands. Instead, the cells are in close association with capillaries, and the cell products (hormones) are secreted into the connective tissue compartment where they are rapidly absorbed by the circulatory system.

The liver is both an endocrine and exocrine gland. It secretes products such as proteins, lipoproteins and glucose (not hormones) into the blood, but it has recently been shown to produce the hormone somatomedin and a prohormone angiotensinogen.

The pancreas, kidneys, testis, ovaries, placenta, brain, lungs, and gastrointestinal tract are known to produce hormones, but they have other functions as well. The traditional endocrine glands are the pituitary, thyroid, pineal, parathyroid and adrenal glands as well as the pancreatic islets of Langerhans.

A. Thyroid Gland

Slide 27 (human thyroid)

When you are looking for the thyroid, also examine slide 21, it often has a section of the thyroid gland on it. The thyroid is composed of two lobes containing many follicles, the basic histological unit. The wall of each follicle is composed of squamous to columnar epithelial cells. The center of each follicle contains colloid secreted by the follicle cells. The follicles are surrounded by connective tissue elements and a rich capillary network.

The quantity and consistency of the colloid and the shape of the cells in the epithelium depends on the functional state of the gland. In the presence of low levels of thyroid stimulating hormone (THS) the follicle cells are squamous and the colloid is dense and abundant. When stimulated by high levels of THS the cells of the epithelium become more columnar in shape and the amount of colloid is reduced, stains less intensely and may appear vacuolated or "foamy" around the periphery.

In addition to the follicle cells, the thyroid contains "light cells" sometimes called parafollicular or C- cells. These cells may be hard to find on our slides. These C-cells originated in the ultimobranchial body, and only in mammals do they migrate into the thyroid. The parafollicular cells produce calcitonin, a hormone that regulates plasma calcium levels by lowering the plasma concentration. Note the capsule, the arrangement of the follicles and the and the vascularized connective tissue.

B. Parathyroid Gland

Slide 27 (parathyroid and thyroid gland)

Examine slide 27 and identify the principle cells (chief cells) which are the most common cell type. These cells secrete PTH (also called parahormone) which acts to raise plasma calcium levels. PTH acts by increasing the rate of breakdown of bone by osteoclasts, increasing the reabsorption of calcium, inhibiting the reabsorption of phosphate ions (kidney) and promotion absorption of calcium from the small intestine.

Another type of parenchymal cell is the oxyphil which are larger cells with dark (may be lighter) acidophilic cytoplasm. They usually occur in groups or nests, but may be isolated. These cells have been identified in primates and cattle but not in other animals. The oxyphils, in common with the parietal cells of the stomach contain large numbers of mitochondria. The function of oxyphils is unknown. Note that the cells of the parathyroid are arranged in irregular anastomosing cords.

C. Adrenal Gland

Slide 65 (adrenal gland)

Examine slide 65 and with the aid of your text and atlases identify the connective tissue capsule which sends septa into the organ, the cortex and the medulla. Within the cortex find the following zones:

1. zona glomerulosa

2. zona fasciculata

3. zona reticularis

The zona glomerulosa secretes mainly aldosterone, a mineralocorticoid. The zona fasciculata secretes mainly cortisol. The zona reticularis may secrets some to the sex hormones (androgenic hormones). Over 30 different hormones have been isolated from the adrenal cortex but aldosterone and cortisol are of greatest importance. Note the arrangement of cells in each zone. In the medulla, note that the cells are arranged in elongated strands, which differ from the cell arrangements of the cortex. The main cellular components of the medulla are the chromaffin cells which are arranged in irregular groups.

The vascular system of the cortex consists of a network of sinusoids supplied by branches of a capsular plexus of vessels. The sinusoids pass between "cords" of epithelial cells. The sinusoids are fenestrated and are bordered by a basal lamina. The arrangement of sinusoids and cells is similar in many ways to that in the liver. A perisinusoidal space separates the lining cells form the parenchymal cells and microvilli or the parenchymal cells extend into this space.

D. Pituitary Gland

Slide 26 (hypophysis)

The pituitary gland is composed of several glandular epithelial components lumped under the name adenohypophysis (derived from Rathke's pouch) and nervous parts collectively called the neurohypophysis (derived from the floor of the diencephalon). The adenohypophysis can be subdivided into three regions: pars distalis, pars tuberalis and pars intermedia. The neurohypophysis also has three parts: pars nervosa, infundibular stem and median eminence. The whole gland is covered by a connective tissue capsule.

There is a great difference in the gross appearance of the pituitary gland in different vertebrates. Identify under low power the adenohypophysis, neurohypophysis and pars intermedia. In the adenohypophysis note three types of cells:

1. Chromophobes or chief cells which are the most numerous. They are small cells which stain rather lightly and often possess little cytoplasm.

2. Eosinophilic or acidophilic cells.

3. Basophilic or basophils which are the least numerous.

Note that the distribution of the cell types is irregular within the adenohypophysis and made up of individual cords. Pay attention to the arrangement of cells in this network of anastomosing cords separated by sinusoidal capillaries. The pars intermedia consists of only one type of epithelial cell. The pars nervosa is composed of many neuroglial cells and fibers but no true nerve cells are present.