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The Urinary System

Objectives

  1. Understand the arrangement of kidney cortical and medullary components.
  2. To be able to identify the individual components of the renal nephron and collecting tubule system.
  3. Understand the anatomical arrangement of the renal vasculature.

Kidney: Slide #74 (even)

The slide from odd-numbered boxes has different staining properties and these will be described below.

This tissue is presented as a truncated right triangle with the kidney capsule covering the wider portion. The shift in staining from dark to light pink about 4 mm from the base is an artifact. See diagram for orientation. The cortical tissue found just below the capsule is divided into medullary rays and cortical labyrinth. Medullary rays, are regions where parallel arrays of straight tubules travel perpendicular to the capsule and extend from the cortex to the medulla. The cortical labyrinth contains renal corpuscles and tubules. A central band of cortical labyrinth separates the medullary rays. Each medullary ray with half of the adjacent cortical labyrinth on either side is a lobule.

Under low power the cortex is divisible into alternating bands called the cortical labyrinth, which is recognized by the presence of numerous renal corpuscles and medullary rays, relatively straight collections of epithelial tubules oriented perpendicular to the capsule. The renal corpuscle is formed by two structures, an epithelial capsule, and a tuft of capillaries, the glomerulus. The capsule has two layers, parietal and visceral. The squamous parietal epithelium is continuous with the proximal convoluted tubule at the urinary pole (the point at which the proximal convoluted tubule joins the renal corpuscle). This may not be visible on the renal corpuscle you are examining, and you may have to examine many to see it.

   

The visceral layer of Bowman's capsule covers the capillaries. On your slides, it will be difficult to distinguish between endothelial cells of the capillaries and the cells of the visceral epithelium (podocytes) by cellular morphology alone, however, you should be able to distinguish the capillaries because they contain red blood cells.

Afferent arterioles are those entering the vascular pole of the renal corpuscle. The efferent arteriole, leaves the renal corpuscle and gives rise to a capillary bed called the peritubular capillary plexus which surrounds the tubules of the cortical labyrinth. You will not have to distinguish between afferent and efferent vessels but should be able to identify the vascular pole.

The proximal convoluted tubule follows a tortuous course in the vicinity of the renal corpuscle from which it originated and will be cut in cross section many times. Four characteristics distinguish the proximal convoluted tubules from other cortical tubular structures present on the slide: (1) It is the most commonly seen tubule in the cortex, (2) the cytoplasm of its cells is more acidophilic than that of the distal and collecting tubules, (3) lateral cell boundaries are not seen by light microscopy, and (4) the apical cell surface has a brush border. The material in the tubule lumen is due to poor fixation.

 

The next part of the nephron is the loop of Henle which can be subdivided into 3 portions: (1) the descending thick limb; (2) the thin loop and (3) the ascending thick limb. The descending thick limb is a straight continuation of the proximal convoluted tubule which it resembles closely. It is found in the medullary ray and a short distance into the medulla, where there is an abrupt change of the descending thick limb into the thin loop of Henle. The thin loop of the medulla is composed of squamous cells and they may sometimes be confused with capillaries. However, they do not contain blood, they have a thicker epithelium, their lumen is usually wider and their nuclei are larger and protrude into the lumen more than endothelial nuclei. The thin loop turns back towards the cortex and is continuous with the ascending thick limb of Henle's loop, which is found in the medulla and the medullary rays. The cells of the ascending thick limb are distinctly smaller than those of the descending thick limb, do not stain as intensely and lack a brush border. Lateral cell boundaries are not usually distinct.

   

The ascending thick limb travels from the medullary ray to the vascular pole of the renal corpuscle where it continues as the distal convoluted tubule. Cells of the distal convoluted tubule: 1) are cuboidal, 2) do not stain as eosinophilic as the proximal tubule, and 3) do not have distinct cell borders. The macula densa is a region of the distal tubule adjacent to the vascular pole of the renal corpuscle. It consists of cells that are taller and thinner than those of the opposite side of the distal tubule, giving the appearance of more tightly packed nuclei. The macula densa is not easy to find and you may have to examine many renal corpuscles to locate one. If you cannot find one after several minutes of observation, continue on with the remainder of the laboratory.

   

The distal convoluted tubule of each nephron discharges into a single collecting tubule in the cortical labyrinth. Collecting tubule cells are columnar, pale staining with very distinct borders between adjacent cells and have rounded apical cell surfaces. Compare the collecting tubules in the medullary rays to the descending and ascending thick tubules. The collecting tubule continues to the medulla, where it fuses with other collecting tubules to eventually form a collecting, or papillary,duct in the papilla. The collecting ducts empty into the minor calyx at the very tip of the papilla. The minor calyx is lined with a transitional epithelium.

Kidney: Slide #74 (odd)

The slide contains material that is preserved better than that observed on slide 74 even. Because of this, the staining properties of the renal tubules are different than those described above. Therefore, the student should read the description for slide 74 even, then consider the following differences.

The proximal convoluted tubule and the descending thick limb can be distinguished from the other tubules by the presence of numerous, large red cytoplasmic granules. These granules are not present in either the distal convoluted tubule or the collecting tubule. In this slide, it is relatively easy to distinguished the urinary pole since the proximal tubule has cytoplasmic granules. The distal tubule and the ascending thick limb (in the medullary ray) have a more cuboidal epithelium and, unlike, slide 74 odd, there is some staining of their cell borders. The collecting tubules on this slide are observed almost exclusively in the medullary rays. They are stained lighter than the other tubules and have distinct cell borders. The medulla is found in the lighter staining region near the tip of the triangular piece of tissue. There are longitudinal sections of thin loop here. Distinguish these from the collecting tubules, and the ascending and descending thick limbs. The thin loops found here will not be as easy to distinguish as those found in cross-section on slide 74 even.

Blood Vessels in the Kidney

In slide #75, the blood vessels of the kidney were injected with red latex before sectioning. It would be to your advantage to read the following material before you begin examination of the slide. If you should have difficulty in identifying the blood vessels when starting from #1, begin with the glomerulus (#6) and work backwards. Do not examine this slide with oil immersion.

   

(1) Arcuate arteries and veins are found at the junction between cortex and medulla.

(2) Interlobular arteries are branches of the arcuate arteries, and run through the cortical labyrinth perpendicular to the kidney capsule, between the medullary rays.

   

(3) Intralobular arteries are branches of the interlobular arteries, and run in a direction generally parallel to the kidney surface. They can branch to give rise to two or more afferent arterioles, however, it is not uncommon for interlobular arteries to give rise to afferent arterioles directly, especially to glomeruli near them. It will only be possible for you to differentiate intralobular from afferent vessels when the intralobular branches can be visualized.

(4) Afferent arterioles arise either from intralobular vessels or directly from interlobular vessels. Afferent arterioles terminate in the glomerulus.

(5) Efferent arterioles are formed by the union of the capillaries from the glomerulus. Although the efferent arterioles differ from afferent arterioles in having a narrower lumen and a thinner wall you will not have to distinguish between them. The efferent arterioles in most of the cortex give rise to the peritubular capillary plexus, in the cortex. Efferent arterioles exiting from glomeruli near the medulla give rise to long straight vessels collectively called the vasa recta which run parallel to the thin loops in the medulla.

(6) Glomerulus is a group of looped capillaries interposed between the afferent and efferent arterioles.

Extrarenal Organs

Urinary Bladder

Examine slides 9 and #76 odd which are sections of urinary bladder. There is a transitional epithelium and the cells lining the lumen are larger. The epithelium is attached to a thin basement membrane which may is difficult to visualize and below the basement membrane is a thick layer of connective tissue. The layers of smooth muscle consist of interwoven bundles that do not appear as distinct as the layers seen in the digestive tract. The inner layer is arranged longitudinally and the outer layer is arranged in a circular manner (the reverse of the arrangement of the intestinal wall). In the lower third of the ureter and in the bladder, there is also an outer longitudinal layer. It will be very difficult to distinguish the different muscle layers.

Ureter

Examine a section of ureter slide #77 in both odd (human) and even (dog) slides. Identify the epithelium, lamina propria and muscular layers. Note the shape of the surface epithelial cells with their apical surface bulging into the lumen.

Female Urethra

Examine the section of female urethra (slide #78). Since the material was taken from both ends of the urethra, slides in both the odd and even boxes will have to be examined. Slide #78 odd is from the urethra at its distal end and has a stratified squamous epithelium. Study the lamina propria and observe the relatively large number of thin-walled veins filled with unstained erythrocytes. Slide #78 even is taken from that portion of the urethra closest to the bladder and has both transitional and stratified squamous epithelium. The male urethra will be studied with the male reproductive tract.