The kidneys, ureters, urinary bladder and urethra are the main components of the urinary system. A function of the urinary system that immediately comes to mind is the excretion of waste products from the body. This is only one of many functions of the system. Others are
- elimination of foreign substances
- regulation of the amount of water in the body
- control of the concentration of most compounds in the extracellular fluid
Most of these tasks are performed in the kidneys. Functionally the processes can be divided into two steps, each of which have their anatomical correlate:
- filtration - glomeruli of the kidney
- selective resorption and excretion - tubular system of the kidney
In addition, the kidney also functions as an endocrine organ. Fibrocytes in the cortex release the hormone erythropoietin, which stimulates the formation of red blood. Modified fibrocytes of the medulla secrete prostaglandins which are able to decrease bloodpressure.
Glomeruli and the tubular system are both part of the basic functional unit of the kidney, the nephron.
The Glomerulus (or renal corpuscle)
The glomerulus is the round (~0.2 mm in diameter) blind beginning of the nephron. It is invaginated by a tuft of capillaries at the vascular pole of the glomerulus. The tuft of capillaries and other cells in contact with them form the anatomical glomerulus. Substances which leave the capillaries enter the renal tubule at the urinary pole of the glomerulus.
The anatomical glomerulus is enclosed by two layers of epithelium, Bowman's capsule. Cells of the outer or parietal layer of Bowman's capsule form a simple squamous epithelium. Cells of the inner layer, podocytes in the visceral layer, are extremely complex in shape. Small foot-like processes (pedicles) of their cytoplasm form a fenestrated epithelium around the fenestrated capillaries of the glomerulus. The openings between the pedicles are called filtration slits. They are spanned by a
thin membrane, the filtration slit membrane. Between the podocytes and the endothelial cells of the capillaries we find a comparatively thick basal lamina, which can be subdivided into an outer lamina rara externa, a middle lamina densa and an inner lamina rara interna. The basal lamina and the slit membranes form the glomerular filtration barrier, which prevents some large molecules from entering the capsular space between the outer and inner epithelial layers
of Bowman's capsule.
Mesangial cells in the glomerulus form the connective tissue that gives structural support to podocytes and vessels.
Blood pressure is the driving force in the formation of about 125 ml of glomerular filtrate per minute. About 124 ml of the glomerular filtrate is reabsorbed in the tubules of the nephron.
Lab: Slides KIDNEY T.S. H&E. Locate the cortex of the kidney and scan over the tissue at low magnification. Note the presence of numerous glomeruli and the apparent absence of any preferred orientation of the tubules visible between the glomeruli (convoluted parts of proximal and distal tubuli). You should be able to identify the vascular pole of a good glomerulus by the attachment of the capillary tuft to the wall of the glomerulus. What would make your glomerulus VERY good would be the presence of a tubulus which contains a dense row of nuclei in the part of its wall closest to the
vascular pole of the glomerulus, the macula densa. The nuclei are located side by side or may even overlap. It should be possible to find this structure in all slides. It is also very likely that it may take you a few minutes of carefully scanning the tissue at high magnification before you will find it. Proximal tubules are characterised by their eosinophilic (pink) low, columnar cells and by large amounts of fuzzy material, which may fill the entire lumen of the tubulus. This fuzzy material represents the remains of the brush border of the cells of the proximal tubules, which is difficult to preserve during the
preparation of the tissue.
Draw a glomerulus and label its components (anatomical glomerulus, parietal blade of Bowman's capsule, podocyte nuclei (fairly large and light), mesangial cell nuclei (smaller and darker), vascular pole. Include a proximal tubulus and a distal tubulus in your drawing.
The tubular system can be divided into proximal and distal tubules, which in turn have convoluted and straight portions. Thin tubules connect the proximal and distal tubules. Running from the cortex of the kidney towards the medulla (descending), then turning and running back towards the cortex (ascending), the tubules form the loop of Henle.
The proximal tubule is the longest section of the nephron (about 14 mm). The convoluted part of the proximal tubules coils close to the glomerulus in the cortex of the kidney. The external diameter of proximal tubules is ~65 µm. Their walls are formed by a low columnar epithelium. The eosinophilic cells of the epithelium have a wide brush border (long microvilli - What is their function?) and are active in endocytosis. They almost completely resorb substances of nutritional value from the glomerular filtrate (glucose, amino acids, protein,
vitamins etc. - Which organelles would you expect to be numerous in proximal tubule cells?). In the proximal tubules the volume of the glomerular filtrate is reduced by about 75%. Sodium ions are actively resorbed from the glomerular filtrate. Sodium ions are followed by passively diffusing chloride ions and the osmotic absorption of water. The straight portion of the proximal tubule descends towards the medulla of the kidney.
The straight portion of the proximal tubule merges with the thin segment of the loop of Henle. A flattened epithelium (only about 1-2 µm high) forms the thin tubule, which is only about 15 µm wide. Descending parts of the straight proximal and thin tubules are permeable to water but not to solutes.
The thin segment of Henle's loop leads into the straight part of the distal tubule, which is formed by low cuboidal cells without a brush border. A few short microvilli are present, but they may be difficult to distinguish in the light microscope. The diameter of the tubule gradually expands to about 35 µm. The prime function of the epithelial cells in the ascending parts of the thin and straight distal tubules cells is the transport of chloride (active) and sodium ions (passive) out of the tubular lumen into the surrounding peritubular space. The epithelium can not be penetrated by water. Consequently, the transport of ions over the
epithelium sets up a gradient in osmotic pressure, which serves as driving force in the further concentration of the urine.
The straight portion of the distal tubule contacts the glomerulus forming the macula densa. Thereafter, the distal tubule forms its convoluted portion (about 5 mm long). Cells in the distal tubulus are sensitive to the hormone aldosterone, which is produced in the zona glomerulosa of the adrenal glands. Aldosterone stimulates the active resorption of sodium ions and the excretion of potassium ions. The convoluted distal tubule merges with the collecting ducts of the kidney. In the presence of antidiuretic hormone (ADH), the epithelia of the collecting ducts are permeable to
water but not to sodium ions. Osmotic forces move water out of the lumen of the tubules as they pass through the medulla of the kidney, where cells of the ascending thin and straight distal tubules of the loop of Henle have established high concentrations of sodium in the extracellular space.
Collecting ducts merge to form papillary ducts (of Bellini), which terminate on the tips of the renal papilla and empty into a distended, funnel shaped part (minor calyx) of the ureter.
Lab: Slide KIDNEY T.S. H&E. Find a good spot in the medulla of the kidney. What is good? Preferably a spot in which you are able to identify a collecting ducts (columnar cells, well-defined boundaries between cells, cytoplasm only weakly stained or unstained, large ducts) and a thin (very flat epithelium, nuclei bulge into the lumen of the tubulus, diameter of the duct is small) and distal tubule (cuboidal epithelium, cells stain weakly pink). Both transversely or longitudinally cut tubules are suitable. Note that it will be almost impossible to identify ALL tubules that are
visible.
In most of our sections only little medulla is present - try to scan along the margins of the tissue and see if you can find some medulla. If that should not be possible take a look at medullary rays instead, although they will contain few, if any, good thin tubules.
Draw the tubules you could identify and label them. Also include some tubules which could not be identified in your drawing, which will give you a better impression of how the medulla of the kidney looks than just three circles on a white background.
As mentioned above, the distal tubule contacts the glomerulus forming a specialized section of tubular epithelium, the macula densa. At the point of contact with the glomerulus, the distal tubule is always in close contact with the endothelial cells of the efferent and afferent arterioles of the glomerulus.
Other parts of the juxtaglomerular apparatus are extraglomerular mesangial cells (or lacis cell) and the juxtaglomerular cells surrounding the afferent arteriole (modified smooth muscle cells), which produce and secrete renin. Renin activates angiotensinogen, a precursor found in the bloodstream, leading to the formation of angiotensin I, which is converted to angiotensin II. Angiotensin II is the most potent vasoconstrictor known. It also stimulates the
secretion of aldosterone.
Different theories exist that try to explain the interactions between the cells that eventually lead to the release of renin. One of them, the baroreceptor theory, assumes that the juxtaglomerular cells function as stretch receptors (high blood pressure would inhibit the release of renin). Another theory, the macula densa theory, claims that the secretion of renin is regulated by the composition of the fluid in the distal tubule and/or the afferent arteriole (low sodium would increase in the release of renin).
The minor calyces merge to form major calyces within the kidney, which in turn merge to form the renal pelvis (still within the kidney). The urine flows through these structures to the ureter and is channelled to the bladder.
The basic structure of all these components is the same. The mucosa is lined with a transitional epithelium, which occurs exclusively in the urinary system. The epithelium is virtually impenetrable to any components of the urine, which consequently does not change in composition as it passes through the excretory passages. The lamina propria consists mainly of dense connective tissue, with many bundles of coarse collagenous fibres. The muscularis usually consists of an inner longitudinal and outer circular layer of smooth muscle cells. In lower parts of the ureter and the bladder an additional outer longitudinal
layer of muscles is added to the first two.
The bladder is finally emptied through the urethra. Initially, the urethra
is lined by a transitional epithelium in males and females. In males, it is
replaced by a preudostratified or stratified columnar epithelium below the
openings of the ejaculatory ducts into the urethra. The distal parts of the
female urethra and the distal end of the male urethra are lined by a stratified
squamous epithelium. The lamina propria contains loose connective tissue.
Smooth muscle cells in the muscularis are mainly oriented longitudinally.
They are surrounded, in the middle part of the urethra (below
the prostate in males), by striated muscle
cells of the sphincter urethrae.
Lab: Slide Ureter T.S. H&E and BLADDER H&E.You should focus your attention first on the epithelium and, second, on the general appearance of the musculature in the walls of the ureter and bladder. Find the place in your tissue (either bladder or ureter) in which the transitional epithelium has the most textbook-like appearance. Although the precise orientation of the muscles in the wall of the bladder depends on where the tissue block was taken and at which
angle it was sectioned, it should be possible to identify three tiers of muscle bundles at least in some parts of the bladder wall. Note also that the epithelium is smooth, without appreciable crypts or folds.
Draw these features of the muscular wall and epithelium. You may want to include a similar low-magnification drawing of the ureter.
