School of Anatomy and Human Biology - The University of Western Australia
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School of Anatomy and Human Biology - The University of Western Australia |
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| Blue Histology - Urinary System |
| Topics |
Lab Guides and Images |
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| Kidney - Cortex - H&E, methenamine silver |
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| Kidney - Medulla - H&E |
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| Ureter and Bladder - H&E |
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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
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:
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 cells. Modified fibrocytes of the medulla secrete prostaglandins which are able to decrease blood pressure.
Glomeruli and the tubular system are both part of the basic functional unit of the kidney, the nephron.
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.
Kidney - 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: the anatomical
glomerulus, the parietal blade of Bowman's capsule (squamous cells), podocytes
(fairly large and light nuclei ), endothelial cells (smaller and darker nuclei),
vascular pole. Include a proximal tubulus and a distal tubulus in your drawing.
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The tubular system can be divided into proximal and distal tubules, which in turn have convoluted and straight portions. Intermediate 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. The 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. They are followed by passively diffusing chloride ions and the osmotic absorption of water. The straight portion of the proximal tubule descends towards the medulla. The straight portion of the proximal tubule merges with the intermediate tubule (thin segment of the loop of Henle). A flattened, only ~1-2 µm high epithelium forms the intermediate tubule, which is only ~15 µm wide. Descending parts of the straight proximal and intermediate tubules are permeable to water but not to solutes. |
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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 are difficult to see in the light microscope. The diameter of the tubule expands to ~35 µm. Epithelial cells in the ascending parts of the intermediate and straight distal tubules cells transport 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, via connecting tubules, with the collecting ducts. 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, where cells of the ascending intermediate 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.
Kidney, human - 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 (cuboidal to columnar
cells, well-defined boundaries between cells, cytoplasm only weakly stained
or unstained, large ducts) and an intermediate (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 difficult
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.
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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 efferent and afferent arterioles of the glomerulus.
Other parts of the juxtaglomerular apparatus are extraglomerular mesangial cells 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 pseudostratified 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.
Ureter, primate - H&E and Bladder,
human - H&E |
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page content and construction: Lutz Slomianka
last updated:
6/08/09
