Urinary Tract - Kidney
Ureter, with typically stellate lumen. The transitional epithelium rests on a blue c.t. lamina propria. There is no boundary between lamina propria and the deeper c.t. submucosa. The muscularis externa stains a grayer blue in this slide; there are inner lonlitudinal and outer circular muscle layers visible here. On the outside is the blue c.t. adventitia with quite a bit of fat. (Mallory stain)
Ureter stained with H&E. Most layers of this wall are not well fixed, but the transitional epithelium and the stellate lumen are characteristic.
Transitional epithelium -- diagnostic for urinary tract. The surface cells are characteristically dome-shaped and puffy.
Transitional epithelium from a distended urinary bladder; compare with the contracted wall in the preceding slide. When stretched, the epithelium becomes very thin, with fewer layers of cells, and the surface cells tend to be flattened. There's only the one layer of flatter cells, however, which is quite different from the appearance of stratifi ed squamous epithelium (with which this might be confused.) See next slide.
Stratified squamous epithelium (non-keratinized) for comparison. Note that there are many layers of flattened cells, and, in fact, the whole epithelium is quite thick.
Anatomy of kidney. In the lower picture, notice positions of cortex, medullary pyramids, calyces, pelvis, blood vessels. Beginning with the minor calyces, and continuing on through the ureter, the lining is transitional epithelium.
Vascular arrangement. Follow renal artery to interlobar arteries to arcuate arteries to interlobular arteries. A kidney lobule lies between two interlobular arteries.
Papilla (right) projecting into calyx (left) lined with transitional epithelium.
Nephrons emptying into a collecting tubule. Notice that the closer to the medulla a glomerulus lies, the longer the loop of Henle is. Also notice that the inner zone of the medulla (lowest section of the picture) contains only thin limbs of the loop of Henle, plus collecting ducts. This is the area where the counter-current mechanism for urine concentration (carried out between the tubules and the surrounding peritubular capillaries) is most active. To make his drawing clear, the artist has made one accommodation that is not quite accurate histologically; namely, within the cortex, the straight positions of the nephrons (that is, the thick and thin portions of the loops of Henle) shoul d lie immediately next to the collecting duct, thus making up the medullary ray. The glomeruli and convoluted portions of the nephrons would then lie on either side of the ray. The ray is the central axis of a lobule.
Cortex of kidney -- alternating straight rays and convoluted portions (with glomeruli). One longitudinally cut interlobular artery (pale pink contents) can be seen near the extreme left border of the picture, running up the middle of a convoluted portion, among glomeruli.
Bar = the width of a straight (medullary) ray which constitutes the center of a lobule
Renal corpuscle with connection to proximal tubule at lower border. This, then, would be a cut through the urinary pole of the corpuscle.
Detail of renal corpuscle. Dark pink epithelium = proximal tubule. Lighter pink (as at upper top left) = distal tubule.
Dark pink = proximal tubule. Lighter, low cuboidal epithelium (as at top left) = distal tubule.
Proximal and distal convoluted tubules (EM). Distal has no brush border. Peritubular capillaries lie in the connective issue between tubules.
Higher EM of proximal tubule with its brush border (arrow), which indicates absorption by the cell.
EM of base of epithelium of proximal convoluted tubule. Note basement (basal) lamina and the great infolding of the cell membrane. These folds, plus the many mitochondria lying in them, tend to give the cytoplasm a striated look in light microscopy. The many folds also provide increased cell surface for passage of absorbed fluid and ions into t he peritubular capillary below.
Thin segment of Loop of Henle (in the middle), with a simple squamous lining.
Medullary region. There are some longitudinal cuts of pale collecting tubules (at left center) and several blood vessels filled with pale pink fluid (to the right). Epithelium of collecting tubules is regular, block-like, simple cuboidal, with unusually clear cell walls. Other tubules in the field are thick and thin limbs of loops of Henle.
Cross cuts in medulla. Two pale collecting tubules in the middle. Simple squamous lining indicates thin loops of Henle. Compare these with blood vessels, which contain r.b.c.'s. (Look for vessels up near top center and to right; also in lower left quadrant of field.) Notice the blue c.t. stroma in between the tubules.
Large pale tubules are collecting tubules, with clear epithelial cell boundaries. Brighter pink tubules are thick portions of loops of Henle; these are basically like distal convoluted tubules in their histology, so would be ascending limbs.
Renal corpuscle in Mallory stain to show bright orange r.b.c.'s in both the glomerular capillaries and the peritubular capillaries (among the convoluted tubules). Notice also the clear simple squamous epithelium of the parietal layer of Bowman's capsule outlining the whole renal corpuscle. The nuclei seen in the glomerulus could belong to endoth elium, podocytes, or mesangial cells.
Long thin artery leading to glomerulus (look in lower mid-picture). (Note long, thin endothelial nuclei lining the lumen. Circular muscle fibers have been cross-cut and look almost like a simple cuboidal epithelium outside the endothelium.)
Detail of wall of renal corpuscle. The space is the lumen of Bowman's capsule that receives glomerular filtrate from the capillary loops. Left wall is simple squamous parietal lining. The visceral lining of podocytes on the right wall of the space is too irregular to be seen clearly in light microscopy because it is following the curves of the individual capillaries.
EM of triangular shaped podocyte with its many terminal end feet (foot processes) touching the basement membrane (dark) which is shared on its other surface by endothelium of a capillary.
Detail of end feet of podocyte on the basement membrane. The basement membrane (basal lamina) is continuous, but the fenestrated capillary endothelium has pores. Glomerular filtrate passes from the capillary lumen, through the layers seen here, into the lumen of Bowman's capsule (where the foot processes are lying). Between the foot processes a re thin slit membranes.
EM of different capillary endothelia for further comparisons with other capillary walls.
- A) continuous, endothelium in capillary.
- B) fenestrated endothelium
- C) continuous, thin endothelium of lung capillary.
Note: basal lamina (1) under each one. Also pinocytotic vesicles (PV) and fenestrations (arrows).
Diagram of capillary loops in glomerulus and location of point of contact with distal convoluted tubule at base of loops. A histological section would show a closely packed portion of epithelial cells lining the distal tubule - the macula densa. Juxtaglomerular cells (J-G) would lie nearby in the wall of the afferent arteriole. The wedge-shaped space just above the distal tubule is called the polar cushion and would be filled with mesangial cells which are sometimes called Polkissen or Lacis cells in this location. Other mesangial cells extend up among the capillary loops of the glomerulus. By definition, the term.juxtaglomerular apparatus includes the J-G cells, macula densa, and pol ar cushion.
Diagram (top) of relation of juxtaglomerular (JG) cells to macula densa. Both areas sense the concentration of urine being produced, and by their action can alter it.
At lower right pole of glomerulus, note a triangular wedge of Polkissen cells just to the left of the straight row of macula densa cells. The latter are part of the epithelial wall of the distal tubule.
EM photo showing dark particles, which are secretory granules in the cytoplasm of juxtaglomerular cells. These cells are modified smooth muscle cells and secrete the hormone renin.
This slide, and the two that follow, show kidney tissue that has been imbedded in methacrylate and is unusually well preserved. This one shows the vascular pole of a renal corpuscle. The blood vessel is presumably an efferent arteriole because no "cuboidal" J-G cells are seen in its wall. (Note the biconcave shape of the rbc's in the vessel.) N otice also the clear parietal layer of Bowman's capsule. Surrounding the renal corpuscle are several cuts of the proximal tubule, with clear brush borders. There is one small distal tubule at bottom center of the field, slightly to the left.
This renal corpuscle has been sectioned through the urinary pole where the proximal tubule is continuous with the urinary space containing the urinary filtrate. Notice two cuts of distal tubule, one at left top edge of field, and one near right bottom edge.
A field of convoluted tubules, some with striated (brush) borders and some without. Notice that their cytoplasm often looks darkly striped or striated, from base to apex of the cells; this is because of all the mitochondria lined up between the basal infoldings of cell membrane. (Remember the EM you saw earlier in this set?) Notice also the small peritubular lying between the proximal and distal tubules; these branched off the efferent arteriole after it left the glomerulus; they will drain eventually into an interlobular vein.
