Page 290 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

272 Veterinary Histology of Domestic Mammals and Birds

into the peri-tubular tissue where it is taken up by medul- epithelium of the collecting duct transforms into a double
VetBooks.ir lary vessels (refer to physiology textbooks for more detail). layer of columnar epithelium in the papillary duct (diame-
ter 200–300 μm). Fat droplets may be seen in the collecting
Species variation
ducts of older animals and goblet cells may be present in
Birds: The loops of Henle and associated counter-cur- horses. At the papilla, the epithelium consists of two layers
rent mechanism are less extensively developed in birds, (three in the pig).
thus the capacity to concentrate urine is lower than The final stages of urine concentration take place
in mammals. in the collecting duct system. The collecting ducts are
surrounded by the hypertonic environment of the renal
Distal straight tubule and distal convoluted medulla. Water follows the osmotic gradient into the inter-
tubule stitium. Only small quantities of urea and electrolytes pass
The ascending thin limb continues as the distal straight into the tubular cells. Recent research has shown that the
tubule, also referred to as the thick ascending limb of the collecting ducts are the main target of aldosterone in regu-
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loop of Henle (Figures 12.8 and 12.18). The convoluted lating transport of Na and water.
portion is located near its associated glomerulus. Both the Antidiuretic hormone (ADH), secreted by the poste-
straight and convoluted portions have a wide lumen (30–45 rior pituitary gland, increases permeability to water. Lack
μm). The epithelium is somewhat lower than that of the of ADH or epithelial ADH receptors results in excretion of
proximal tubule. Instead of a brush border, the apical surface large volumes of relatively dilute urine (diabetes insipidus).
bears a few irregularly spaced, small microvilli. The basal
labyrinth is particularly prominent, with numerous mito- Juxtaglomerular apparatus (complexus
chondria occupying the infoldings of the plasmalemma. juxtaglomerularis)
Na ions (via basally located ion pumps) are transported The juxtaglomerular apparatus (Figure 12.12) consists of:
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into the interstitium in exchange for K , H or NH . The dis-
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tal tubule thus plays an important role in maintaining water · the macula densa,
and electrolyte balance. These mechanisms are hormonally · juxtaglomerular cells and
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regulated. Aldosterone influences epithelial Na and K · extraglomerular mesangial cells (Goormaghtigh
transport (see also below). Antidiuretic hormone increases cells, lacis cells).
the permeability of the epithelium to water. Calcitonin
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raises, and parathyroid hormone lowers, excretion of Ca2 . Located at the vascular pole of the renal corpuscle, the
juxtaglomerular apparatus regulates blood pressure and
Collecting duct system urinary sodium concentration by modifying glomerular
The collecting duct system (Figure 12.19) comprises the: filtration rate and sodium reabsorption (renin–angioten-
sin system).
· arched collecting tubule,
· straight collecting tubule (collecting duct) and Macula densa
· papillary duct. The macula densa is a region of up to 40 modified epithe-
lial cells located in the portion of the distal straight tubule
The collecting duct system begins with the short, arched that lies adjacent to the vascular pole of the glomerulus,
collecting tubule, which opens into the straight collecting between the afferent and efferent arterioles and the extra-
tubule or collecting duct (Figure 12.6). Up to ten nephrons glomerular mesangial cells (Figure 12.10). Beyond this
drain into each collecting duct. In the cortex, the collect- point, the distal tubule becomes convoluted.
ing ducts are grouped together in the medullary rays. The epithelial cells are columnar with dense cytoplasm.
They continue to run parallel to one another within the Short microvilli are present on the cell surface. Basally
medulla. In the inner zone of the medulla, the collecting there is a relatively poorly developed basal labyrinth and
ducts merge to form the papillary ducts, which empty a continuous basal lamina. The macula densa acts as a
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by a series of openings at the tip of the renal papilla/crest chemoreceptor, detecting the Na ion concentration
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(area cribrosa) into the renal calyces/pelvis (Figure 12.3). in the tubular fluid. A fall in tubular Na triggers release
In contrast to the nephron, the collecting duct appara- of renin by the juxtaglomerular cells. This activates the
tus constitutes a branched system of tubules in which the angiotensin system which brings about increased Na reab-
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epithelial lining changes gradually from the cortex to the sorption, with water following by osmosis.
papilla. The epithelial cells are predominantly cuboidal in
the cortex and increase in height towards the papilla. The Juxtaglomerular cells
cells have a centrally located nucleus, pale cytoplasm and Juxtaglomerular cells are modified smooth muscle cells
clear cell boundaries. The simple cuboidal to columnar found in the afferent arteriole, near its entry into the

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