الفهرس 
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Abstract
This study was carried on seventy rabbit’s fetuses collected from uteri of New Zealand white rabbits at embryonic days (E) E10, 12, 14, 16, 18, 20, 23, 25, and 30 days for the morphological, histological, histochemical, SEM, semithin sections and TEM investigations to explain the developmental changes of rabbit kidneys during their prenatal life.
The rabbit pronephros consisted of tubule-like structures, which were believed to be important in the pronephric-mesonephric duct formation. The pronephric duct observed at E10, canalized and extended caudally as mesonephric duct to initiate the formation of the mesonephros.
The mesonephros of rabbit was large, long and located in the thoracolumbar region. At E10, the mesonephros was observed and reached to complete developing at E14. At E10, the mesonephric duct was presented laterally and the mesonephric tubules were located medially. These tubules showed different stages of development.
The mesonephric tubules at E12 were differentiated into proximal secretory part and distal collecting part. Morphologically, the proximal tubules were larger and longer than the distal ones. The proximal tubules were lined by low columnar epithelium with basely located nucleus, while the distal tubules were lined by cuboidal epithelium with centrally located nucleus. According to the position, the proximal tubules were medially located and connected to the mesonephric corpuscles, while the distal tubules were laterally located and connected to the mesonephric duct.
The mesonephric corpuscles were observed at E12. They were formed by the medial end of the proximal tubules, which were ambulated and invaginated by mesenchymal mass to form the Bowman’s capsule. The mesenchymal mass developed into arterioles, which formed the glomerulus.
At E14, the mesonephros reached its maximum size and differentiation. The glomerulus became highly vascular and the podocyte developed at the visceral layer of the Bowman’s capsule. At E16, the right mesonephros was mechanically migrated and slightly located cranial to the left one. The progressive degeneration began at E16 and disappeared at E23.
Concerning the vasculature of the mesonephros, the aorta gave on either side large renal arteries, which enter the medial aspect of the mesonephros and terminated by afferent arterioles of the glomerulus. Also, the posterior cardinal vein was branched into a network of sinusoids in the tubules. These latter channels were continued in turn with the subcardinal veins. As the result of the presence of branches of both veins, the renal portal circulation was formed.
At E14, the two metanephroi were located at the pelvic inlet at the same level, while at E16, the right metanephros was located somewhat cranial and dorsal to the left one. Finally, at E20, the two metanephroi reached their permanent position. The two metanephroi were separated from each other by the ventral extension of the sublumbar muscles. Therefore, the wide space was occupied dorsally by the sublumbar muscles, ventrally by the intestine in addition to the aorta, caudal vena cava and ureter in the middle. The position of the hilus depended upon the degree of rotation of the kidney along its longitudinal axis. At E14, the ureter entered the ventral aspect of the metanephros. While at E16, the hilus of the metanephros located on the middle of the medial border and at E18, the hilus became dorsomedially. The structures that passed within the hilus were the renal blood vessels and ureter. The ureter was directed ventrally toward the urinary bladder.
The metanephros originated from the ureteric bud and the nephrogenic tissue. At E14, the ureteric bud originated from the mesonephric duct before its entrance into the urogenital sinus. Then directed craniodorsally within the pelvic cavity to be capped by the nephrogenic tissue. At E16, the metanephros differentiated into outer dark cortex and inner pale medulla. The cortex contained superficially the nephrogenic tissue followed by solid spherules, vesicles and renal tubules. The tubules had attached and free ends. The attached end connected with the terminal collecting tubule and the free end became ambulated to form the Bowman’s capsule, which with the mesenchymal mass formed the primitive metanephric corpuscle. The medulla contained the stems of the collecting tubules.
The different parts of the nephron became differentiated with different stages of development at E18. The metanephric corpuscle consisted of Bowman’s capsule and glomerulus. The Bowman’s capsule consisted of outer parietal layer, inner visceral layer and were separated from each other by Bowman’s space. The development of the glomerulus showed different stages of glomerulogenisis. The metanephric corpuscles reached their adult form and structure at E20 and the growth of these corpuscles was correlated with the age of the embryo. While, these corpuscles at the cortico-medullary junction more developed and differentiated than the newly formed ones, which were located peripherally. The juxtaglomerular apparatus was completed at E25 and composed of the macula densa, extraglomerular mesangial cells and the granular juxtaglomerular cells of the afferent arteriole
The renal tubules differentiated into proximal and distal convoluted tubules at E18. The proximal tubules were larger, darker and numerous than the distal ones. The proximal convoluted tubules were lined by cuboidal epithelium, while the distal convoluted tubules lined by low columnar epithelium. At E20 and E23, the tubules became convoluted and formed with the metanephric corpuscles, the convoluted part of the cortex, which alternated with the radiated medullary part of the cortex. At E25 and E30, the convoluted tubules with adjacent renal corpuscles extended between the renal pyramids to form the renal columns of the cortex. Consequently, the cortex reached its adult form concerning its parts, where it consisted of convoluted part, radiated part and renal columns.
The juxtamedullary nephrons had long loop of Henle, which extended in the medulla and surrounded the collecting tubules. The loop of Henle consisted of descending thin and ascending thick loops. The stems of the collecting tubules at E18 were located in the medulla and its terminal tubules extended within the cortex to form the primitive medullary rays. At E20, the collecting tubules were lined by simple columnar epithelial cells, while at E23, the collecting tubules were lined by two different cell types; the principle columnar cells and intercalated cells. The intercalated cells were elongated cells with apical microprojections.
The proximal convoluted tubules reacted positively with PAS, bromophenol blue, acid fuchsin of Crossmon’s trichrome and silver stains in addition to numerous lipid droplets were recorded in the tubules. The distal convoluted tubules reacted positively with Alcian blue, bromophenol blue, acid fuchsin of Crossmon’s trichrome and silver stains but in less degree than the proximal tubules. While, the collecting tubules gave positive reaction with these stains. The intensity of these reactions increased with increase in fetal age.
The kidney of rabbit was unipapillary. Although at E23, the kidney appeared multipapillary due to each group of the collecting tubules in the medulla converged toward the renal pelvis to form a papilla. While at E25, the renal papillae fused to form a common renal papilla, occupied most of the renal pelvis and separated from it by a narrow crescentic recess. The blood supply of rabbit kidney began by the renal artery that entered the kidney through the hilus and subdivided into three or four branches. These branches gave off interlobar arteries, which subdivided the kidney into renal lobes. At the corticomedullary junction, the interlobar arteries gave off the arcuate arteries to be extended transversally between the cortical and medullary parts of each lobe. The arcuate arteries gave off the interlobular arteries, which subdivided the cortex into lobules. On either side, the interlobular artery gave off the afferent arterioles, which shared in the formation of the glomerulus of the renal corpuscle. Finally, the interlobular arteries terminated superficially to form the subcapsular plexus.
The results of SEM showed that the surface aspect of renal glomeruli was covered by irregular flat polyhedral cells, which demarcated by short marginal microvilli and long solitary cilium protruded from the center of every cell. Then the cell bodies of podocytes were observed at E20, which were connected with each other by distinct interdigitations and the surface view of podocytes revealed labyrinth-like appearance of cytoplasmic processes of these podocytes. The consecutive branching of a podocyte cytoplasm resulted in four generation of processes. SEM of proximal convoluted tubules revealed that they had narrow lumen and two types of cells; the columnar and vacuolated cells. Moreover, the apical border showed brush border. At the basal border, the basal ridges beard a basal striation. Then at the oldest ages, the proximal tubules became well-developed with occluded lumen by the vacuolated material. The lateral aspect of the adjacent cells was interdigitated with each other and every cell beard well-developed luminal brush border.
The results of TEM showed that the immature renal corpuscle composed of Bowman’s capsule invaginated by mesenchymal cells. The podocytes of the visceral layer of Bowman’s capsule composed of cell body and few foot processes. These cell bodies of the podocytes connected with each other by tight junction. Then, the junctions between the podocytes disappeared in the growing ages. The podocytes rested on the glomerular basement membranes and formed slit membrane between contiguous foot processes. The filtration barrier was consisted of fenestrated endothelium of the arterioles, glomerulus basement membrane and filtration slits. The mesangial cells, which derived from the mesenchymal cells accompanied the primitive glomerular capillary were initially poorly differentiated. Then the mature mesangial cells appeared as highly irregular polyhedral cells with numerous cytoplasmic projections.
The proximal convoluted tubules consisted of columnar epithelial cells. The apical border of these cells joined together by tight junction and beard short microvilli. The basal border beard basal interdigitation. Then at the older ages, the apical border occupied by dense microvilli. The basal striation occupied by mitochondria. The cytoplasm contained many profiles of rER, ribosomes and abundant lipid droplets. Adjacent cells joined by tight junction and desmosomes. The basal half of the cells of distal convoluted tubules occupied by abundant elongated mitochondria, while the apical border was provided by short microvilli. Then the basal border of the oldest ages contained numerous elongated mitochondria situated between the basal striation.
The collecting tubules composed of low columnar cells. The apical border beard very short microvilli. These cells connected together by tight junction and desmosomes. Then at older ages, the collecting tubules composed of two types of cells; light principle and dark intercalated cells. The cytoplasm of principle cells was poor in organelles, while its of the intercalated cells contained more numerous mitochondria, apical microvilli, ribosomes and rER. The renal interstitial tissue contained the fibroblasts mainly, macrophages and telocytes. The fibroblast was irregular cell body with many cell processes and the cytoplasm was poor in organelles. The telocytes were spindle cells with two long telopodes. The telopodes composed of dilated thick podoms and fibrillar-like thin podomeres. The processes of telocyte connected with the processes of fibroblast and macrophage to form a network within the interstitium.