Histology of urinary system. Urinary System consists of two kidneys, two ureters, a bladder, and a urethra

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1 Histology of urinary system Urinary System consists of two kidneys, two ureters, a bladder, and a urethra 1- kidney: Hilus contains renal artery, renal vein, and renal pelvis surrounded by renal sinus Darker outer region of kidney is cortex; lighter inner region is medulla Medulla contains numerous pyramids, which face the cortex at corticomedullary junction Round apex of each pyramid extends toward renal pelvis as renal papilla Cortex that extends on each side of renal pyramid constitutes the renal columns Each papilla is surrounded by a minor calyx that joins to form a major calyx. 1

2 Major calyces join to form the funnel-shaped renal pelvis that narrows into the muscular ureter Urine is formed as a result of blood filtration, and absorption from and excretion into the filtrate, almost all filtrate is reabsorbed into the systemic circulation and about 1% of filtrate is voided as urine Produces renin that regulates filtration pressure and erythropoietin for erythrocyte production Uriniferous Tubules and Nephrons Functional unit of kidney is uriniferous tubule, consists of nephron and collecting duct. Two types of nephrons: cortical nephrons (in cortex) and juxtamedullary nephrons (near the junction of the cortex and medulla of the kidney). Nephron is subdivided into renal corpuscle and renal tubules. o Renal Corpuscle Consists of fenestrated capillaries called glomerulus and double-layered glomerular (Bowman s) capsule (parietal layer, outside and visceral layer, inside). Parietal layer is lined by simple squamous epithelium of the glomerular capsule. Visceral layer of capsule contains podocytes that surround glomerular capillaries. Podocytes exhibit primary processes and pedicles that form filtration slits around capillaries Between parietal and visceral layers is the capsular (urinary) space that holds glomerular filtrate Blood is filtered in the glomerular capillaries of the corpuscle to form ultrafiltrate. At vascular pole, afferent and efferent arterioles enter and exit the renal corpuscle At opposite urinary pole, ultrafiltrate enters the proximal convoluted tubule 2

3 o Mesangial cells In addition to podocytes that surround the capillaries, there are other specialized cells in the glomerulus called mesangial cells. These cells are also attached to the capillaries and perform several important functions. Mesangial cells synthesize the extracellular matrix and function as macrophages in the intraglomerular regions and phagocytose antigen antibody complexes and the material that accumulate on the glomerular filter, thus preventing its clogging with filtered matter and keeping the glomerular filter free of debris. Some of the mesangial cells are also located outside the renal corpuscle in the vascular pole region, between the afferent and efferent arterioles. Here, they are called the extraglomerular mesangial cells, also called lacis cells, and form part of the juxtaglomerular apparatus. o juxtaglomerular apparatus. This apparatus consists of three components: the juxtaglomerular cells, the macula densa, and the extraglomerular mesangial cells (or lacis cells). Juxtaglomerular cells are a group of modified smooth muscle cells located in the wall of the afferent arteriole of the vascular pole of the renal corpuscle before it penetrates the glomerular capsule to form the glomerulus. The cytoplasm of these cells contains membrane-bound secretory granules of the enzyme renin, which is synthesized, stored, and released into the blood stream when needed. 3

4 Opposite the afferent arteriole is the macula densa, a group of modifi ed distal convoluted tubule cells that form a dense cluster. The macula densa cells and juxtaglomerular cells are in close proximity to each other and are separated only by a thin basement membrane. This proximity of juxtaglomerular cells to the macula densa allows for integration of their functions. o Renal Tubules Glomerular filtrate leaves renal corpuscle and enters renal tubules that extend to collecting ducts. Initial tubule is the proximal convoluted tubule that starts at the urinary pole of renal corpuscle. Loop of Henle consists of thick descending, a thin loop, and thick ascending tubules. Distal convoluted tubule ascends into kidney cortex and joins the collecting tubule. Juxtamedullary nephrons (a) have very long loops of Henle. Note: the collecting tubules (collecting ducts) are not part of nephron, but join larger collecting ducts to form papillary ducts Deep in medulla, papillary ducts are lined by columnar epithelium and exit in area cribrosa Medullary rays in cortex are collecting ducts, blood vessels, and straight portions of nephrons. Renal Blood Supply To understand the functional correlation of the kidney, it becomes important to understand the blood supply of the organ. Each kidney is supplied by a renal artery that divides in the hilus into several segmental branches, which branch into several interlobar arteries. The interlobar arteries continue in the kidney between the pyramids toward the cortex. At the corticomedullary junction, the interlobar arteries branch into arcuate arteries, which arch over the base of the pyramids and give rise to interlobular arteries. These branch further 4

5 into the afferent arterioles, which give rise to the capillaries in the glomeruli of renal corpuscles. Efferent arterioles leave the renal corpuscles and form a complex peritubular capillary network around the tubules in the cortex and long, straight capillary vessels or vasa recta in the medulla that loops back to the corticomedullary region. The vasa recta forms loops that are parallel to the loops of Henle. The interstitium is drained by interlobular veins that continue toward the arcuate veins. Blood supply to the kidney. 5

6 Renal corpuscles. (a): The renal corpuscle is a small mass of capillaries called the glomerulus housed within a bulbous glomerular capsule. The internal lining of the capsule is composed of complex epithelial cells called podocytes, which cover each capillary, forming filtration slits between interdigitating processes called pedicels. Blood enters and leaves the glomerulus through the afferent and efferent arterioles respectively. (b): The micrograph shows the major histological features of a renal corpuscle. H&E. X300. (c): Filtrate is produced in the corpuscle when blood plasma is forced under pressure across the filtration membrane of the glomerular capillary wall and through the filtration slits between the pedicels of podocyte processes. (d): The SEM shows the distinctive appearance of podocytes and their processes covering glomerular capillaries. X800 6

7 2- Ureter Lined by transitional epithelium and consists of mucosa, muscularis, and adventitia Upper part lined by inner longitudinal and middle circular smooth muscle layers Third longitudinal smooth muscle layer added in the lower third of ureter Connective tissue adventitia surrounds the ureter 3- Bladder Thick muscular wall with three indistinct layers of smooth muscle Serosa lines superior surface and adventitia covers the inferior surface Transitional epithelium in empty bladder exhibits about six layers of cells When stretched, transitional epithelium appears stratified squamous Changes in epithelium caused by thicker plasma membrane of superficial cells and plaques Plaques act like hinges, allow cell to expand during stretching; cells become squamous Thicker plasma membrane and transitional epithelium provide osmotic barrier to urine 7