Histology of the Urinary system Lufukuja. G1 The urinary system The urinary system, also known as the renal system, consists of the kidneys, ureters, urinary bladder, and the urethra. Each kidney consists of millions of functional units called nephrons. The purpose of the renal system is to eliminate wastes from the body, regulate blood volume and blood pressure, control levels of electrolytes and metabolites, and regulate blood pH. Lufukuja. G2 Kidneys The kidneys are enclosed in a fibro-connective tissue capsule which may be striped easily from the underlying parenchyma, an indication that no septa are present. FCT, is tissue made up of high-strength, slightly stretchy fibers. These fibers consist mainly of collagen, water, and complex strands of carbohydrates called polysaccharides.carbohydrates 3Lufukuja. G STRUCTURE OF KIDNEYS In longitudinal section, the kidney is divided into two zones; an outer cortex and an inner medulla. The cortex lies superficial, between renal capsule and the base of renal pyramids. The cortex extends into the renal medulla between adjacent pyramids as the renal column. 3/2/20164 RENAL CORTEX Each kidney is composed of millions of nephrons. Each nephron consists of a dilated portion, the renal corpuscle; the proximal convoluted tubule; the thin and thick limbs of Henle's loop; the distal convoluted tubule; and the collecting tubules and ducts. 5Lufukuja. G 6 RENAL CORTEX Renal Corpuscles & Blood Filtration Each renal corpuscle consists of a tuft of capillaries, the glomerulus, surrounded by a double-walled epithelial capsule called glomerular (Bowman's) capsule. The internal layer (the visceral layer) of the capsule envelops the capillaries of the glomerulus. The external layer forms the outer limit of the renal corpuscle and is called the parietal layer of Bowman's capsule. Lufukuja. G7 Renal Corpusclescont Between the two layers of Bowman's capsule is the urinary space, which receives the fluid filtered through the capillary wall and the visceral layer. Each renal corpuscle has a vascular pole, where the afferent arteriole enters and the efferent arteriole leaves, and a urinary pole, where the proximal convoluted tubule begins. After entering the renal corpuscle, the afferent arteriole usually divides into two to five primary branches, each subdividing into capillaries and forming the renal glomerulus. Lufukuja. G8 BOWMANS CAPSULE Cells of the visceral layer are known as podocytes (flattened satellite cells). The cells of the visceral layer podocytes (podos, foot + -cyte, cell) are connected by gap junction and have many process which are known as foot-processes. The process interdigitate with each other, and between these processes there is narrow intercellular slits or gaps covered by slit membrane (slit diaphragm) that prevent passage of molecules like albumin and gamma globulin Damage to the podocytes can lead to a serious problems such as massive protenuria. 9Lufukuja. G 10 MEDICAL APPLICATION In diseases such as diabetes mellitus and glomerulonephritis, the glomerular filter is altered and becomes much more permeable to proteins, with the subsequent release of protein into the urine (proteinuria). Lufukuja. G11 Mesangium In addition to endothelial cells and podocytes, the glomerular capillaries have mesangial (Gr. mesos, middle, + angeion, vessel) cells adhering to their walls. Mesangial cells are contractile and have receptors for angiotensin II. When these receptors are activated, the glomerular flow is reduced. 12Lufukuja. G Mesangium In the vascular pole but outside the glomerulus, there are the so- called extraglomerular mesangial cells that form part of the juxtaglomerular apparatus (macula densa, a part of the distal convoluted tubule of the same nephron. The juxtaglomerular cells, which secrete renin, specialized smooth muscle cells of the afferent arteriole, and extraglomerular mesangial cells)macula densadistal convoluted tubulejuxtaglomerular cellsreninafferent arterioleextraglomerular mesangial cells 13Lufukuja. G RENAL MEDULLA The renal medulla contains the conducting part of the kidney made of collecting tubules which open into minor calyces. This conducting part develops from ureteric diverticulum arising from the distal end of mesonephric duct or wolffian duct. The renal medulla forms the renal pyramids and the medullary rays The medullary rays (striated medullary substances from the base of renal pyramids formed by straight portions of nephrons, collecting tubules and blood vessels) and interlobular arteries and veins. These are microscopic and can be viewed during histology classes. 14Lufukuja. G 15 RENAL MEDULLA Lufukuja. G16 RENAL PYRAMIDS The renal pyramids have a broad base that borders the renal cortex and the apex called papilla. The renal papilla projects into the funnel like structure, the minor calyx. There may be one to three papillae in one minor calyx. Between adjacent renal pyramids are cortical materials extending from the cortex forming the renal columns. 17Lufukuja. G Kidneys 18Lufukuja. G RENAL PYRAMIDS The papillae are pierced by the collecting ducts or ducts of bellini, which are contained in the papillae. Because many ducts pierce the tip of papillae they give rise into sieve like appearance that is known as cribrosa. Urine is therefore poured in the area of cribrosa, which opens into minor calyces. 19Lufukuja. G Kidneys (renal lobe) A renal lobe consists of a renal pyramid, the overlying area of renal cortex, and adjacent tissues of the renal columns Lufukuja. G20 Lufukuja. G21 Kidneys (renal lobe)... kidneys. Urine production occurs in the renal lobes. Ducts within each renal papilla discharge urine into a cup-shaped drain called a minor calyx. Lined by transitional epithelium (also known as urothelium), which is a type of tissue consisting of multiple layers of epithelial cells which can contract and expand. Lufukuja. G22 URINIFEROUS TUBULES Lufukuja. G25 Lufukuja. G26 Lufukuja. G27 Renal tubules The renal tubule extends from Bowmans capsule to its junction with a collecting duct. The renal tubule is approximately 55 mm long in humans and is lined by a single layer of epithelial cells. The primary function of the renal tubule is selective re- absorption of water, inorganic ions and other molecules from the glomerular filtrate. Also some inorganic ions are secreted directly from blood into the lumen of the tubule. In humans, glomerular filtrate is produced at a steady rate of approximately 120mls/min; of this, 1 ml is reabsorbed by the renal tubules. The renal tubule has a convoluted shape and has four distinct histophysiological zones, each of which has a different role in tubular function. Lufukuja. G28 The proximal convoluted tubule Is the longest and most convoluted section of the renal tubule, commencing at the urinary pole of a renal corpuscle. Is lined by mixture of simple cuboidal and columnar cells that have numerous microvilli on their luminal surface. The microvilli form the brush border and increase the surface area for absorption. The basal and lateral surface exhibits numerous in folding between which are elongated mitochondria. Lufukuja. G29 The PCT The PCT is responsible for re-absorption of molecules like amino acids, sugars, proteins, and vitamins; approximately 85% of Na + and water are reabsorbed here. Although reabsorption is the primary function of the PCT, the epithelial cells can also secrete substances into the lumen, e.g H +, NH3, hippuric acid and creatine Lufukuja. G30 The loop of Henle It is a U-shaped tubule that receives urine filtered from the PCT. It includes three parts; the distal straight part of the proximal tubule or pars recta, thin U-shaped limb and thick ascending limb. The main function of the loops of Henle is to produce urine that is hypertonic to plasma, the mechanism by which this is achieved is known as the counter- current multiplier system. Lufukuja. G31 loop of Henle Straight descending limb: Loop of Henle is lined by low columnar cells to simple cuboidal cells. The apical surface of the cells have long microvilli and the height of the microvilli is equal to that of the cells. Lufukuja. G32 loop of Henle Thin U-shaped limb: In this part the epithelium changes from simple cuboidal to simple squamous cells. The cells show only short microvilli and occasionally single cillium. The segment is involved mainly in water and sodium re- absorption Lufukuja. G33 loop of Henle Thick ascending limb: In this part the epithelium changes from low squamous cells to cuboidal cells. Its structure is similar to distal convoluted tubule. The epical membrane is smooth, and the cells have densely packed mitochondria that provide energy for pumping ions particularly sodium against an osmotic gradient. Lufukuja. G34 The distal convoluted tubule The DCT is a continuation of the thick limb of loop of Henle. Is shorter and less convoluted than the PCT. DCT is responsible for reabsorption of Na+ by an active process which is controlled by adrenocortical hormone aldosterone. Sodium reabsorption is coupled with secretion of hydrogen or potassium ions into the DCT, resulting in a net loss of acid from the body. DCT is lined by coboidal cells. Lufukuja. G35 The collecting tubule A number of smaller connecting tubules join to form the collecting ducts. In the cortex they are distinct because of their lightly stained cuboidal cells and cell membrane. Collecting tubules conduct urine from the nephrons to the ureteric pelvis and also play a role in the concentration of urine by water absorption, this being controlled by antidiuretic hormone(ADH). The epithelium lining the collecting system begins with simple cuboidal cells in the connecting tubules and changes to a columnar epithelium in the collecting and papillary ducts. Lufukuja. G36 Lufukuja. G37 The collecting tubule