Histology Of Renal System PDF

histology of renal System Introduction o Functions of renal system:  Regulate homeostasis:  Regulation of fluid & salts levels in the body.  Via filtration, selective reabsorption & secretion.  Regulation of blood pressure: via Renin production.  Stimulate RBCs...

histology of renal System Introduction o Functions of renal system:  Regulate homeostasis:  Regulation of fluid & salts levels in the body.  Via filtration, selective reabsorption & secretion.  Regulation of blood pressure: via Renin production.  Stimulate RBCs production: via Erythropoietin (secreted by fibrocytes of peritubular region). o Components of Urinary System:  2 Kidneys (Filtration & Reabsorption).  2 Ureters (Conductive).  One Urinary bladder (Reservoir).  One Urethra (Excretory). o Blood flow to kidneys is about 1.2-1.3 Liters/minute (All blood is filtered every 4-5 minutes). Histology Of Nephrons o Introduction:  Each kidney contains ~1 million nephrons (constant):  Number does not increase after birth.  Cannot be replaced if damaged.  Consists of: Renal corpuscle & renal tubules (PCT, loop of Henle, DCT, & collecting duct). A. Renal Corpuscle:  Each renal corpuscle is about 200 µm in diameter.  Consists of: 1. Glomerulus: tuft of capillaries. 2. Glomerular (Bowman’s) capsule: ▪ Double-walled epithelial capsule surrounds the glomerulus. ▪ Urinary space:  Between the two layers (parietal & visceral layers) of Bowman's capsule.  Receives the fluid filtered through the capillary wall & visceral layer. 1  Each renal corpuscle has: ✓ Vascular pole:  Where the afferent arteriole enters and the efferent arteriole leaves.  After entering the renal corpuscle, afferent arteriole usually divides into 2-5 primary branches, each subdividing into capillaries and forming renal glomerulus. ✓ Urinary pole: where the proximal convoluted tubule begins.  Histology of Bowman's capsule: ✓ Parietal layer:  Consists of a simple squamous epithelium.  Supported by a basal lamina & a thin layer of reticular fibers.  At the urinary pole, the epithelium changes to the simple cuboidal, or low columnar. ✓ Visceral layer:  Cells of internal layer are called Podocytes.  Have a cell body from which arise several primary processes.  Each primary process gives rise to numerous secondary processes “pedicels” (embrace capillaries of the glomerulus).  At a periodic distance of 25 nm, secondary processes are in direct contact with basal lamina of capillaries.  Cell bodies of podocytes and their primary processes do not touch the basement membrane.  Secondary processes of Podocytes interdigitate forming the filtration slits.  Bridging between filtration slits is known as diaphragm (about 6 nm thick).  Podocytes have bundles of actin filaments in their cytoplasm → give them a contractile capacity.  Basement membrane: - Between fenestrated endothelial cells of glomerular capillaries & Podocytes. - Believed to be the filtration barrier that separates urinary space & blood in the capillaries. - Derived from the fusion of capillary- and podocyte-produced basal laminae. ▪ Renal Filter Made off: - Fenestrated Capillaries. - Underlying Basement Membrane. - Slits created by criss-crossing of secondary processes of Podocytes surrounding capillaries. 2   Mesangial cell: ✓ Specialized cells around blood vessels in the kidneys. ✓ Usually divided into two types: 1. Extraglomerular mesangial cells: - AKA: Lacis cells. - Light-staining cells in kidney. - Found outside the glomerulus, near the vascular pole & macula densa. - Form the Juxtaglomerular apparatus in combination with two other types of cells. 2. Intraglomerular mesangial cells: - Specialized pericytes located among glomerular capillaries within a renal corpuscle. - Functions of intraglomerular mesangial cells:  Filtration.  Phagocytosis.  Structural support. B. Proximal Convoluted Tubule:  At the urinary pole of renal corpuscle, squamous epithelium of parietal layer of Bowman's capsule is continuous with cuboidal (low columnar) epithelium of proximal convoluted tubule.  Longer than distal convoluted tubule → more frequently seen near renal corpuscles in renal cortex.  Cells of cuboidal epithelium have an acidophilic cytoplasm due to presence of numerous elongated mitochondria.  Cell apex has abundant Microvilli (about 1 µm in length) → form a brush border.  Because the cells are large, each transverse section of a PCT contains only 3-5 spherical nuclei. C. Henle's Loop:  U-shaped structure.  Consists of a thin descending limb & thick ascending limb (lined with simple cuboidal epithelium).  In the outer medulla, the thick descending limb, with an outer diameter of about 60 µm, suddenly narrows to about 12 µm & continues as thin descending limb.  Lumen of this segment of the nephron (thin descending limb) is wide because the wall consists of squamous epithelial cells whose nuclei protrude only slightly into the lumen. D. Distal Convoluted Tubule:  Lined with simple cuboidal epithelium.  Differ from PCT (No Brush Border, No Apical Canaliculi, and smaller cells).  More nuclei are seen in DCT than in PCT (because DCT cells are flatter and smaller). 3   Cells of DCT have elaborate basal membrane invaginations & associated mitochondria (Striation) indicative of their Ion-transporting function.  Contact with vascular pole of renal corpuscle:  In this Juxtaglomerular region, cells of DCT usually become columnar, and their nuclei are closely packed together → Macula Densa.  Cells of macula densa are sensitive to ionic content & water volume of the tubular fluid. E. Collecting Tubules & Ducts:  Urine passes from DCT to collecting tubules:  Collecting tubules join each other to form larger, straight collecting ducts.  Collecting ducts approach the tips of medullary pyramids.  Smaller collecting tubules: ✓ Lined with cuboidal epithelium & have a diameter of approximately 40 µm. ✓ As they penetrate deeper into medulla → cells increase in height → become columnar.  Collecting duct: diameter reaches 200 µm near the tips of medullary pyramids. o Juxtaglomerular Apparatus (JGA):  Composed of: 1) Macula Densa of DCT. 2) Extraglomerular mesangial cells. 3) Juxtaglomerular cells of afferent arteriole: ▪ Adjacent to renal corpuscle, tunica media of afferent arteriole has modified smooth muscle cells → Juxtaglomerular (JG) Cells. ▪ JGC have a cytoplasm full of secretory granules of Renin. ▪ Secretions of JGC monitor BP:  Act as chemoreceptor & baroreceptor.  If BP falls, they secret Renin.  Reduction in BP results in reduced glomerular filtration rate.  Renin converts Angiotensinogen to Angiotensin-I.  Angiotensin-I converted to Angiotensin-II (potent vasoconstrictor).  Angiotensin-II release Aldosterone from suprarenal glands → increase reabsorption of sodium & water from DCT. 4 Histology Of Excretory Passages o Include:  Calyces.  Renal Pelvis.  Ureter.  Urinary Bladder.  Urethra. o Basic structure of all these components is the SAME:  Mucosa: ✓ Lined with a Transitional Epithelium (occurs exclusively in urinary system). ✓ Epithelium is virtually impenetrable to any components of urine.  Lamina propria: consists mainly of connective tissue.  Muscularis: usually consists of an inner longitudinal, middle circular, & outer longitudinal layers of smooth muscle cells. o Urethra:  Lined by a transitional epithelium in males & females.  Distal parts of female & male urethra are lined by a stratified squamous epithelium.  Smooth muscle cells in the muscularis are replaced in the middle part of the urethra (below the prostate in males) by striated muscle cells of Sphincter Urethrae. 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