Renal Histology
Histology of the Nephron – outer cortical nephrons have short thick and little or no thin limbs, inner have long both
- renal corpuscle: 150 um (cortical) to 250 um (juxtamedullary) – afferent and efferent arterioles at vascular pole
- fenestrated capillary epithelium surrounded by 2-layered tubule (Bowman’s capsule)
- inner glomerular epithelium (visceral layer) – podocytes (cover capillaries like glove fingers)
- primary and secondary processes divide extensively into secondary pedicles or foot processes
- interdigitate with one another (much surface area) – separated by thin (75-250 A) slit-pore complex
- "closed" by thin layer (50-70 A) structure with central knob
- separated from capillaries by 3-layered basement membrane (320-340 A) derived from podocytes
- outer capsular epithelium (parietal layer): simple squamous, elaborate lateral membrane interdigit. (to contain ultrafiltrate) resting on basement membrane; continuous with tubular lining (at urinary pole)
- functional morphology of corpuscle – three-part filtration barrier:
- (1) capillary endothelial pores restrict RBC’s but transmits ferritin (450 kD)
- (2) basement membrane restricts ferritin, and catalase (240 kD), but transmits myeloperoxidase (170 kD)
- (3) slit-pore restricts myeloperoxidse, but transmits horseradish peroxidase (40 kD) to Bowman’s space
- proximal convoluted tubule (40-60 um): most numerous element in immediate region of any glomerulus
- most acidophilic cytoplasm
- truncated pyramidal cell bodies with prominent lateral membrane interdigitations (ill-defined boundaries)
- apical surface covered by brush border (thick mat of PAS+ surface coating on densely packed microvilli)
- endocytotic activity – proteins in glomerular filtrate are trapped by thick glycocalyx, to small apical vesicles Þ apical vacuoles Þ fuse with primary lysosomes Þ digestion for amino acid absortion
- residual bodies are presumably excreted luminally by membrane fusion
- lateral interdigitations of membrane are most developed along basal, and extend under adjacent cells with extensive basal infoldings which contain numerous long mitochondria (suggest active transport)
- prominent basement membrane separates cells PCT from interstitital space and the capillary network
- Loop of Henle: descending thick limbs continuous with PCT; thin limbs (20-40 um)
- outer cortical (86%): short or missing (i.e., only thick limbs); if present, only on descending aspect
- juxtamedulary (14%): penetrating to innermost aspects of medulla, both descending and ascending limbs
- simple squamous epithelium with complex lateral interdigitations and junctions on a normal basement membrane
- scattered short microvilli on luminal surface
- distal tubules (20-50 um): ascending thick limb of Loop of Henle
- cuboidal, less acidophilic, larger lumen – also smaller so more nuclei/cross section (5-9 vs. 3-4 for PCT)
- no brush border, scattered short microvilli, thinner less prominent glycocalyx
- prominent basal striations (basal membrane invagination and interdigitations with many slender mitochondria)
- lateral cell boundaries are straighter and therefore more evident
- macula densa – region of taller more nucleated epith.at site of DT contact with afferent/efferent arterioles
- thereafter, DT becomes DCT – but few convolutions and thus not as freqently observed as PCT
- arched collecting duct passing to straight collecting ducts (both 40-200 um)
- pale cuboidal well-demarcated cells – 2 types:
- (1) dark cells (intercal.) – few short microvilli, apical vesicles, few basal striations, vary with acid-base bal.
- (2) light cells (principal cells) – no microvilli or apical vesicles, some basal striations.
- straight collecting ducts pass to papillary ducts (of Bellini) – cells change from cuboidal to tall columnar
- duct empties from tip of medullary pyramid at the "area cribrosa" into cavity of minor calyx of ureter
- juxtaglomerular apparatus: macula densa – modified media of afferent arteriole, also some mesangial cells
- afferent arteriole loses elastica interna, adventitia, has weak intima
- media well-developed (modified smooth muscle with round basophilic cells with PAS+ granules and renin)
- cellular apex abuts arteriolar endothelium: juxtaglomerular cells (base rests against macula densa of DCT with thin intervening basement membrane); communicate by macula MD Golgi or enzymatic shift
- mesangial cells: poorly understood – three classes of cells near arterioles and capillaries where no podocytes:
- (1) phagocytic—resorb effete basement membrane from between endothelium and podocytes
- (2) contractile properties (extensive filaments) esp. in a perivascular orientation along efferent arteriole
- (3) other cells show enzymatic shifts with secretory activity of the JG cells termed polkissen cells
- papillae, calyx, ureter – transitional epithelium
Renal Interstitium – mostly in medulla
- fibroblasts and mononuclear phagocytes are found around cortical vessels
- fine collagen bundles, some reticular fibers, and fluid reabsorbate in transit from tubular lumen to capillaries fills interstitium in cortex
- medullary interstitium contains numerous elongated branching cells that appear perpendicular to long axis of tubules and vasa recta and contain lipid droplets (variable number), RER, and numerous lysosomes.
- proposed functions for these cells:
- elaborate interstitial matrix (collagen fibers and ground substance)
- are contractile (possess filaments) and by contraction may play role in urine concentration (??)
- are phagocytic (at least to extravasated proteins)
- produce vasodepressor substances (prostaglandins E2 and A2) found in some renal interstitium
- are stem cells for tissue maintenance (not likely)
Extrarenal Duct System
- ureter: begins as minor calyces form major calyces
- transitional epithelium on collagenous lamina propria connect directly to smooth muscle coat; no submucosa
- lumen folded longitudinally (cross section=stellate)
- two smooth muscle coats: inner longitudinal, outer circular; much more CT in muscle here than in gut
- muscle layers extend into calyces, terminate as follows:
- inner longitudinal muscle in minor calyx at papilla
- outer circular-forms ring around papilla-"milking action"
- extra outer longitudinal develops at bladder insertion
- outer fibrous CT adventitia-firmly anchors ureter to body wall
- large blood vessels in CT coat supply muscle layers and produce rich capillary plexus in lamina propria
- rich lymphatic drainage in lamina propria
- intrinsic nerve plexuses in walls of ureter and pelvis
- unmyelinated fibers mainly to blood vessel and muscle coats
- myelinated fibers to mucosa, lose myelin, terminate as naked pain endings in epithelium
- no true glands at any level
- urinary bladder: similar to ureter
- lined by transitional epithelium on collagenous lamina propria, again direct to sm. muscle so no submuc.
- muscularis consists of three indistinctly separable coats; inner longitudinal, middle circular, outer long.
- similar distribution as cardiac tissue—facilitates three-dimensional contraction of vessel
- CT adventitia and/or serosa-numerous BV, Iymphatics, and both unmyelinated (sympathetic) and myelinated (sensory) nerves (just like ureter)
- male urethra: three portions: prostatic, membranous, and cavernous
- mucous membrane varies widely along tract
- epithelium – transitional in prostatic portion, stratified columnar or pseudostratified up to fossa navicularis, although stratified squamous may be found almost at random along length
- separated by thin basement membrane from broad lamina propria with stroma of loose CT with numerous elastic fibers (frequently scattered smooth muscle)
- submucosa proper absent in all portions
- glands present in two epithelial locations
- intraepithelial glands-mucous glands, usually found in pseudo or stratified columnar regions
- in cavernous portion-outpocketings of mucosa (lacunae of Morgagni) may continue into stroma of lamina propria as Glands of Littre- branched tubular mucousa
- muscularis varies widely by portion of tract
- prostatic – surrounded by fibromuscular prostatic tissue-keeps closed lumen
- membranous – surrounded by skeletal muscle sphincter derived from deep transverse perineal
- cavernous – no true muscularis, surrounded by corpus-spongiosum cylindrical mass of erectile tissue
- female urethra
- mucous membrane
- epithelium varies widely—transitional near bladder, otherwise stratified or pseudostratified
- no true submucosa-scant Glands of Littre
- lamina propria contains complex of BV, scattered smooth musc1e fibers, and thin erectile tissue coat
- muscularis – embedded in anterior vaginal wall
- muscular coats continue as in bladder
- skeletal muscle sphincter at terminus