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

• 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