Urinary System

BIO 408

Histology

Dr. D. L. Daley

Kidney

n Large reddish, bean-shaped organs - retroperitoneally located on the posterior abdominal wall

n Concave region - hilum - the ureter, renal vein and renal artery and lymph vessels enter the kidney in this region

n Ureter is expanded in the region of the hilum - forms the renal pelvis

Kidney - Hemisected

n Shows the outer cortex and inner medulla

n Cortex is dark brown

n Medulla has 6 to 12 discrete, pyramid-shaped, pale, striated regions - renal pyramids

n Base of each is oriented toward the cortex

n Apex - renal papilla - points toward the hilum

n Apex surrounded by cup-like minor calyx which when it joins 2 or 3 other minor calyces - form a major calyx

n The 3 or 4 major calyces empty into the renal pelvis

n Neighboring pyramids are separated by material resembling the cortex - cortical columns

Uriniferous Tubules

n Nephron

n 1.3 million in each kidney

n Cortical nephrons - shorter

n Juxtamedullary nephrons - longer

n Renal corpuscle in cortex - tubular parts in the medulla

n Collecting duct - largest ones called the ducts of Bellini

n Drains several nephrons

Each Nephron

n Glomerular capsule (Bowman's capsule)

n Surrounds the glomerulus (tuft of capillaries)

n Together - renal corpuscle

n Proximal convoluted tubule (PCT)

n Loop of Henle

n Distal convoluted tubule (DCT)

Renal Corpuscle

n Oval to round structure - 200 to 250mm in diameter

n Glomerulus invaginated into Bowman’s capsule

n Glomerulus is in close contact with the visceral layer of Bowman’s capsule - composed of modified epithelial cells called podocytes

n The outer wall surrounding Bowman’s capsule is simple squamous epithelial cells - parietal layer

n Glomerulus is supplied by a short afferent arteriole and drained by an efferent arteriole

Glomerulus

n Formed as several tufts of anastomosing capillaries arising from the afferent arteriole

n Mesangial cells - connective tissue cells of Bowman’s capsule

n Extraglomerular mesangial cells - located at the vascular pole of the corpuscle -

n Intraglomerular mesangial cells - situated within the renal corpuscle - may be phagocytic and contractile

Glomerulus

n Capillaries of the glomerulus are similar to fenestrated capillaries

n Endothelial cells of the capillaries are highly attenuated - pores are usually not covered by a diaphragm

n Pores 70 to 90 mm in diameter

n Basal lamina - three layers

n about 300 nm thick

Visceral Layer of Bowman’s Capsule

n Podocytes - bear numerous long tentacle-like cytoplasmic extensions - primary (major) processes

n Each primary process bears secondary processes called pedicels

n Pedicels completely envelope the capillaries - interdigitating with neighboring processes

Visceral Layer of Bowman’s Capsule

n Pedicels have well-developed glycocalyx of negatively charged sialoprotein - podocalyxin

n Pedicel interdigitation occurs such that narrow clefts, 20 to 40 nm in width form filtration slits - between adjacent pedicels

n The slits are covered by a thin (6nm)slit diaphragm that extends between neighboring pedicels and acts a part of the filtration barrier

Proximal Tubule

n Tubule is composed of simple cuboidal epithelial cells - eosinophilic cytoplasm

n Elaborate striated boarders with interlocking and interwoven lateral cell processes

n The cells range from low cuboidal to high cuboidal epithelium

n 60 mm in diameter and 14 mm long

n High tortuous - pars convoluta (proximal convoluted tubule)

n Straighter portion - pars recta (descending thick limb of Henle’s loop) - descends into the medulla - becomes continuous with the loop of Henle

Proximal Tubule Cells

n The first 2/3 of the pars convoluta is designated S₁

n Closely packed microvillae & apical canaliculi (extend the apical cytoplasm) - many mitochondria

_n Remainder of the pars convulta and much of the pars recta is designated S₂

n Cells similar to S1 - but fewer mitochondria and less apical caniculi

n The remainder of the pars recta is designated S3

n Low cuboidal cells - few mitochondria

Thin Limbs of Henle’s Loop

n The pars recta of the proximal tubule continues as the thin limb of Henle’s loop

n Diameter is 15 to 20 mm

n Squamous epithelial cells with an average height of 1.5 to 2 mm

n Length depending on location

n Cortical nephrons - 1 to 2 mm - thin segment

n Juxtamedullary nephrons - 9 -10 mm - thin segment

n A hairpin loop is formed at the end of the thin limb (Henle’s loop)

Distal Tubule

n Divided into

n Pars recta - continuation of the of the ascending thin limb of Henle’s loop - known as the ascending thick limb of Henle’s loop

n Pars convoluta - distal convoluted tubule

n Interspersed between the ascending thick limb and the DCT is macula densa

Distal Tubule

n Ascending thick limb - 9 to 10 mm in length - 30 to 40 mm in diameter

n Low cuboidal cells make up most of the thick ascending limb

n Cells are not permeable to water or urea

n Active transport of Na ions occurs in these cells

n Macula densa - as the ascending limb passes the its own renal corpuscle at the region of the afferent arteriole - cells are tall and narrow

Distal Tubule

n Distal convoluted tubule - short (4 to 5 mm and 25 to 45 mm in diameter)

n Low cuboidal cells

n Open lumen

Juxtaglomerular Apparatus

n Composed of macula densa of the distal tubule, juxtaglomerular cells of the afferent arteriole and extraglomerular mesoangial cells

n Macula densa cells - tall narrow pale cells - centrally placed nuclei - may appear in the light microscope as a dense spot

n Juxtaglomerular cells - modified smooth muscle cells - in tunica media of the afferent (sometime efferent) arteriole

n Produce renin & also ACE & angiotensin I & II

Collecting Tubules (Ducts)

n Not part of the nephron

n Several distal convoluted tubules join to form a short connecting tubule that leads to a collecting tubule or duct

n Collecting tubules are about 20 mm in length and have 3 regions

n Cortical

n Medullary

n Papillary

Collecting Tubules (Ducts)

n Cortical Collecting tubules - located in the medullary rays - cuboidal epithelial cells

n Principal cells - oval central nuclei & sparse microvilli - basal membranes have numerous infoldings - function unknown

n Intercalated cells - display numerous apical vesicles (50 to 200 nm), microplicae on their apical surface - nuclei round and centrally located - actively transport & secrete hydrogen ions

Collecting Tubules (Ducts)

n Medullary collecting tubules - large caliber - formed as the union of several cortical collecting tubules

n In the outer medullary zone - tubules like cortical collecting tubules in structure

n Inner medullary area - only see principal cells composing the tubules

n Papillary Collecting tubules (ducts of Bellini) - formed by confluence of several medullary collecting tubules - large duct (200 to 300 mm)

n Composed of tall columnar principle cells

Renal Circulation: Arterial Supply

n Each kidney receives 10% of the total blood volume per minute via a branch of the abdominal aorta - renal artery

n The renal artery quickly divides after entering the kidney into 5 segmental arteries

n These divide into lobar arteries

n These branch into 2 or 3 interlobar arteries - travel through the renal pyramids

Renal Circulation: Arterial Supply

n Interlobar arteries form a series of arcuate arteries - these then divide to form interlobular arteries

n Branches of these form the afferent glomerular arterioles

n Efferent glomerular arterioles drain the glomeruli

n These form peritubular capillaries - cortical nephrons

n Vasa recta - hairpin-like capillaries of the juxtaglomerular nephrons - parallel the loops of Henle

Renal Circulation: Venous Drainage

n The venous side of the vasa recta deliver blood to arcuate veins (parallel the arcuate arteries) in the medulla

n Stellate veins collect cortical blood that merge into interlobular veins

n These deliver blood to arcuate veins - thus they drain both the cortex and medulla

n Arcuate veins deliver blood to interlobar veins that unite in the hilum to form the renal vein

n No lobar or segmental veins

Calyces

n The renal papilla of each renal pyramid fits into a minor calyx (funnel shaped) - accepts urine from the ducts of Bellini

n The apex of the pyramids is transitional epithelium

n Deep to the lamina propria -muscular layer - propels urine to the major calyx - one of 3 or 4 larger funnel-shaped chambers

n These collect urine from several minor calyces

n Major calyces join to form the renal pelvis

Ureter

n 3 to 4 mm in diameter - hollow tubes - 25 to 30 cm long - join the urinary bladder

n Mucosa

n Several folds project into the lumen - transitional epithelium - 3 to 5 cell layers thick

n Muscularis

n Proximal 2/3rds - two predominantly inseparable layers of smooth muscle - inner logitudinal layer and outer circular layer

n Lower 1/3 - third layer oriented longitudinally

n Fibrous outer coat - tough connective tissue

Urinary Bladder

n Organ for storing urine

n Mucosa acts as a osmotic barrier between the urine and the lamina propria

n Mucosa arranged in numerous folds that disappear when the bladder is distended

n During distention, the large round, diamond-shaped cells of the transitional epithelium become stretched and they become flattened

Urinary Bladder

n Cell shape changes can occur because the cell membrane of these cells is a mosasic of specialized thickened regions, plaques interspersed by normal membrane regions

n When empty the plaque regions are folded into irregular, angular contours, that disappear when the cell becomes stretched

n Rigid plaque regions are anchored to intracytoplsmic filaments - like gap junctions

n Plaques are impermeable to water and salts - act as an osmotic barrier

n Desmosomes hold the cells together

Urinary Bladder

n Muscular coat of the bladder - three interlaced layers of smooth muscle

n Middle circular layer of smooth muscle form the internal sphincter muscle around the internal orifice of the urethra

Urethra

n Coveys urine from the bladder to the outside

n Female urethra

n 4 to 5 cm in length - 5 to 6 mm in diameter

n Extends from the bladder to the external urethral orifice just anterior to the vaginal opening

n Lined with transitional epithelium near the bladder and then with stratified squamous epithelium

n Along the length of the urethra are clear mucous-secreting glands of Littre

n Small erectile coat surrounds the mucosa

n Muscular layer is continuous with the bladder

n Skeletal muscle form an external sphincter

Urethra

n Male urethra (15 to 20 cm long)

n Prostatic urethra - 3 to 4 cm long - transitional epithelium with openings to many tiny ducts of the prostate

n Membranous urethra - 1 to 2 cm long - where it passes through the perineal membrane (urogenital diaphragm)

n Stratified columnar epithelium and some pseudostratified columnar epithelium

n Spongy urethra (penile urethra) - about 15 cm long - passes through the penis - terminating the external urethral opening - stratified columnar epithelium and stratified squamous epithelium