Urinary System Microanatomy- Pt 2

Distal Convoluted Tubules (DCT)

• The DCT continues from the thick ascending segment of the loop of Henle
• Composed of a single layer of cuboidal epithelial cells with basal striations, but lacks microvilli (brush border)
• Found only in the cortex of the kidneys, and is the site of action for aldosterone
• Contains specialized chemoreceptor cells of the macula densa

Collecting Ducts

• Connect the DCT to the renal papillae/crest, and are not considered part of the nephron
• Have a lumen containing primitive urine
• Lined by simple cuboidal to low columnar epithelium, composed of principal and intercalated cells
• Principal cells function to reabsorb sodium and water
• Intercalated cells participate in acid-base balance
• Site of action for ADH (anti-diuretic hormone) via aquaporin receptors

Renal Papilla (Papillary Ducts)

• Terminal portion of the collecting ducts, emptying at the area cribrosa of the renal crest (or renal papilla)
• Renal crest may be referred to as the renal papilla, depending on the species
• Vasa recta functions to take away water passing through collecting and papillary ducts

Vasculature

• High blood supply (25% cardiac output) at the terminal (end) artery system
• Artery vasculature includes: renal artery, interlobular artery, arcuate artery, interlobular artery, and intralobular (arcuate) artery
• Arcuate artery is located directly between the cortex and medulla of the kidneys
• Vein vasculature includes: intralobular vein, interlobular vein, and arcuate vein
• Afferent arterioles include the glomerulus (capillaries)
• Efferent arterioles include the peritubular capillaries (surround tubules), and vasa recta (surround loops of Henle)

Urinary System: General Info

• Interstitium is sparse in the cortex and supports kidney cell functionality
• More interstitium is present in the medulla
• Interstitial cells include fibroblasts, bone marrow-derived cells, and a unique lipid-laden interstitial cell (stellate-shaped) prominent in the inner medulla
• Medulla is the site of prostaglandin (E2) production
• Lymphatics are found in the interstitium surrounding the intraarterial arteries
• Kidneys have efferent innervation to the smooth muscle arteries, afferent and efferent arterioles, and descending vasa recta
• Tubular organs have 4 layers (tunics/tela), which are: tunica mucosa, tunica submucosa, tunica muscularis, and tunica serosa/adventitia

Juxtaglomerular Apparatus

• Macula densa cells (part of DCT wall) are sodium chemoreceptors that sense sodium concentrations in filtrate
• Juxtaglomerular cells are modified smooth muscle cells, mainly of afferent and slightly efferent arteriole
• Detect variations in blood pressure (baroreceptors) and secrete renin into vessel lumen
• Renin is secreted in response to low sodium concentrations in filtrate and low blood pressure

Cause and Effect

• Decreased blood pressure at afferent arterioles stimulates juxtaglomerular cells to release renin
• Decreased sodium levels in DCT stimulates Macula Densa, leading to juxtaglomerular cells releasing renin
• Renin increases blood pressure by converting angiotensinogen into angiotensin 1, then angiotensin 2, and finally aldosterone
• Aldosterone works to increase sodium and water retention at the DCT, resulting in increased blood pressure

Ureters

• Function to convey urine from the renal pelvis to the bladder
• Tunica mucosa is composed of urothelium (transitional epithelium) which flattens when stretched
• Tunica submucosa is composed of lamina propria lacking a lamina muscularis
• Tunica muscularis is composed of 3 layers of smooth muscle and autonomic innervation for peristalsis
• Tunica adventitia is the outermost layer

Urinary Bladder

• Functions to store urine
• Composed of all 4 tunics
• Tunica mucosa is composed of urothelium and has lamina present with lamina muscularis (as thin incomplete bands of smooth muscle)
• Tunica submucosa is above the tunica mucosa layer
• Tunica muscularis is above the tunica submucosa layer and has 3 layers of smooth muscle
• Smooth muscle makes up the detrusor muscle
• Skeletal muscle makes up the sphincter near the urethra
• Tunica serosa/adventitia is above the tunica muscularis

Urethra

• Structure similar to ureters and urinary bladder
• Urothelium proximally, and stratified squamous epithelium distally
• Has accessory sex glands and mucous glands
• Mucous glands are diffuse in ruminant, boar, and cats
• In males, has a vascular stratum (corpus spongiosum) and tunica muscularis/urethral sphincter
• In females, urethra is shorter than in males
• Lined initially with transitional epithelium (urothelial) and then stratified squamous epithelium near the external urethral orifice