Genitourinary Microanatomy

Renal Microscopic Anatomy I

• Renal corpuscle: filtering unit of each nephron in a kidney consisting of:
- Glomerulus: network of capillaries in which blood plasma is filtered out. Blood is delivered in by the afferent arterioles and out by the efferent arterioles with both located at the vascular pole.
- Bowman’s capsule: sac-like structure that encloses the glomerulus, collects the filtrates and delivers it into the proximal tubule at the urinary pole
- Mesangium: the matrix between the capillaries
• Cells of the renal corpuscle:
- Mesangial cells: specialized cells located in the connective tissue between endothelial cells of the capillaries of the blood vessels and help support the structure of the glomerulus, regulate blood flow by contraction of lumen and finally phagocytose clots and debris that form.
- Lacis cells (EGM): cells found outside the glomerulus near the vascular pole and macula densa. It plays a role in the juxtaglomerular apparatus.
- Macula densa: concentrated groups of cells located in the wall of the distal convoluted tubule against the vascular pole of the glomerulus. Its function is to detect the ionic content and water volume of the filtrate and through the JG apparatus regulate blood pressure to maintain homeostasis.
- Juxtaglomerular cells: modified smooth muscle cells located at the walls of the afferent arteriole and is responsible for response action to signals and consequently production of rennin to regulate blood pressure
- Podocytes: cells that form a tight indigitating network of foot process extensions called pedicels which cover the outer surface of all capillaries, controlling the filtration of proteins into Bowman’s space.
- Glomerular epithelial cells: visceral layer of the corpuscle covering the glomerulus
- Capsular epithelium: parietal layer of the corpuscle made up of squamous epithelial cells
• Juxtaglomerular apparatus: made up of JG cells, lacis cells and macula densa. These three groups of cells co-ordinate to regulate blood pressure through ionic content and volume of the filtrates.
• Filtration of renal corpuscles: 1700 L of blood flows through the capillary and around the 180 L filtrate per day. Basement membrane is around 0.1 cm thick.
- Fenestrated endothelium: the glomerulus capillaries are discontinuous with gaps of 50 nm and provide the first barrier of filtration – filters formed elements as small as platelets
- Lamina rara interna: inner layer of the basement membrane filters any components above 14 nm of size
- Lamina densa: central layer will filter 9 -10 nm
- Lamina rara external 14 nm
- Slit pores: formed from the pedicels of the podocytes (25 nm wide) and along with the membrane matrix that lines the gaps, the slit pores can selectively filter substances that passes through, i.e. slit itself are negative charged so only allows positive charged substance to pass through.
• Development of renal corpuscle:
- Network of capillaries comes into contact with the kidney tubules with the tubules folding around the capillaries forming the renal corpuscle.
- Inner surface of the tubule against the capillaries forms the visceral layer
- Outer surface of the tubule forms the parietal layer

Renal Microscopic Anatomy II

• Nephron: a single function unit (tubular system) of the kidney
- Structure: consists of a renal corpuscle and the renal tubules
- Function: regulate body fluid volume, electrolyte and metabolite concentration, blood pH by filtering blood plasma and reabsorbing essential components while expelling the unwanted content in the form of urine
• Types of nephron:
- Cortical nephron: 85% of renal nephron has their renal corpuscle located in the cortex and perform excretory and regulatory functions
- Juxtamedullary nephron: 15% of renal nephron has their renal corpuscle located in the medulla and used to concentrate or dilute urine
• Renal tubule:
- Proximal tubule: proximal convoluted tubule passing on to proximal straight tubule (thick descending limb)
- Loop of henle: U shaped layout of the thin tubules located in the medulla made up of thin descending limb and thin ascending limb.
- Distal tubule: distal straight tubule (thick ascending limb) progress into distal convoluted tubule which passes directly next to the renal corpuscle and eventually emptying into the collecting duct in the cortex
• Collecting tubules: long straight tubule in which the nephrons of a kidney lobule empty into
• Histology of proximal tubule:
- simple cuboidal cells lining the tubule with microvilli in the apical surface and also multiple folding in the basolateral membrane.
- Folding at the base of the cell house a long profile of mitochondria for active transport.
- A primary cilium projects from the apical surface into the lumen
- Pinocytotic vesicles pinching off the microvilli and digestive lysosomes are visible
- Tight junction does occur between cells but are leaky and allows water to pass through
- Neighbouring cells join through interdigitation of the cells
• Histology of loop of henle:
- Thin squamous epithelial cells
- Primary cilium projects from each cell into the lumen
• Histology of distal tubule:
- All features same as proximal tubule cells except there are no microvilli
• Anatomic features of the kidney:
- Cortex: outer portion of the kidney containing majority of the renal corpuscles
- Medulla: innermost part of the kidney occupied mostly by loop of Henle along with their ascending and descending thick limb
- Pyramid: cone shaped tissues making up a portion of each renal lobe and converge inwards into a papilla
- Renal column: medullary extension of the renal cortex between the pyramids allowing the cortex to be anchored
- Medullary rays: stripe structures apparent in the cortex and middle of the renal lobes due to the arrangement of the collecting duct and the straight tubules that empties into it.
- Minor calyces: spaces in which the papilla empty the urine into
- Major calyces: spaces in which the minor calyces converges into
- Renal pelvis: funnel-like dilated proximal part of the ureter and is the convergence point of two or three major calyces allowing a passageway of the urine into the ureter
• Anatomic division of the medulla: division made at the transition between thin and thick limbs
- Outer medulla: section of the medulla containing the thick limbs of the loop of henle
- Inner medulla: section of the medulla containing the thin limbs of the loop of henle
• Anatomic features of the renal lobule: a lobule is composed of a central collecting tube and its converging straight tubules (medullary rays) surrounded by aggregation of convoluted tubules, both proximal and distal, and corpuscles (cortical labyrinth). At the very edge of a lobule are interlobular arteries that supply blood to each lobule.

Renal Microscopic Anatomy III

• Renal blood supply:

• Histology of ureter and bladder:
- Transitional epithelium is the first layer against the lumen of the ureter and bladder. The epithelial cell is able to stretch going from columnar to squamous. The epithelial also has a protective layer that protect against damaging urine
- Muscle of ureter has 2 layers with inner circular and outer longitudinal. Bladder muscles are thicker
- Adventitia contains fibroblasts with elastic connective tissue
• Polycystic kidney disease: autosomal dominant problem affecting 1 in 500 individuals. It develops later in life and patients go on dialysis. The disease is caused by lack of protein in making primary cilium which senses the ionic concentration and flow of content of the tubule.
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