Session 2 Histology Flashcards
Recognise the histology of a Uriniferous Tubule
Recognize the histology of a nephron
Recognize the histology of a renal corpuscle
- Vascular pole: afferent/efferent arterioles, glomerulus
- Urinary pole: Bowman’s capsule, where the PCT begins – point of entry into the tubular system
- During development, the primitive renal tubule is derived from the ureteric bud. It is blind-ending and grows into primordium of true kidney. The tubule envelopes the growing glomerulus resulting in a double-layered cover, the Bowman’s capsule.
Recognize the histology of a proximal convoluted tubule
PCT microvillus: The brush border enhances reabsorption of fluid and solutes from the lumen through or between the cuboidal epithelial cells and into capillaries. These tubules also secrete organic bases and H+ ions. The brush border of microvillus enables the tubules to absorb about 70% of the total Glomerular filtrate. The membranes contain Na+ H+ and Cl- antiporter exchangers and enzymes that digest small amino acids.
- The convoluted part of the proximal tubules coils close to the glomerulus in the cortex of the kidney.
Recognize the histology of the Loop of Henle
- The limbs of Henle’s loop are recognisable by the association of thin walled tubules (T) and parallel capillaries that represent the ascending vasa recta (VR). In a section of the kidney showing loop of Henle there will also be visible collecting ducts (CD) which is a much wider section of the uriniferous tube. In addition proximal tubules (PT) which belong to other nephrons will also be visible.
- In the outer medulla the Vasa recta form vascular bundles mainly surrounded by thick and thin limbs of the loop of Henle. Vasa recta are columns of capillaries that are predominant in the medulla. They are mixtures of descending vessels that originate from efferent glomerular arterioles which extend to the medulla and return as venous or ascending vessels that drain into the corticomedullary veins.
- The vesa recta have a slow blood flow and are responsible for operating a counter current exchange between their plasma and the interstitial fluid.
- The collecting ducts which lie in the inner medulla have wide lumens and no brush border They have an orderly cuboidal epithelium and prominent plasma membrane borders between the cells. These ducts reabsorb water and urea and partly determine urine volume and concentration. They are predominately regulated by the Antidiuretic hormone (ADH) and also aldosterone.
Recognize the histology of the distal convoluted tubule
These tubules have no brush borders. However the mitochondria are numerous indicating their rôle in active transport. This nephron segment is relatively impermeable to water and in not homologous in either morphology or function. It is a transitional tubule, which leads to the collecting segment of the tubule.
Recognize the histology of the juxtaglomerular apparatus
The juxtaglomerular apparatus is a specialized structure formed by the distal convoluted tubule and the glomerular afferent arteriole. It is located near the vascular pole of the glomerulus and its main function is to regulate blood pressure and the filtration rate of the glomerulus. Consists of
- The macula densa is a collection of specialized epithelial cells in the distal convoluted tubule that detect sodium concentration of the fluid in the tubule. In response to elevated sodium, the macula densa cells trigger contraction of the afferent arteriole, reducing flow of blood to the glomerulus and the glomerular filtration rate.
- The juxtaglomerular cells, derived from smooth muscle cells, of the afferent arteriole secrete renin when blood pressure in the arteriole falls. Renin increases blood pressure via the renin-angiotensin-aldosterone system. They contain dark granules
- Lacis cells, also called extraglomerular mesangial cells, are flat and elongated cells located near the macula densa. Their function remains unclear.
Recognize the histology of the collecting duct
Continuation of DCT via collecting tubule.
Similar appearance to thick limbs of Henle’s loop
But lumen is larger and tend to be more irregular rather than circular
Recognize the histology of the collecting system (renal pyramid)
Progressively larger ducts formed by merging of collecting merge and terminate to form papillary ducts of Bellini (B) which open sieve-like at area cribosa, empty at the renal papilla (into the minor calyx). The cavity of the minor calyx which is funnel shaped (MC) and is line with transitional epithelium and conveys urine from the papilla to the renal pelvis and urethra.The minor calyces merge to form major calyces within the kidney, which in turn merge to form the renal pelvis (still within the kidney). The urine flows through these structures to the ureter and is channelled to the bladder.
Recognize the histology of the ureter
Urine passes along the ureter in peristaltic waves when the lumen changes from the stellate to the ovoid. The proximal ureter contains the typical transitional epithelium (T) as well as the elastic lamina propria and the circular muscularis externa (ME). The distal ureter has smooth epithelium without crypts or folds.
- A muscular tube
- 2 layers of smooth muscle. A third appears in the lower 1/3 of the ureter.
- Lined by specialized epithelium – transitional epithelium aka urinary epithelium or urothelium
- Urothelium is stratified epithelium – “umbrella cells” on the surface layer which makes the epithelium impermeable.
Recognize the histology of the bladder
The bladder is finally emptied through the urethra.
- 3 layers of muscle
- Outer adventitia
- Transitional epithelium
Recognise the histology of the urethra
Initially, the urethra is lined by a transitional epithelium in males and females. In males, it is replaced by a pseudostratified or stratified columnar epithelium below the openings of the ejaculatory ducts into the urethra (see reproduction module). The distal parts of the female urethra and the distal end of the male urethra are lined by a stratified squamous epithelium. The lamina propria contains loose connective tissue. Smooth muscle cells in the muscularis are mainly oriented longitudinally. They are surrounded, in the middle part of the urethra (below the prostate in males), by striated muscle cells of the sphincter urethrae.
Describe the microscopic arrangement of the kidney
- The nephron is a long tube, lined with epithelial cells, with the glomerulus at one end, and a connection to the renal pelvis at the other.
- It consist of the renal corpuscle (Glomerulus and Bowman’s capsule) + PCT, LH and DCT.
- Filtrate formed by the glomerulus passes first through the proximal convoluted tubule (PCT), which is also in the cortex. The nephron then dips down into the medulla forming a hairpin loop, known as the loop of Henle. As the nephron ascends, it forms the distal convoluted tubule, located in the cortex. Finally the nephron joins the collecting duct that passes through the medulla to the renal pelvis.
- Specialised afferent and efferent arterioles maintain a relatively high, constant filtration pressure in capillaries at the glomerulus, which drives water and small molecules out of the plasma.
- Glomerular filtration rate is 125ml/min or 180L/day
Describe the PCT
- Longest most convoluted section of the tubule.
- Simple cuboidal epithelium with pronounced brush border.
- Absorbs 70-80% of most ions and water, plus most or all of the small organic molecules, not destined for excretion (e.g. glucose, amino acids).
- Reabsorption in this section of the tubule is not tightly regulated.
- Water follows the reabsorption of solute so the filtrate remains isotonic with extracellular fluid.
- The epithelial cells lining the nephron are polarized with different properties on the membrane facing the filtrate – the apical or luminal membrane, and that facing the extra-cellular surface – the basolateral membrane.
- The absorption of many substances is linked to the active movement of sodium ions.
- Sodium ions are moved by active transport across the basolateral membrane, so generating a concentration gradient between the intra-cellular fluid and filtrate. This then drives the reabsorption of most ions and small molecules, with water following passively by osmosis.
Describe the Loop of Henle including the four parts
Pars recta (thick descending portion)
Thin descending limb
Thin ascending limb
Thick ascending lim
- The four parts are described on the basis of appearance / epithelial lining and each part has a specialized function.
- The thin limb dips down into the medulla and has simple squamous epithelium: no active transport, looks a lot like a small capillary, but there are no RBCS and no brush border
- The thick ascending limb (best seen in medulla, interspersed with thin limbs, vasa recta and collecting ducts) have simple cuboidal epithelium and no brush border and active transport occurs.
- Filtrate passes through the Loop of Henle and solute is selectively reabsorbed.
- There is overall less reabsorption of water, so the fluid leaving the loop is hypotonic.
- As the fluid moves first down the descending limb and then up the ascending limb, however, a process of counter current multiplication generates a very hypertonic environment within the kidney medulla.
Describe the filtration barrier
The filtration barrier in the kidney is made up of the capillary endothelium and the visceral layer of Bowman’s capsule, podocytes. Podocytes are ‘cells with feet’, and look like lots of tentacles wrapped around the capillaries.
- The capillary endothelium is fenestrated (it is the leakiest capillary in the body, with lots of holes), and podocytes invest in it, making filtration slits.
- Podocytes and the fenestrated capillary endothelium share a basement membrane.
The parietal layer (simple squamous epithelium) of Bowman’s capsule makes a ‘funnel’ to collect the ultrafiltrate to drain into the PCT at the urinary pole.
