Kidney histology

Renal corpuscle

The renal corpuscle is the filtration apparatus of the nephron. Each corpuscle consists of two main elements; the glomerulus and glomerular (Bowman's) capsule. The glomerulus is a network of capillaries formed by branches of the renal artery (afferent and efferent arterioles). 

The glomerular capsule surrounds the glomerulus. It consists of two layers (parietal and visceral), which bound a cavity called the glomerular capsular space (Bowman’s / urinary space). The inner visceral layer is made of special cells called podocytes. Podocytes cover the walls of glomerular capillaries, interdigitating with each other and forming narrow slits between their projections. The outer parietal layer is made of simple squamous epithelium and is continuous with the nephron tubules. The afferent and efferent arterioles enter the renal corpuscle at the vascular pole, while the site where the glomerular capsule narrows and continues as the proximal thick segment of the nephron is called the urinary pole.

Glomerular filtration

The renal corpuscle is the starting point of urine formation. Systemic blood passes through the glomerular capillary system, and is filtered to form primary urine (ultrafiltrate). It does this via a special filtration barrier which selectively filters water and solutes from the blood passing through the glomerular capillaries. The glomerular ultrafiltrate is collected by the glomerular space, and passes into the kidney tubules. 
 
The kidney filtration apparatus is formed by three layers of tissue; endothelium of the glomerular capillaries, glomerular basement membrane (GBM) and podocytes (visceral layer of renal capsule). Glomerular capillaries are composed of fenestrated endothelium. Fenestrations function as pores. The GBM is more complex than other epithelial basal membranes. It consists of three layers; a thick central lamina densa and two thinner layers (lamina rara interna and lamina rara externa).

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Podocytes cover the walls of the glomerular capillaries. Their finger like projections (pedicles) interdigitate, with narrow filtration slits (filtration slit diaphragm) forming between the projections. Together, these three layers function as a selective filter, allowing only molecules below a certain size, and of a certain charge, to pass from the blood and enter the renal tubular system. For example, blood cells, platelets, some proteins and some anions are prevented from leaving the glomerular capillaries, while water and solutes pass through. The remaining unfiltered blood is carried out of the glomerulus by the efferent arteriole, and passes back into the venous system. 

Renal tubule system

The tubule system is the part of the nephron which processes glomerular ultrafiltrate into urine by reabsorbing necessary molecules and secreting the unnecessary and waste substances. It consists of three parts;

• Proximal tubule; convoluted proximal tubules and straight proximal tubule
• Nephron loop; descending and ascending limbs
• Distal tubule; straight distal tubule and convoluted distal tubule

Proximal tubule

The proximal tubule is the first part of the tubular system. It consists of convoluted and straight segments. The proximal convoluted tubule is located within the renal cortex and is continuous with the capsular space.

The straight proximal tubule (or thick descending limb) extends down into the medulla. Both parts are composed of simple cuboidal epithelium, rich in mitochondria and microvilli (brush border). This morphology is adapted to the proximal tubule function of absorption and secretion. More than half of the previously filtered water and molecules are returned to the blood (reabsorption) by the proximal tubules.

Nephron loop

The nephron loop is the U-shaped bend of a nephron which extends through the medulla of the kidney. Histologically, it consists of two parts; thin descending and thin ascending limbs.

Both limbs are composed of simple squamous epithelium. The cells have few organelles, little to no microvilli and low secretion abilities. The two limbs work in parallel, with the surrounding vasa recta capillaries, to adjust the filtrate’s salt (e.g. sodium, chloride, potassium) and water levels. More specifically, the descending limb is highly permeable to water, less permeable to solutes, while the ascending limb is the opposite. Some authors consider the nephron loop to be synonymous with the loop of Henle, while other authors include the proximal straight tubule, nephron loop and distal straight tubule in this term.

Distal tubule

The distal tubule also consists of straight and convoluted segments. The straight distal tubule (thick ascending limb) continues on from the thin ascending limb of the nephron loop at the level between the inner and outer medulla. The convoluted distal tubule projects into the cortex. Both parts of the distal tubule are composed of simple cuboidal epithelium, similar in morphology to the proximal tubule.

A key difference between them is that the epithelium of the distal tubule has less well-developed microvilli. Reabsorption and secretion occurs here, albeit to a lesser degree than in the proximal tubule. By having lots of mitochondria the straight distal tubules can reabsorb any useful substances (electrolytes), and secrete any remaining waste products using active transport. Of particular note is the absorption of sodium, under the regulation of aldosterone.

Collecting system

The collecting system of the kidney is a series of tubes that moves urine from the nephrons into the minor calices. Several distal convoluted tubules from neighbouring nephrons drain into a collecting duct via connecting/collecting tubules. Collecting ducts then travel through the kidney medulla, converging at the apex of each renal pyramid. Here, several ducts merge to form a single large papillary duct (of Bellini), which opens into the minor calix through the area cribrosa.

Collecting ducts are termed cortical or medullary, depending on which part of the kidney parenchyma that part of the duct is located. They are made of epithelial cells, which get progressively taller as the ducts get larger.

• Cortical collecting ducts - simple cuboidal epithelium
• Medullary collecting ducts - simple columnar epithelium
• Papillary ducts - simple columnar epithelium

Two additional types of cells are distinguishable in these ducts. The principal cells, which are pale staining and play a role in ion transport. Darker staining intercalated cells are scattered amongst the principal cells and are responsible for acid-base balance. Collecting ducts are the last chance site for water and electrolyte reabsorption from the filtrate further concentrating the urine, particularly under the influence of antidiuretic hormone (ADH). No more reabsorption takes place past the medullary collecting ducts.

Juxtaglomerular apparatus

Nestled into the vascular pole of the nephron is a collection of cells called the juxtaglomerular apparatus (JGA). It is formed by 3 types of cells; macula densa, juxtaglomerular granular (JG) cells and extraglomerular mesangial (Lacis) cells.
 
The macula densa is located in the wall of the distal tubule, at the point where the tubule comes in contact with the glomerulus. Here the regular cuboidal epithelium of the distal tubule crowd together and become columnar in shape. The juxtaglomerular granular (JG) cells are modified smooth muscle cells found surrounding the afferent, and sometimes efferent, arteriole. The third cell type of the JGA are the extraglomerular mesangial (lacis) cells. These are located in the triangular space between the afferent and efferent arterioles.
 
The juxtaglomerular apparatus has two key functions;

• regulates glomerular blood flow and filtration rate
• regulates systemic blood pressure

Glomerular blood flow is regulated by a feedback mechanism, whereby the macula densa responds to high sodium chloride levels in the filtrate by releasing vasoconstrictor chemicals. These chemicals cause the afferent arteriole to vasoconstrict, thus lowering glomerular pressure and, in turn, filtration rate. This system maintains a mostly constant pressure within the nephrons. Systemic blood pressure is regulated through the renin-angiotensin-aldosterone system. Low systemic blood pressure, recognised by baroreceptors, triggers the juxtaglomerular granular cells to secrete an enzyme called renin. Renin, in turn, activates the renin-angiotensin-aldosterone system, raising systemic blood pressure through the actions of angiotensin and aldosterone. 

Don't forget to quiz yourself on the renal corpuscle and juxtaglomerular apparatus to consolidate your knowledge!