EM 364 Renal Corpuscle

Renal Corpuscle

Transmission electron micrograph (TEM) of a renal corpuscle in a kidney.

Electron microscopy enables high-resolution examination of tissue, but it can be challenging to observe large structures. This remarkable micrograph of a renal corpuscle is an exceptionally large montage of 81 x 76 images, totaling 5,230 images and covering an area of 144 x 134 µm.

• Glomerulus
• Capillaries (red) - lumen (pink)
• Podocytes (blue and green)
• Mesangial Cells (orange)/Matrix (tan)
• Bowman's Capsule (brown)
• Proximal Convoluted Tubule (tan, upper right) - part of the initial segment

Cell Structures

• Nuclei (blue) / Nuclear Envelope (purple)
• Golgi Apparatus (yellow)
• Mitochondria (red)
• Endoplasmic Reticulum (cyan)
• Basal Lamina (purple)

Courtesy of Bernard van den Berg and Antonius Johannes Rabelink, Department of Internal Medicine, Leiden University Medical Center, Leiden, Netherlands.

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Glomerulus

The glomerulus is a tuft of capillaries that extends into the sac formed by the terminal end of the renal tubule. This cup-shaped structure is called Bowman's capsule.

A single afferent arteriole enters the glomerulus, forms about 20 to 40 capillary loops (red cells with pink lumen) and then emerges as an afferent arteriole. The point where the blood vessels enter and exit the glomerulus is known as the vascular pole of the renal corpuscle.

Bowman's capsule is composed of two main components:

• Parietal Layer (#1, #2) – outer layer composed of a simple squamous epithelium (brown) continuous with renal tubule.
• Visceral Layer (#1, #2) – inner layer composed of a thin, specialized layer of epithelial cells called podocytes (blue and green).

The exit point of Bowman's capsule into the renal tubule is known as the urinary pole of the renal corpuscle.

Glomerular Filtration

The glomerulus is a tuft of capillaries where water, ions, and small molecules are filtered out of the blood into the space made by Bowman's capsule.

The permeability barrier for filtering plasma is made up of three closely apposed parts:

• Fenestrated Capillaries (#1, #2, and , #3; red cells with pink lumen) - lined by endothelial cells with numerous pores (or fenestrations) with an average diameter of 70 nm.
• Filters plasma from blood
• Fenestrations (#1, #2, and , #3) lack diaphragms
• Some red blood cells (dark red) and white blood cells (gray) are seen in the lumens of some capillaries
• Basement Membrane (#1, #2, and , #3; purple) - sandwiched between the capillaries and podocytes
• Barrier to plasma proteins (such as albumin)
• Formed by the endothelial cells and adjacent podocytes
• Much thicker (300 to 400 nm) than in most tissues
• Podocytes (#1 and #2; blue and green) - highly specialized epithelial cells with long processes that wrap around the capillary loops (i.e., the visceral layer of Bowman's capsule).
• Each podocyte has several primary branches, which give rise to many secondary processes (pedicles or foot processes).
• Pedicles (#1 and #2) of adjacent podocytes interdigitate and form filtration slits about 20 to 25 nm wide
• Thin, non-membranous diaphragms span each filtration slit

The podocyte slit diaphragm is the main structure that controls the permeability of the glomerulus. Small molecules, such as glucose, can easily pass through, while larger molecules like albumin (69 kDa) can barely cross the barrier. Negatively charged, large molecules pass through less easily compared to positively charged ones of the same size.

Mesangium

The mesangium is a connective tissue framework that provides structural support to the tuft.

• Mesangial Cells (#1 and #2; orange) - pericyte-like cells located between the capillary loops
• Phagocytic role in maintaining the basement membrane
• Can contract to regulate blood flow in glomerular capillaries
• Mesangial Matrix (tan) - extracellular matrix secreted by mesangial cells

Immune complexes may be phagocytosed by mesangial cells and, together with increased mesangial matrix production, may contribute to glomerular dysfunction.

Extraglomerular mesangial cells form an outward extension at the vascular pole and are a component of the juxtaglomerular apparatus (not visible).

Proximal Convoluted Tubule

Proximal Convoluted Tubules arise from the urinary poles of the Bowman's capsule. They are highly convoluted in the cortex and are sectioned randomly in circular, elliptical, or U-shaped profiles.

A small segment of the proximal convoluted tubule (tan, upper right) is seen exiting the renal corpuscle at the urinary pole.

• Proximal convoluted tubules have a simple cuboidal epithelium with a brush border of tall microvilli, reflecting its role in reabsorption and transport.
• 60 to 70% of the glomerular filtrate
• Most of the water, sodium, glucose, amino acids, potassium, and citrate
• Regulates the pH of the filtrate by secreting H+ into the lumen and reabsorbing bicarbonate
• Endocytic Vesicles - arise from canaliculi in the apical cytoplasm that reabsorb and digest proteins
• Proteins digested to amino acids
• Basal Infoldings (#1 and #2) - extensive deep invaginations into the cell's cytoplasm, which greatly increases the surface area compared to a non-folded membrane
• Mitochondria packed between infoldings create a pattern of basal striations
• Allows close proximity of plasma cell membrane and mitochondria
• Absorbed molecules and amino acids are transported out of the cell into the peritubular capillaries
• Na⁺, K⁺> -ATPase pumps Na+ out of the cell