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Renal Corpuscle

Key Points:

The renal corpuscles lie within the renal cortex.
They comprise the glomerular, aka, Bowman's capsule and capillaries

The capsule is a double-layer sac of epithelium: — The outer parietal layer folds upon itself to form the visceral layer. — The inner visceral layer envelops the glomerular capillaries.

As blood passes through the glomerular capillaries, aka, glomerulus, specific components, including water and wastes, are filtered to create ultrafiltrate.
The filtration barrier, which determines ultrafiltrate composition,

comprises glomerular capillary endothelia, a basement membrane, and the visceral layer of the glomerular capsule.

Nephron tubules modify the ultrafiltrate to form urine.

Overview Diagram:

Tuft of glomerular capillaries; blood enters the capillaries via the afferent arteriole, and exits via efferent arteriole.
The visceral layer of the glomerular capsule envelops the capillaries, then folds outwards to become the parietal layer.
The capsular space lies between the parietal and visceral layers; this space fills with ultrafiltrate.
Vascular pole = where the arterioles pass through the capsule
Urinary pole = where the nephron tubule begins
Distal tubule passes by the afferent arteriole.

Details of Capillary and Visceral Layer:

Fenestrated glomerular capillary; fenestrations are small openings, aka, pores, in the endothelium that confer permeability.
Thick basement membrane overlies capillaries
Visceral layer comprises podocytes:

— Cell bodies — Cytoplasmic extensions, called primary processes, give rise to secondary foot processes, aka, pedicles.

The pedicles interdigitate to form filtration slits; molecules pass through these slits to form the ultrafiltrate in the capsular space.
Subpodocyte space; healthy podocytes do not adhere to the basement membrane.

Clinical Correlation:

Podocyte injury causes dramatic changes in shape, and, therefore, their ability to filter substances from the blood.

Cross Section of Renal Corpuscle:

Center = fenestrated capillary walls
Basement membrane comprises elements from both the endothelia of the capillaries and the epithelia of the visceral layer of the glomerular capsule.
Podocytes and processes are external to the basement membrane
Parietal layer comprises simple squamous epithelia resting on a basement membrane.

Notice that there are two basement membranes in our drawing; one envelopes the glomerular capillaries, the other comprises the outer surface of the renal corpuscle.

Capsular space between the parietal and visceral layers of the capsule.
Afferent and efferent arterioles pass through the vascular pole
Mesangial matrix and cells fill the spaces between the capillaries and arterioles.

Mesangium supports the capillaries, secretes vasoactive factors, and contains phagocytes that eliminate immune complexes

Juxtaglomerular apparatus, aka, JGA

Located at the vascular pole
Participates in systemic blood pressure regulation via the renin-angiotensin-aldosterone system (RAAS)
Comprises:

Juxtaglomerular cells, which are specialized cells within the smooth muscle of the afferent arteriole, and the macula densa, which is a specialized area of the distal convoluted tubule that comprises taller, columnar-like cells packed tightly together. Extraglomerular mesangial cells fill the space between the arterioles and the macula densa. These cells participate in tubuloglomerular feedback to regulate renal blood flow.

Filtration barrier:

Fenestrated capillary endothelium
Three layers of the basement membrane:

— Lamina rara interna, which abuts the capillary endothelium — Lamina densa, which lies in the middle — Lamina rara externa, which is the outermost layer

Podocytes and their processes

— Filtration slit, and the diaphragm that covers it.

Capsular space.

Key functions of each of these layers:

The capillary endothelium blocks the passage of large molecules, including red blood cells, from entering the capsular space
The basement membrane blocks intermediate-sized molecules and negatively charged molecules
The podocytes of the visceral layer block smaller molecules
Thus, only the smallest, positively charged molecules enter the capsular space to become ultrafiltrate.

Features in a histological sample:

Identify the renal corpuscle as the collection of cells set off by the capsular space.
Label the parietal and visceral layers that enclose the capsular space; we can see the parietal epithelial cell nuclei and a podocyte of the visceral layer.
Indicate the tightly packed cells of the the macula densa, and the nearby afferent arteriole, which is characterized by its thick wall of smooth muscle. Notice the mesangial cells in the interstitial space.
Within the glomerulus, identify a capillary lumen; we can see the brighter-staining red blood cells in this preparation.
Overlying the capillaries, identify the glomerular basement membrane.

Images:

Histology (Mark Braun, MD, & Indiana University: http://medsci.indiana.edu/c602web/602/c602web/toc.htm; http://www.indiana.edu/~anat215/virtualscope2/start.htm)

Renal Corpuscle

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