The Juxtaglomerular Apparatus Is the Site of Renin Production

Each nephron forms a loop, and the thick ascending limb touches the vascular pole of the glomerulus (see Fig. 23.2). At this site is the juxtaglomerular apparatus, a region com-

Macula densa

Thick ascending limb

Granular cell

Nerve

Afferent arteriole

Macula densa

Afferent arteriole

Juxtaglomerular Apparatus

Efferent arteriole

Glomerular capillary

Glomerular capillary

Efferent arteriole

Extraglomerular mesangial cell

Bowman's capsule

Mesangial cell

Histological appearance of the juxtaglomerular apparatus. A cross section through a thick ascending limb is on top and part of a glomerulus is below. The juxtaglomerular apparatus consists of the macula densa, extraglomerular mesangial cells, and granular cells. (From Taugner R, Hackenthal E. The Juxtaglomerular Apparatus: Structure and Function. Berlin: Springer, 1989.)

prised of the macula densa, extraglomerular mesangial cells, and granular cells (Fig. 23.4). The macula densa (dense spot) consists of densely crowded tubular epithelial cells on the side of the thick ascending limb that faces the glomerular tuft; these cells monitor the composition of the fluid in the tubule lumen at this point. The extraglomerular mesangial cells are continuous with mesangial cells of the glomerulus; they may transmit information from macula densa cells to the granular cells. The granular cells are modified vascular smooth muscle cells with an epithelioid appearance, located mainly in the afferent arterioles close to the glomerulus. These cells synthesize and release renin, a proteolytic enzyme that results in angiotensin formation (see Chapter 24).

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