The juxtaglomerular apparatus: anatomical considerations in feedback control of glomerular filtration rate.

The juxtaglomerular apparatus is an anatomical unit located at the hilus of the glomerulus and is believed to be involved in feedback control of renal blood flow and glomerular filtration rate. In the mammal, it consists of the glomerular arterioles and extraglomerular mesangium (the vascular component) and the macula densa (the tubular component). Juxtaglomerular granular cells are modified smooth muscle cells and are believed to be the source of renin. They are present in the vascular component, being most numerous in the afferent arteriole. The macula densa segment of the distal tubule is characterized by closely packed nuclei, the basal or lateral position of the Golgi apparatus, and dispersed mitochondria that show little association with the reduced infoldings of the basal membrane. In addition to these morphological differences, there are marked histochemical differences between the macula densa and the adjacent segments of the distal tubule. Tubule-vessel contact appears to vary in extent and ultrastructure. The macula densa forms extensive and complex contact with the extraglomerular mesangial region. Contact with the arterioles may consist of simple apposition of basement membranes. Fluorescent histochemistry and electron microscopy demonstrate a consistent monoaminergic innervation of the juxtaglomerular apparatus. Autoradiography shows that axons associated with the juxtaglomerular apparatus are able to incorporate exogenous tritiated norepinephrine. Nerve endings are observed on cells of the vascular component and less frequently on the tubules of the juxtaglomerular region.