The structure and innervation of the juxtaglomerular apparatus are examined by both light and electron microscopy, using series of semithin and thin sections. At the vascular pole of the renal corpuscle, granulated and nongranulated epithelioid cells, Goormaghtigh's cells and mesangial cells, as well as adjacent smooth muscle cells, are all connected to each other. Due to their fine structure, their topographical conti- nuity, and the numerous gradual stages of transformation, the different components of the vascular wall are defined as ramified smooth muscle cells (epithelioid cells), which...