The three "classical" components of the juxtaglomerular apparatus are the myoepithelioid cells, the macula densa cells and the lacis cells, which have been studied for over half a century. This complex multicellular structure is situated in the periglomerular position of every glomerulus, and is responsible for the intrinsic renal control of glomerular filtration rate and tubular reabsorption of sodium, and the regulation of body fluid volume and blood pressure. Complex cellular interactions occur within the juxtaglomerular apparatus. These cells also exhibit physiological relationships with other tissues of the body; but detailed mechanism are still unclear. Ten years ago, a distinctive type of cell, the peripolar cell, was recognised in close anatomical relationship with the juxtaglomerular apparatus. Morphological evidence indicates that the peripolar cell is a secretory type of cell but the substance of the secretory product is unknown. The available data suggests that the peripolar cell might be a previously unrecognised but integral component of the juxtaglomerular apparatus. Its secretory product may be responsible for the hormonal control of renal proximal tubular reabsorption, although this remains speculative
