The macula densa, The afferent arteriole, and The lacis cells

By Supriyadi Pro -
The macula densa - As the distal tubule approaches its own glomerulus, the cells on the side facing the vascular pole of the glomerulus become modified to form the macula densa. The cells are taller and narrower than normal distal tubule cells, although this feature is less well marked in the human kidney than in other species.

The infoldings of cell membrane near the base of the cell become less deep and the mitochondria diminish in size and number. The intercellular spaces may he dilated, an appearance which in other situations is known to indicate transepithelial transport of salt and water.

Experimental evidence suggests that the macula densa is the receptor through which changes in the composition of distal tubular fluid can affect the activities of the juxtaglomerular apparatus and it is presumed that this involves the transepithelial transport of ions.

However, Beeuwkcs et al. (1975) have found that while normal distal tubule cells show high Na-K-ATP-ase activity, this enzyme cannot be demonstrated in the macula densa cells, either under normal conditions or in sodium loading or deprivation.

The afferent arteriole
The structure of the proximal part of the afferent arteriole has been described on page 7. As it approaches the glomerulus, it loses it selastic tissue, and its smooth muscle cells are gradually replaced by more or less spherical epithelioid cells (juxtaglomerular cells), many of which contain characteristic granules.

The granules are spherical or oval in shape and vary in number according to the species. They are not particularly prominent in the human kidney. Various other types of cytoplasmic bodies may be present, in particular lysosomes, residual bodies and lipofuscin granules (see Rouiller & Orci 1971 for references).

The granules do, in fact, show many similarities to lysosomes in their cytochemical reactions and it has been suggested that the two types of organelle are identical. A number of important differences have, however, been demonstrated (Cantin et al. 1975).

The lacis cells
These cells, sometimes known as Goormaghtigh cells, fill in the rather conical space bounded by the afferent and efferent arterioles and the macula densa. On the glomerular side, the lacis cells are continuous with the glomerular mesangial cells and it may be difficult to distinguish between them. The lacis cells themselves are arranged in stacks and have long slender cytoplasmic processes. They are enmeshed in a network of basement membrane.

Read the full article: The Anatomy of the Renal Circulation

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