Macula densa, granular cells and extraglomerular mesangium
Expansion
Three components at the vascular pole, where the distal tubule returns to touch its own glomerulus:
- Macula densa: specialised cells of the thick ascending limb, appearing as a patch of crowded, taller cells with closely packed nuclei. Senses luminal sodium chloride
- Juxtaglomerular (granular) cells: modified smooth muscle in the afferent arteriole wall, containing renin granules that stain with PAS
- Extraglomerular mesangial (lacis) cells: between the two
This is the only site in the body where a tubule contacts its own arteriole, which is what permits tubuloglomerular feedback.
Histological changes are diagnostically useful: hyperplasia of granular cells occurs in renal artery stenosis, Bartter syndrome and any state of chronic renin stimulation, while they are reduced in primary hyperaldosteronism.
The macula densa is best seen on a PAS stain, which also highlights the tubular basement membranes and allows the segments to be distinguished.
Renin-secreting tumours (reninomas) arise from these cells and cause severe hypertension with hypokalaemia in a young patient.