Juxtaglomerular

Juxtaglomerular refers to the juxtaglomerular apparatus and its cells, a specialized structure in the kidney that regulates nephron filtration rate and blood pressure. It consists of three main components: juxtaglomerular (JG) cells, which are modified smooth muscle cells of the afferent arteriole; the macula densa, a group of tall, closely packed cells in the distal tubule; and extraglomerular mesangial (Lacis) cells. The apparatus sits at the vascular pole of each glomerulus, where the afferent arteriole contacts the glomerular capillaries.

Functionally, the juxtaglomerular apparatus controls the glomerular filtration rate (GFR) and renal blood flow through tubuloglomerular feedback and the renin–angiotensin–aldosterone system (RAAS). JG cells synthesize and release renin, a key enzyme in RAAS. Macula densa cells sense sodium chloride delivery to the distal tubule and communicate with JG cells to regulate renin release. Low NaCl delivery or increased sympathetic activity (beta-1 adrenergic stimulation) promotes renin release, aided by prostaglandin E2; high NaCl delivery or adenosine signaling from the macula densa inhibits renin release. Renin initiates a cascade that produces angiotensin II and aldosterone, increasing blood pressure and adjusting GFR.

Clinical relevance includes potential involvement in hypertension and kidney function disorders. Rare renin-secreting tumors (reninomas) arise from JG cells and can cause secondary hypertension. The term juxtaglomerular denotes proximity to the glomerulus and is used to describe both the cells and the regulating apparatus.