juxtaglomerular cell

**Location/Structure:** JG cells are specialized, granular, myoepithelioid (modified vascular smooth muscle) cells located mainly in the tunica media of the afferent arteriole, just before it enters the glomerulus. Together with the macula densa (specialized distal tubule cells in the thick ascending limb/early DCT) and extraglomerular mesangial (lacis/Goormaghtigh) cells, they form the juxtaglomerular apparatus (JGA).

**Function:** Synthesize, store (in secretory granules), and release renin, an aspartyl protease. Renin cleaves angiotensinogen (from liver) to angiotensin I → ACE (lung endothelium) converts to angiotensin II → vasoconstriction (efferent > afferent), aldosterone release (zona glomerulosa), ADH release, thirst, and increased Na+/H2O reabsorption (proximal tubule via Na+/H+ exchange). Net effect: raises BP and GFR/filtration fraction.

**Three main stimuli for renin release:** 1. ↓ Renal perfusion pressure (intrarenal baroreceptor function of JG cells; afferent arteriolar stretch ↓) 2. ↑ Sympathetic tone via β1-receptors (Gs → ↑cAMP) 3. ↓ NaCl delivery to the macula densa (sensed via Na+/K+/2Cl− cotransporter, NKCC2) → macula densa releases prostaglandins (PGE2/PGI2, via COX-2) and NO → ↑ renin release. (Macula densa also mediates tubuloglomerular feedback: ↑NaCl → adenosine → afferent vasoconstriction.)

**Inhibition of renin release:** Angiotensin II (negative feedback), ↑ afferent arteriolar pressure, ↑ NaCl at macula densa, ↑ adenosine, ANP, and β-blockers (block β1), and direct renin inhibitors (aliskiren). NSAIDs reduce prostaglandin-mediated renin release (can cause hyporeninemic hypoaldosteronism/type 4 RTA and hyperkalemia) and constrict the afferent arteriole → AKI risk.

**Clinical correlations:** - **Renin-secreting tumor (reninoma/JG cell tumor):** rare; young hypertensive patient with ↑ renin, ↑ aldosterone, hypokalemia, and metabolic alkalosis (secondary hyperaldosteronism). - **Renal artery stenosis (renovascular hypertension):** ↓ perfusion → ↑ renin → secondary hyperaldosteronism; "string of beads" in fibromuscular dysplasia (young women), atherosclerosis in older patients. ACE inhibitors/ARBs can precipitate AKI in bilateral RAS (efferent dilation drops GFR). - **Hypovolemia/hemorrhage/CHF/cirrhosis:** ↑ renin. - **Primary hyperaldosteronism (Conn):** ↓ renin (suppressed by volume expansion); high aldosterone-to-renin ratio. - **Diabetic nephropathy/hyporeninemic hypoaldosteronism (type 4 RTA):** JG cell dysfunction → ↓ renin, hyperkalemia, non-anion gap metabolic acidosis. - **Bartter syndrome (NKCC2 defect, mimics loop diuretics)**: macula densa senses "low NaCl" → ↑ renin, hypokalemic metabolic alkalosis, hypercalciuria. Loop diuretics also stimulate renin release by blocking NKCC2.

**Histology tip:** JG cells appear with prominent eosinophilic/granular cytoplasm (renin granules) and are innervated by sympathetic nerves. Macula densa cells appear as a crowded, darker-staining "dense" nuclear patch.

**High-yield memory aids:** "JG cells = Juice (renin) Generators"; macula densa = "salt sensor"; JG cells = "pressure sensor" (baroreceptor).