Renal Histology Notes: Mesangium & Juxtaglomerular Apparatus

Q1. Functions of the Mesangium (with Diagram)

Diagram of the Mesangium

Fig. 19-7: Mesangial cell (green) in the interstice between glomerular capillaries, surrounded by basement membrane, podocyte processes, and endothelial cells. — Junqueira's Basic Histology, 17e

Functions of Mesangial Cells

Clinical note: In diseases such as IgA nephropathy and lupus nephritis, mesangial cells proliferate abnormally due to trapping of immune complexes in the GBM. — Histology: A Text and Atlas (Ross & Pawlina), 9e

Q2. Process of Mesangium Formation

Structure

• Mesangial cells reside within the renal corpuscle, enclosed by the GBM shared among several capillaries
• They are most prominent at the vascular stalk of the glomerulus and at the interstices of adjoining glomerular capillaries
• The mesangial cells are not confined entirely to the renal corpuscle — some extend outside at the vascular pole, where they are called lacis cells (extraglomerular mesangial cells), forming part of the juxtaglomerular apparatus

Cellular Characteristics

• Resemble vascular pericytes — they have contractile properties and produce components of an external lamina
• In routine sections, mesangial cells are difficult to distinguish from podocytes but often stain more darkly
• PAS stain (Periodic Acid-Schiff) clearly visualizes the GBM and positions mesangial nuclei between capillary loops, more toward the center of the glomerulus

Formation Process

1. Mesangial matrix is produced by the mesangial cells themselves — it differs substantially in composition from the GBM and allows larger molecules to pass
2. Mesangial cells extend contractile processes along capillaries — some processes pass between endothelial cells into the capillary lumen, where they endocytose adherent protein aggregates
3. The matrix provides scaffolding where the GBM is not present, bridging multiple capillary loops into a functional filtration unit

Fig 1.14B: Electron micrograph of a juxtaglomerular (granular) cell — the modified smooth muscle cell filled with renin granules. — Comprehensive Clinical Nephrology, 7e

Q3. Juxtaglomerular Apparatus (JGA) — Structure & Development

Components of the JGA

Fig 27.11: Structure of the juxtaglomerular apparatus showing its feedback role in nephron control. — Guyton & Hall Medical Physiology, 14e

Detailed Development of Each Component

• A plaque of specialized epithelial cells in the wall of the thick ascending limb of Henle where it attaches to the extraglomerular mesangium of the parent glomerulus
• Cells are narrowly packed with large nuclei (hence "macula densa" = dense spot)
• Anchored to a basement membrane that blends with the extraglomerular mesangial matrix
• Cells are joined by tight junctions with very low permeability and have prominent lateral intercellular spaces whose width varies under different functional conditions
• Contain high amounts of nNOS (neuronal nitric oxide synthase) and COX-2 (cyclooxygenase-2)

• Assembled in clusters within the terminal portion of the afferent arteriole, replacing ordinary smooth muscle cells
• Store renin in specific cytoplasmic granules; renin is released by exocytosis into surrounding interstitium
• Connected to extraglomerular mesangial cells, adjacent smooth muscle cells, and endothelium via gap junctions
• Under conditions of volume depletion or renal artery stenosis, additional upstream smooth muscle cells transform into granular cells to increase renin synthesis

• Firmly attached to the basal aspect of the macula densa
• Solid complex of cells and matrix penetrated by neither blood vessels nor lymphatic capillaries
• Processes interconnected by gap junctions; contain prominent bundles of microfilaments
• Connected to Bowman's capsule basement membrane and walls of both arterioles
• Interconnects all structures of the glomerular entrance as a signaling intermediary

Functions of the JGA

1. Tubuloglomerular Feedback (TGF):
   • Increased GFR → increased NaCl delivery to macula densa → activates Na⁺/K⁺/2Cl⁻ cotransporter → depolarization → ATP release → converted to adenosine → afferent arteriole constriction → decreased GFR (negative feedback loop)
   • Also suppresses renin release when NaCl is high
2. Renin-Angiotensin-Aldosterone System (RAAS):
   • Decreased GFR or blood pressure → decreased NaCl at macula densa → renin release from granular cells → angiotensin I → angiotensin II → systemic vasoconstriction + aldosterone release → increases blood pressure and extracellular fluid volume
3. Autoregulation of GFR — maintains relatively constant GFR despite fluctuations in systemic blood pressure, through both the TGF mechanism and myogenic responses of the afferent arteriole.

• Junqueira's Basic Histology: Text and Atlas, 17e, pp. 962–964
• Histology: A Text and Atlas (Ross & Pawlina), 9e, pp. 1885–1886
• Comprehensive Clinical Nephrology, 7e, pp. 31–32
• Guyton and Hall Textbook of Medical Physiology, 14e, pp. 3819–3831