Mesangial cells are specialised cells in the kidney that make up the mesangium of the glomerulus. Together with the mesangial matrix, they form the vascular pole of the renal corpuscle. The mesangial cell population accounts for approximately 30-40% of the total cells in the glomerulus. Mesangial cells can be categorized as either extraglomerular mesangial cells or intraglomerular mesangial cells, based on their relative location to the glomerulus. The extraglomerular mesangial cells are found between the afferent and efferent arterioles towards the vascular pole of the glomerulus. The extraglomerular mesangial cells are adjacent to the intraglomerular mesangial cells that are located inside the glomerulus and in between the capillaries. The primary function of mesangial cells is to remove trapped residues and aggregated protein from the basement membrane thus keeping the filter free of debris. The contractile properties of mesangial cells have been shown to be insignificant in changing the filtration pressure of the glomerulus.
Structure
thumb|468x468px|Mesangial cells (colored in purple) as seen within intraglomerular and extraglomerular mesangium.
Mesangial cells have irregular shapes with flattened-cylinder-like cell bodies and processes at both ends containing actin, myosin and actinin, giving mesangial cells contractile properties. The anchoring filaments from mesangial cells to the glomerular basement membrane can alter capillary flow by changing glomerular ultrafiltration surface area.
Development
It is unclear whether the mesangial cells originate from mesenchymal or stromal cells. However there is evidence suggesting that they originate elsewhere outside of the glomerulus and then migrate into the glomerulus during development. Human foetal and infant kidneys stained for alpha smooth muscle actin (α-SMA), a marker for mesangial cells, demonstrated that α-SMA-positive mesenchymal cells migrate towards the glomerulus and during a later stage they can be found within the mesangium.
Function
Formation of capillary loops during development
During development mesangial cells are important in the formation of convoluted capillaries allowing for efficient diffusion to occur. Endothelial precursor cells secrete platelet-derived growth factor (PDGF)-B and mesangial cells have receptors for PDGF. This induces mesangial cells to attach to endothelial cells causing developing blood vessels to loop resulting in convoluted capillaries.
Interactions with other renal cells
Mesangial cells form a glomerular functional unit with glomerular endothelial cells and podocytes through interactions of molecular signalling pathways which are essential for the formation of the glomerular tuft. Communication between mesangial cells and vascular smooth muscle cells via gap junctions helps regulate the process of tubuloglomerular feedback and urine formation. Damage to mesangial cells using Thy 1-1 antibody specific to mesangial cells causes the vasoconstriction of arterioles mediated by tubuloglomerular feedback to be lost. Contraction of mesangial cells is dependent on cell membrane permeability to calcium ions and relaxation is mediated by paracrine factors, hormones and cAMP. Triglycerides may undergo pinocytosis and antibody IgG complexes may lead to activation of adhesion molecules and chemokines by mesangial cells. Mesangial expansion occurs due to increased deposition of extracellular matrix proteins, for example fibronectin, into the mesangium. Mesangial cells grown on advanced glycosylation end product-modified matrix proteins demonstrate increased production of fibronectin and a decrease in proliferation.
Mesangial pathologies may also develop during the early phase of diabetes. Glomerular hypertension causes mesangial cells to stretch which causes induced expression of GLUT1 leading to increased cellular glucose.