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 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. Extraglomerular mesangial cells are in close connection to afferent and efferent arteriolar cells by gap junctions, allowing for intercellular communication. Mesangial cells are separated by intercellular spaces containing extracellular matrix called the mesangial matrix that is produced by the mesangial cells. Mesangial matrix provides structural support for the mesangium. Mesangial matrix is composed of glomerular matrix proteins such as collagen IV (α1 and α2 chains), collagen V, collagen VI, laminin A, B1, B2, fibronectin, and proteoglycans. 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. It is possible that they share the same origin as supporting cells such as pericytes and vascular smooth muscle cells, or even be a type of specialised vascular smooth muscle cell. Function Formation of capillary loops during development During development mesangial cells are important in the formation of convoluted capillaries allowing for