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. Mice lacking the growth factor PDGF-B or PDGFRβ do not develop mesangial cells. When mesangial cells are absent the blood vessel becomes a single dilated vessel with up to 100-fold decrease in surface area. The transcription factor for PDGFRβ, Tbx18, is crucial for the development of mesangial cells. Without Tbx18 the development of mesangial cells is compromised and results in the formation of dilated loops. Mesangial cell progenitors are also a target of PDGF-B and can be selected for by the signal to then develop into mesangial cells. 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. Mesangial cells aid filtration by constituting part of the glomerular capillary tuft structure that filters fluids to produce urine. 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. Contractions regulate capillary flow Mesangial cells can contract and relax to regulate capillary flow. This is regulated by vasoactive substances. Contraction of mesangial cells is dependent on cell membrane permeability to calcium ions and relaxation is mediated by paracrine factors, hormones and cAMP. In response to capillary stretching, mesangial cells can respond by producing several growth factors: TGF-1, VEGF and connective tissue growth factor. Removal of macromolecules The mesangium is exposed to macromolecules from the capillary lumen as they are separated only by fenestrated endothelium without basement membrane. Mesangial cells play a role in restricting macromolecules from accumulating in the mesangial space by receptor- independent uptake processes of phagocytosis, micro- and macro-pinocytosis, or receptor-dependent processes and then transported along the mesangial stalk. Size, charge, concentration, and affinity for mesangial cell receptors of the macromolecule affects how the macromolecule is removed. Triglycerides may undergo pinocytosis and antibody IgG complexes may lead to activation of adhesion molecules and chemokines by mesangial cells. They also regulate glomerular filtration Clinical significance Diabetic nephropathy The expansion of mesangial matrix is