The mesangium is an inner layer of the glomerulus

Glomerulus

A glomerulus is a capillary tuft that is involved in the first step of filtering blood to form urine.A glomerulus is surrounded by Bowman's capsule, the beginning component of nephrons in the vertebrate kidney. A glomerulus receives its blood supply from an afferent arteriole of the renal...

, within the basement membrane surrounding the glomerular capillaries. This term is often used interchangeably with mesangial cell

Mesangial cell

Mesangial cells are specialized cells around blood vessels in the kidneys, at the mesangium. They are specialized smooth muscle cells that function to regulate blood flow through the capillaries, usually divided into two types, each having a very distinct function and location:* Extraglomerular...

, but in this context refers specifically to the intraglomerular mesangial cells. These cells are phagocytic and secrete the amorphous basement membrane

Basement membrane

The basement membrane is a thin sheet of fibers that underlies the epithelium, which lines the cavities and surfaces of organs including skin, or the endothelium, which lines the interior surface of blood vessels.- Composition :...

-like material known as the mesangial matrix. They are typically separated from the lumen

Lumen (anatomy)

A lumen in biology is the inside space of a tubular structure, such as an artery or intestine...

 of the capillaries by endothelial cells.