basement membrane
/ BAYS-ment MEM-brayn /
Picture wallpaper that needs a thin layer of backing felt between it and the wall — the felt is not the wall and not the paper, but a flat mat that the paper rests on and sticks to. Sheets of cells in the body, like the skin's surface or the lining of your gut, rest on just such a mat. The basement membrane is that thin, tough sheet of matrix lying directly beneath a layer of cells, supporting them and separating them from the tissue underneath.
Despite the name, the basement membrane is not a cell membrane at all — it is a dense, flat layer of extracellular matrix, only tens of nanometers thick. It is woven mainly from a sheet-forming type of collagen (type IV) and from laminin, knitted together with other matrix molecules into a continuous felt. The cells above it anchor onto it, often through hemidesmosomes that grip the laminin via integrins, so the cell sheet is firmly fastened down to this base.
Basement membranes matter as both barrier and boundary. They give epithelial sheets a stable foundation, act as selective filters (in the kidney, the basement membrane is part of what filters blood to make urine), and define the line that normal cells respect. That last point is important in cancer: a tumor is generally considered still local (in situ) as long as it stays above the basement membrane, and crossing or dissolving the basement membrane to invade the tissue below is a defining step toward malignancy. So this thin felt is a quiet but crucial line in the body's geography.
A pathologist deciding whether a breast tumor is still 'in situ' or has become invasive looks at one thing above all: whether the cancer cells have broken through the basement membrane into the tissue below.
Crossing the basement membrane marks the line from local to invasive cancer.
The basement membrane is matrix, not a lipid membrane; the similar-sounding name causes endless confusion with the plasma membrane, which is a completely different thing.