The basement membrane is a very thin sheet of tissue that sits under the epithelium, the layer of cells that lines the skin and the inside of organs. It acts like a foundation, holding the lining cells in place and separating them from the tissue below. Pathologists pay close attention to the basement membrane because it marks the line between an early, non-invasive change and a cancer that has started to spread.
This article explains what the basement membrane is, which cells make it, how pathologists use it to describe invasion, and what basement membrane-like material means in a report.
The basement membrane consists mostly of proteins, including a special type of collagen called type IV collagen and a protein called laminin. These proteins link together to form a strong but flexible mesh. Epithelial cells attach to the top of this mesh, and supporting tissue called stroma lies underneath it. Basement membranes also surround blood vessels, muscle cells, fat cells, and nerves.
The cells that sit directly on top of the basement membrane make most of it. In the skin and other linings with several layers of cells, this job falls to the basal cells, the bottom layer that rests on the basement membrane. In glands such as the salivary glands and the breast, the outer layer of basal or myoepithelial cells makes the basement membrane that wraps around each gland. Cells in the supporting tissue underneath, such as fibroblasts, also add some of these proteins.
Because basal cells and myoepithelial cells make this material, tumors built from these cells often keep the same ability. Some of these tumors make far more basement membrane than normal tissue does, and the pathologist can see the extra material under the microscope.
The basement membrane has several jobs in healthy tissue. The main ones include:
The basement membrane is important in cancer because crossing it is what turns an abnormal growth into an invasive cancer. Cells that stay above an intact basement membrane cannot reach the blood vessels and lymph channels in the stroma. Once tumor cells break through it, they can spread to lymph nodes and other parts of the body.
Pathologists use the basement membrane to sort abnormal growths into a few groups:
The basement membrane is too thin to see easily on routine hematoxylin and eosin slides. The pathologist usually decides whether it is intact by looking at the shape and position of the cells. Special stains or immunohistochemistry can highlight it when the answer is unclear.
The basement membrane can also appear in reports for noncancer conditions, especially in the kidney. In these settings, the pathologist usually describes it as thickened or damaged. Examples include:
Basement membrane-like material is extra basement membrane made by tumor cells, usually tumors that contain basal cells or myoepithelial cells. Under the microscope, it looks like smooth, glassy pink material, and pathologists may also describe it as hyaline material, hyaline globules, or reduplicated basement membrane. The material may form thick bands around groups of tumor cells, small round droplets between the cells, or pools that fill small spaces inside the tumor. Tumors that often make this material include:
Basement membrane-like material helps the pathologist narrow down the tumor type, but it does not by itself mean the tumor is cancerous. Noncancerous and cancerous tumors can both make it, so the pathologist decides the diagnosis using the tumor’s growth pattern, its edges, and other test results.