Basement Membrane: Definition


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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.

What is the basement membrane made of?

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.

Which cells make the basement membrane?

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.

What does the basement membrane do?

The basement membrane has several jobs in healthy tissue. The main ones include:

  • Support. It anchors the lining cells so they stay attached and arranged in an orderly layer.
  • Barrier. It separates the lining cells from the blood vessels and lymph channels below.
  • Filter. In the kidney, a specialized basement membrane in each filtering unit, called the glomerular basement membrane, helps filter waste from the blood.
  • Signals. It sends signals that help nearby cells grow, heal after injury, and stay in the right place.

Why is the basement membrane important in cancer?

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:

  • Precancerous change. Abnormal cells, such as dysplasia, stay above the basement membrane in part of the lining.
  • Carcinoma in situ. Cancer cells fill the lining but stay above the basement membrane, so this stage is also called non-invasive cancer.
  • Invasive cancer. Cancer cells have broken through the basement membrane and grown into the tissue below.

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.

Where else might the basement membrane appear in a pathology report?

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:

What does basement membrane-like material mean in a pathology report?

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:

  • Adenoid cystic carcinoma. This cancer, most common in the salivary glands, forms small round spaces that are often filled with basement membrane-like material.
  • Basal cell tumors of the salivary glands. Both the noncancerous basal cell adenoma and the cancerous basal cell adenocarcinoma can show bands and droplets of this material.
  • Epithelial-myoepithelial carcinoma. In this cancer, myoepithelial cells are often surrounded by a thick layer of basement membrane-like material.
  • Skin adnexal tumors. The noncancerous skin tumors cylindroma and spiradenoma often have thick pink bands around their cell nests and small droplets of the same material inside them.

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.

Questions to ask your doctor

  • Did the abnormal cells stay above the basement membrane?
  • Is my diagnosis a precancerous change, an in situ cancer, or an invasive cancer?
  • If there was invasion, how deep did it go?
  • Were any special stains done to look at the basement membrane?
  • Does the condition of the basement membrane change my treatment?
  • Did my report describe basement membrane-like material, and what does it suggest about my tumor?
  • What follow-up will I need?

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