Basement membrane

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Overview of the main cellular components and tissues.

The basement membrane is a thin, non-cellular, extracellular protein matrix that forms a specialized supporting sheet directly beneath the basal surfaces of all epithelial cells, separating them from underlying supporting tissues. In non-epithelial cells such as muscle, adipocytes and nervous tissue, the basement membrane is typically referred to as the external lamina.

Basement membranes are composed primarily of proteoglycans such as heparan sulphate, chondroitin sulfate, collagens, mainly type IV collagen, and structural glycoproteins including fibronectin, laminins and entactin/nidogen, which are mainly synthesized by the epithelial cells.

Basement membranes typically consist of two layers, the basal and reticular lamina, which are visible on electron microscopy. The dense basal lamina is closest to the basal aspect of the epithelium. It is about 40-120 nm thick and is composed of a network of fine fibrils of mainly laminin and type IV collagen polymers. On the other hand, the reticular laminar which lies beneath the basal lamina is thicker, more diffuse and fibrous with a high concentration of proteoglycans and type III collagen.

The basement membrane mediates physiological interactions between the avascular epithelium and underlying connective tissue. Nerve fibers extend close to, and may traverse, the basement membrane to directly supply epithelial cells. In contrast, capillaries run parallel to the basement membrane, allowing the exchange of oxygen, nutrients and other metabolic substances to and from the epithelium, without breaching the membrane.

The basement membrane performs several key functions:

• Allows epithelial cells to physically attach to underlying connective tissue and offers structural support for epithelial repair and regeneration.
• Establishes the polarity of epithelial cells and serve as a barrier limiting invasion of epithelial tissue into underlying tissues.
• Ensures the nourishment and the exchange of materials and metabolites with the epithelia which are avascular
• Acts as a selective barrier or permeability filter regulating the passage of substances between epithelium and supporting tissue, or from one compartment to another in specific locations
• Plays an important role in cellular processes such as cell adhesion, migration, differentiation, and proliferation