What is the covering of epithelial tissue classified by?

• A. The thickness of the basement membrane
• B. The number of layers of cells and the shape of surface cells
• C. The presence of nerve supply
• D. The vascularity of tissue

Answer: (B)

Epithelial tissue classification hinges on two features: how many layers there are and the shape of the cells at the surface. If there’s a single layer, it’s simple epithelium; multiple layers form stratified epithelium; there’s also pseudostratified epithelium, which looks layered but all cells touch the basement membrane. The shape of the surface cells—squamous (flat), cuboidal (cube-like), or columnar (tall)—further defines the type, leading to names like simple squamous, stratified squamous, simple cuboidal, or simple columnar. The basement membrane’s thickness, nerve supply, or vascularity aren’t used to classify epithelia: epithelia are avascular, receiving nutrients by diffusion from underlying tissue, and the basement membrane is just the boundary between tissues. So the best way to classify the covering of epithelial tissue is by the number of layers and the shape of the surface cells.