Biology: Epithelial Tissue Features

General Features of Epithelial Tissue

• Intercellular space is very limited, approximately 20 nm.
• Epithelial tissue is usually avascular, and cells receive nourishment through diffusion from adjacent supporting tissue.
• Cells rest on a basement membrane.
• Cells exhibit polarity and surface modifications.
• Epithelial tissue has a good regenerative capacity, and metaplasia (change from one type to another) can occur.
• Epithelial tissue is derived from all three germ layers.

Types of Epithelia

• Epithelial tissue can be divided into two main types: covering (lining) epithelia and secretory (glandular) epithelia.

Covering Epithelia

• Classified into two categories based on the number of cell layers and cell morphology in the outer layer: simple epithelia and stratified epithelia.
• Simple epithelia consist of a single cell layer resting on the basement membrane.
• Stratified epithelia consist of two or more cell layers, with the basal layer resting on the basement membrane.

Types of Simple Epithelia

• Classified into three categories based on cell shape:

Simple Squamous Epithelia

• Composed of a single layer of flattened cells.
• Cell height is very little compared to cell width.
• Nuclei are oval and centrally located.
• Functions: facilitates rapid transport of substances, diffusion of gases, and filtration of fluids.
• Examples: Alveoli of the lungs, endothelium lining blood vessels.

Simple Cuboidal Epithelia

• Cells have approximately equal height and width.
• Nuclei are usually rounded and centrally placed.
• Functions: primarily concerned with secretory, absorptive, and excretory functions.
• Examples: Thyroid gland follicles, ducts of large glands.

Simple Columnar Epithelia

• Cells are taller than they are wide.
• Nuclei are elongated and located in the lower half of the cells.
• In some cases, the cell surface bears cilia.
• Examples: Lining epithelium of the fallopian tube.