Define epithelium and list its common features. Classify epithelia by cell shape, layers, nucleus, and surface specializations, providing examples and functions.

1. Define "epithelium" and list the features common to all epithelia.

Epithelium is a type of animal tissue that forms the outer covering of all internal and external body surfaces and lines body cavities and hollow organs. It is primarily involved in protection, secretion, absorption, excretion, filtration, diffusion, and sensory reception.

Common features of all epithelia:

• Cellularity: Composed almost entirely of cells with very little extracellular material.
• Polarity: Cells have distinct apical (free) and basal (attached) surfaces.
• Specialized Contacts: Cells are joined by tight junctions and desmosomes.
• Supported by Connective Tissue: Rests on a basement membrane, which is an adhesive sheet formed by both epithelial and connective tissue cells.
• Avascular but Innervated: Contains no blood vessels but is supplied by nerve fibers.
• Regeneration: Has a high capacity for regeneration.

2. In a tabular form, classify epithelia using the following features:

• a. Cell Shape and layers (Squamous, Cuboidal, Columnar, Pseudocolumnar, and Transitional)
• b. Nucleus Shape & Position
• c. Surface Specialisations if present
• d. Examples and Functions

| Epithelial Type | Cell Shape & Layers