Tissues - Epithelial Tissue Overview

Tissues are groups of cells with similar structure, working together to perform a common function.
There are four primary tissue types: Nervous, Epithelial, Muscle, and Connective tissue.

Location/Function: Covers exposed surfaces (e.g., skin), lines internal passageways (e.g., GI tract organs), and forms glands.
Cellular Structure: Cells are tightly packed, forming protective barriers. They exhibit structural and functional differences between their Apical (top) and Basal (bottom) surfaces.
Vascularity: Avascular; lacks blood vessels. Nutrients are received from underlying connective tissue via the basement membrane.
Regeneration: Possesses a very fast rate of regeneration.
Innervation: Typically has a rich supply of nerve endings.

Apical Surface Features:
- Goblet cells: Unicellular glands responsible for secretion.
- Cilia: Hair-like projections that move substances across the surface.
- Microvilli: Tiny projections that increase surface area for enhanced absorption and secretion.
- Keratinized cells: Dead cells filled with the protein keratin, providing protection against abrasion (e.g., outer skin layer).
Basal Surface Features (Basement Membrane):
- Composed of collagen and glycoprotein.
- Functions: Attaches epithelium to underlying connective tissue, provides space for cell mitosis, and regulates what enters the epithelium from below.

Epithelial tissues are classified based on the number of cell layers and the shape of the cells.
Based on Layers:
- Simple epithelium: Consists of a single layer of cells, primarily involved in absorption, diffusion, and reducing friction.
- Stratified epithelium: Composed of several layers of cells; named according to the shape of the cells at the free (apical) surface.
Based on Cell Shape:
- Squamous: Thin and flat cells.
- Cuboidal: Square-shaped cells.
- Columnar: Tall, slender, rectangular cells.
Common Types and Locations:
- Simple Squamous Epithelium: Facilitates absorption and diffusion (e.g., lungs, kidney tubules).
  - Mesothelium: A type of simple squamous epithelium that lines body cavities.
  - Endothelium: A type of simple squamous epithelium that lines the heart, blood, and lymphatic vessels.
- Simple Cuboidal Epithelium: Involved in secretion and absorption (e.g., glands, ducts, kidney tubules).
- Simple Columnar Epithelium: Primarily functions in absorption and secretion.