Introduction to Tissues

Definition and Primary Types

Tissues are groups of closely associated cells that perform related functions and are similar in structure. In the human body, there are four primary tissue types, each with distinct roles:

• Epithelium – coverings

• Connective tissue – support

• Nervous tissue – control

• Muscle – movement

Most tissues contain cells suspended in an extracellular matrix, which is material located outside the cell. This matrix is composed of proteins produced and secreted by tissue cells, as well as interstitial fluid.

Introduction to Epithelial Tissue

General Characteristics and Functions

Epithelial tissue covers body surfaces or lines body cavities. It forms boundaries between different environments and is essential for the passage of substances into and out of the body. Most glands are derived from epithelial tissue, and it lines both the outside and inside of most organs. Epithelial tissue contains little to no extracellular matrix.

• Protection – shields underlying tissues from mechanical and chemical injury.

• Absorption, secretion, and ion transport – regulates exchange of substances.

• Filtration – allows selective movement of materials.

Special Characteristics of Epithelia

Structural Features

• Cellularity – Epithelial cells are closely packed with minimal extracellular material.

• Specialized contacts – Adjacent cells are joined by specialized junctions (e.g., tight junctions, desmosomes).

• Polarity – Epithelial cells have an apical (exposed) surface and a basal (attached) surface, which differ structurally and functionally.

• Apical surface modifications – May have cilia (for movement) or microvilli (to increase surface area).

• Avascular – Epithelia lack blood vessels and receive nutrients by diffusion from underlying connective tissue.

• Innervated – Epithelia have nerve endings for sensation.

• Attachment – All epithelial sheets are supported and attached to underlying connective tissue by a basal membrane (composed of basal lamina and reticular fibers).

• Regeneration – Epithelial cells have a high capacity for renewal by cell division.

Example: The lining of the small intestine is composed of epithelial tissue with microvilli to maximize nutrient absorption.

Summary Table: Key Characteristics of Epithelial Tissue

Additional info: The basal membrane provides structural support and acts as a selective filter for molecules passing between the epithelium and underlying tissues.