Epithelial Tissue

Overview

Epithelial tissue is one of the four major tissue types in the human body, serving essential roles in protection, absorption, secretion, and filtration. It forms the covering or lining of all internal and external body surfaces.

• Functions: Protection, absorption, secretion, filtration.

• Locations: Covers body surfaces, lines cavities, forms glands.

• Associated Tissues: Connective (support), muscle (movement), nervous (communication).

Characteristics of Epithelial Tissue

• Polarity: Has an apical (free) surface and a basal (attached) surface.

• Specialized Contacts: Cells are joined by tight junctions and desmosomes.

• Supported by Basement Membrane: The basal surface is attached to a basement membrane (composed of epithelial and connective tissue fibers).

• Avascular but Innervated: Contains no blood vessels but is supplied by nerve fibers.

• High Regeneration Capacity: Rapidly replaces lost or damaged cells.

Classification of Epithelia

Epithelia are classified based on the number of cell layers and the shape of the cells.

• Layers:

  • Simple: One cell layer thick.

  • Stratified: Multiple cell layers.

• Cell Shapes:

  • Squamous: Flat and scale-like.

  • Cuboidal: Cube-shaped.

  • Columnar: Tall and column-like.

• Special Types: Pseudostratified (appears layered but is not), transitional (changes shape).

Examples of Epithelial Types

• Simple Squamous: Diffusion/filtration (lungs, capillaries).

• Simple Cuboidal: Absorption/secretion (kidneys, glands).

• Simple Columnar: Absorption/secretion (digestive tract).

• Pseudostratified Columnar: Secretion, movement (respiratory tract).

• Stratified Squamous: Protection (skin, mouth, esophagus).

• Transitional: Stretch (bladder).

Glandular Epithelium

Glandular epithelium forms glands that produce and secrete substances such as hormones, sweat, and saliva.

• Endocrine Glands: Secrete hormones directly into blood or lymph.

• Exocrine Glands: Secrete products onto body surfaces or into body cavities (e.g., sweat, saliva, mucus).

• Unicellular Glands: Single cells (e.g., goblet cells).

• Multicellular Glands: Composed of many cells, classified by duct structure and secretory units.

• Secretion Types:

  • Merocrine: Secrete by exocytosis (e.g., sweat glands).

  • Holocrine: Entire cell ruptures to release secretion (e.g., sebaceous glands).

  • Apocrine: Apex of cell pinches off (e.g., some sweat glands; additional info: apocrine secretion is less common in humans).

Skin Cancer & Burns

Skin Cancer

Skin cancer is the uncontrolled growth of abnormal skin cells. It is commonly caused by excessive exposure to ultraviolet (UV) radiation.

• Basal Cell Carcinoma: Most common, least malignant, slow-growing.

• Squamous Cell Carcinoma: Readily spreads, can be cured if detected early.

• Melanoma: Most dangerous, spreads rapidly, early detection is critical.

Burns

Burns are injuries to tissues caused by heat, chemicals, electricity, or radiation. They are classified by depth and severity.

• 1st Degree: Affects only the epidermis; redness, mild pain (e.g., sunburn).

• 2nd Degree: Involves epidermis and part of the dermis; blisters, more severe pain.

• 3rd Degree: Full thickness burn; destroys entire skin layer, may appear white or charred, no pain due to nerve damage.

• 4th Degree: Extends into muscle and bone; often requires amputation.

Table: Comparison of Burn Degrees

Key Terms

• Apical Surface: The free surface of epithelial tissue exposed to the body exterior or cavity.

• Basement Membrane: A thin, fibrous layer anchoring epithelium to underlying connective tissue.

• Avascular: Lacking blood vessels.

• Innervated: Supplied by nerve fibers.

• Metastasis: Spread of cancer cells to distant sites.