Tissue: The Living Fabric – Structure and Function of Human Tissues

Introduction to Tissues

Tissues are groups of cells similar in structure that perform a common or related function. The study of tissues is known as histology. Each tissue type is specialized to perform specific functions that contribute to the maintenance of homeostasis in the body.

• Tissue: A group of cells with similar structure and function.

• Histology: The scientific study of tissues.

• There are four basic tissue types in the human body: epithelial, connective, muscle, and nervous tissue.

Overview of the Four Basic Tissue Types

Tissue Type	Main Function	Examples/Locations
Epithelial	Protection, absorption, filtration, excretion, secretion, sensory reception	Skin, lining of digestive tract, glands
Connective	Support, protection, binding, insulation, transportation	Bones, tendons, fat, blood
Muscle	Movement	Skeletal muscles, heart, walls of hollow organs
Nervous	Internal communication	Brain, spinal cord, nerves

Epithelial Tissue

Definition and Forms

Epithelial tissue (or epithelium) is a sheet of cells that covers body surfaces or lines body cavities. It serves as a boundary between different environments and has two main forms:

• Covering and lining epithelia: Found on external and internal surfaces (e.g., skin, mucous membranes).

• Glandular epithelia: Forms the secretory tissue in glands (e.g., salivary glands).

Main functions: Protection, absorption, filtration, excretion, secretion, and sensory reception.

Key Characteristics of Epithelial Tissue

• Polarity: Epithelial cells have an apical (top) surface and a basal (bottom) surface, each with distinct structures and functions.

• Specialized contacts: Cells are closely packed and held together by tight junctions and desmosomes.

• Supported by connective tissue: All epithelial sheets rest upon and are supported by connective tissue (basement membrane).

• Avascular but innervated: Contains no blood vessels but is supplied by nerve fibers; nutrients diffuse from underlying tissues.

• High regenerative capacity: Rapid cell division to replace lost or damaged cells.

Classification of Epithelia

Epithelia are classified by the number of cell layers and the shape of the cells at the apical surface.

• Layers:

  • Simple epithelia: Single layer of cells (for absorption, secretion, filtration).

  • Stratified epithelia: Two or more layers (for protection).

• Cell shapes:

  • Squamous: Flattened and scale-like.

  • Cuboidal: Box-like, as tall as they are wide.

  • Columnar: Tall and column-shaped.

Examples of Epithelial Types and Locations

Type	Structure	Location	Function
Simple Squamous	Single layer, flat cells	Air sacs of lungs, lining of heart, blood vessels	Allows diffusion and filtration
Simple Cuboidal	Single layer, cube-shaped	Kidney tubules, ducts, small glands	Secretion and absorption
Simple Columnar	Single layer, tall cells	Digestive tract, gallbladder	Absorption, secretion of mucus/enzymes
Pseudostratified Columnar	Single layer, appears stratified	Trachea, upper respiratory tract	Secretion, propulsion of mucus
Stratified Squamous	Multiple layers, flat cells at surface	Skin, mouth, esophagus	Protection
Transitional	Multiple layers, shape varies	Urinary bladder	Stretches readily

Glandular Epithelia

• Gland: One or more cells that make and secrete a particular product (secretion).

• Endocrine glands: Ductless; secrete hormones directly into the blood.

• Exocrine glands: Secrete products into ducts (e.g., sweat, oil, salivary glands).

• Unicellular glands: Goblet cells (produce mucus).

• Multicellular glands: Composed of a duct and a secretory unit; classified by structure and mode of secretion.

Modes of Secretion

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

• Holocrine: Accumulate products until the cell ruptures (e.g., sebaceous glands).

• Apocrine: Accumulate products, then the apex of the cell pinches off (controversial in humans).

*Additional info: Apocrine secretion is well-documented in some animals, but its presence in humans is debated.*