Tissues: The Living Fabric – Structure and Classification of Epithelial Tissue

Tissue: The Living Fabric

Introduction to Tissues

Tissues are groups of cells that are similar in structure and perform common or related functions. The study of tissues is known as histology. Each tissue type is specialized to carry out specific functions that help maintain homeostasis in the body.

• Epithelial tissue – Covers body surfaces and lines cavities.

• Connective tissue – Supports, protects, and binds other tissues together.

• Muscle tissue – Produces movement through contraction.

• Nervous tissue – Controls and integrates body functions through electrical signals.

Additional info: These four basic tissue types are foundational to the structure and function of organs throughout the human body.

Epithelial Tissue

Functions and Locations

Epithelial tissue forms the boundaries between different environments, protects, secretes, absorbs, and filters. It is found:

• On the skin surface (epidermis)

• Lining the digestive tract and other hollow organs

• Covering organs and body cavities

Additional info: Epithelial tissue also forms glands and is involved in sensory reception.

Characteristics of Epithelial Tissue

• Polarity: Epithelial cells have an apical surface (upper, free, exposed to exterior or cavity) and a basal surface (lower, attached to underlying connective tissue).

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

• Supported by connective tissue: The basement membrane reinforces the epithelial sheet and defines its boundary.

• Avascular but innervated: Epithelial tissue contains no blood vessels but is supplied by nerve fibers. Nutrients diffuse from underlying connective tissue.

• Regeneration: Epithelial cells have a high regenerative capacity, especially when exposed to friction or injury.

Classification of Epithelia

Epithelial tissues are classified based on two criteria: the number of cell layers and the shape of the cells.

Number of Cell Layers

• Simple epithelium: Single layer of cells; typically involved in absorption, secretion, and filtration.

• Stratified epithelium: Two or more layers; more durable and protective.

Cell Shape

• Squamous: Flat and scale-like.

• Cuboidal: Cube-shaped.

• Columnar: Tall and column-shaped.

Each epithelial tissue is named according to its cell shape and number of layers, e.g., simple squamous epithelium, stratified cuboidal epithelium.

Examples of Epithelial Tissue Types

• Simple squamous epithelium: Single layer of flat cells; found in air sacs of lungs, lining of heart, blood vessels.

• Simple cuboidal epithelium: Single layer of cube-shaped cells; found in kidney tubules, ducts, and small glands.

• Simple columnar epithelium: Single layer of tall cells; found in digestive tract, gallbladder, and some glands.

• Stratified squamous epithelium: Multiple layers of flat cells; found in skin (epidermis), mouth, esophagus, vagina.

Additional info: Some epithelial tissues have specialized structures such as microvilli (increase surface area for absorption) or cilia (move substances across the surface).

Table: Classification of Epithelial Tissues

Type	Number of Layers	Cell Shape	Main Locations	Main Functions
Simple Squamous	1	Flat	Air sacs of lungs, lining of heart, blood vessels	Diffusion, filtration
Simple Cuboidal	1	Cube-shaped	Kidney tubules, ducts, small glands	Secretion, absorption
Simple Columnar	1	Tall, column-shaped	Digestive tract, gallbladder	Absorption, secretion
Stratified Squamous	2 or more	Flat (surface), cuboidal/columnar (deeper)	Skin, mouth, esophagus, vagina	Protection
Stratified Cuboidal	2 or more	Cube-shaped	Some ducts (e.g., sweat glands)	Protection
Stratified Columnar	2 or more	Column-shaped	Male urethra, some glands	Protection, secretion
Transitional	Multiple	Varies (can change shape)	Urinary bladder, ureters	Stretching, protection

Additional info: Transitional epithelium is specialized to stretch and is found in organs of the urinary system.

Key Terms and Definitions

• Histology: The study of tissues.

• Apical surface: The upper free surface of an epithelial cell exposed to the body exterior or cavity.

• Basal surface: The lower surface attached to the underlying connective tissue.

• Basement membrane: A thin layer that anchors epithelium to connective tissue.

• Microvilli: Finger-like extensions of the plasma membrane that increase surface area for absorption.

• Cilia: Hair-like projections that move substances across the epithelial surface.

Summary Table: Four Basic Tissue Types

Tissue Type	Main Function	Example Location
Epithelial	Covers surfaces, lines cavities	Skin, lining of GI tract
Connective	Supports, protects, binds	Bones, tendons, fat
Muscle	Movement	Skeletal muscles, heart
Nervous	Control, integration	Brain, spinal cord, nerves

Applications and Examples

• Simple squamous epithelium in alveoli allows rapid gas exchange.

• Stratified squamous epithelium in skin protects against abrasion.

• Simple columnar epithelium in intestines absorbs nutrients efficiently.

Additional info: Understanding tissue structure and function is essential for recognizing pathological changes in disease states.