Anatomy & Physiology: Tissues and Epithelial Tissue

Tissues in Human Anatomy

Definition and Overview

Tissues are groups of cells that work together to perform specific, common functions in the body. Understanding tissues is fundamental to the study of anatomy and physiology, as they form the building blocks of organs and organ systems.

• Tissue: A group of cells of similar structure that perform a common function.

• Cells work together in functionally related groups called tissues.

Four Basic Tissue Types and Their Functions

The human body is composed of four primary tissue types, each with distinct roles:

• Epithelial tissue – covering (Chapters 4 and 5)

• Connective tissue – support (Chapters 4, 5, 6, and 9)

• Muscle tissue – movement (Chapters 10 and 11)

• Nervous tissue – control (Chapters 12–16 and 25)

Example: The stomach contains all four tissue types: epithelial lining, connective tissue support, muscle for churning, and nervous tissue for control.

Epithelial Tissue

General Characteristics

Epithelial tissue covers body surfaces, lines body cavities, and forms parts of most glands. It serves as a protective barrier and is involved in absorption, secretion, and sensation.

• Protection: Shields underlying tissues from mechanical and chemical injury.

• Secretion: Produces substances such as enzymes, hormones, and mucus.

• Absorption: Takes in substances such as nutrients in the intestines.

• Diffusion: Allows gases and small molecules to pass through (e.g., in lungs).

• Filtration: Filters fluids (e.g., in kidney glomeruli).

• Sensory reception: Contains nerve endings for sensation.

Special Characteristics of Epithelia

• Cellularity: Epithelial tissues are composed almost entirely of tightly packed cells with minimal extracellular material.

• Specialized contacts: Cells are joined by specialized junctions (e.g., tight junctions, desmosomes).

• Polarity: The apical (top) surface differs structurally and functionally from the basal (bottom) surface.

• Support by connective tissue: Epithelia are supported by an underlying layer of connective tissue.

• Avascular but innervated: Epithelial tissues lack blood vessels but are supplied by nerve fibers. Nutrients diffuse from underlying connective tissue.

• Regeneration: Epithelial cells have a high capacity for renewal; lost or damaged cells are rapidly replaced by cell division.

Example: The skin's outer layer (epidermis) is an epithelium that is constantly renewed.

Classification of Epithelial Tissue

By Number of Cell Layers

• Simple epithelia: Single layer of cells attached to the basement membrane. Specialized for absorption, secretion, and filtration.

• Stratified epithelia: Multiple layers of cells; basal layer attached to basement membrane. Specialized for protection.

By Cell Shape

• Squamous: Cells are wider than tall (flat, disc-like).

• Cuboidal: Cells are as wide as they are tall (cube-shaped).

• Columnar: Cells are taller than they are wide (column-shaped).

Types of Epithelial Tissue

Simple Epithelia

• Simple Squamous Epithelium

  • Description: Single layer of flat cells with disc-shaped nuclei.

  • Function: Allows passage of materials by passive diffusion and filtration; secretes lubricating substances in serosae.

  • Location: Kidney glomeruli, air sacs of lungs, lining of heart, blood vessels, lymphatic vessels, lining of ventral body cavity (serosae).

• Simple Cuboidal Epithelium

  • Description: Single layer of cube-like cells with large, spherical central nuclei.

  • Function: Secretion and absorption.

  • Location: Kidney tubules, secretory portions of small glands, ovary surface.

• Simple Columnar Epithelium

  • Description: Single layer of tall, column-shaped cells with round to oval nuclei. Some bear cilia; may contain goblet cells.

  • Function: Absorption; secretion of mucus, enzymes, and other substances. Ciliated type propels mucus or reproductive cells by ciliary action.

  • Location: Nonciliated: lines digestive tract, gallbladder, ducts of some glands. Ciliated: lines small bronchi, uterine tubes, uterus.

• Pseudostratified Columnar Epithelium

  • Description: All cells originate at basement membrane; only tall cells reach apical surface. Nuclei at varying heights; may contain goblet cells and bear cilia. Appears stratified but is not.

  • Function: Secretion of mucus; propulsion of mucus by ciliary action.

  • Location: Nonciliated: ducts of male reproductive tubes, ducts of large glands. Ciliated: lines trachea and most of upper respiratory tract.

Stratified Epithelia

• General Properties: Two or more layers of cells; regenerate from basal layer; major role is protection; named by shape of apical cells.

• Stratified Squamous Epithelium

  • Description: Many layers; apical cells squamous, deeper layers cuboidal or columnar. Thickest epithelial tissue.

  • Types:

    • Keratinized: Contains keratin protein, waterproof, surface cells dead and full of keratin. Location: Epidermis of skin.

    • Nonkeratinized: Forms moist linings of body openings. Location: Esophagus, mouth, anus, vagina, urethra.

  • Function: Protects underlying tissues in areas subject to abrasion.

• Stratified Cuboidal Epithelium

  • Description: Generally two layers of cube-shaped cells.

  • Function: Protection.

  • Location: Ducts of mammary glands, salivary glands, largest sweat glands.

• Stratified Columnar Epithelium

  • Description: Several layers; basal cells usually cuboidal, superficial cells elongated and columnar.

  • Function: Protection and secretion.

  • Location: Rare; found in male urethra and large ducts of some glands.

Summary Table: Epithelial Tissue Types and Functions

Type	Description	Function	Location
Simple Squamous	Single layer, flat cells	Diffusion, filtration, secretion	Kidney glomeruli, air sacs of lungs, lining of heart, blood vessels
Simple Cuboidal	Single layer, cube-shaped cells	Secretion, absorption	Kidney tubules, small glands, ovary surface
Simple Columnar	Single layer, tall cells; may have cilia/goblet cells	Absorption, secretion, propulsion (ciliated)	Digestive tract, gallbladder, bronchi, uterine tubes
Pseudostratified Columnar	Single layer, varying cell heights; appears stratified	Secretion, propulsion of mucus	Trachea, upper respiratory tract, male reproductive ducts
Stratified Squamous	Multiple layers; apical cells squamous	Protection	Epidermis (keratinized), mouth, esophagus, vagina (nonkeratinized)
Stratified Cuboidal	Two layers, cube-shaped cells	Protection	Ducts of sweat, mammary, and salivary glands
Stratified Columnar	Several layers; apical cells columnar	Protection, secretion	Male urethra, large gland ducts