Chapter 4: Tissues – Structure, Function, and Repair

Tissues of the Human Body

Introduction to Tissues

Tissues are groups of cells with similar structure and function, forming the basic building blocks of organs and body systems. There are four principal types of body tissues, each with distinct characteristics, locations, and functions.

• Epithelial tissue: Covers body surfaces, lines cavities, and forms glands.

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

• Muscle tissue: Contracts to produce movement.

• Nervous tissue: Initiates and transmits electrical impulses for communication.

Epithelial Tissue

Characteristics and Locations

• Cellularity: Composed almost entirely of closely packed cells.

• Polarity: Has an apical (top) surface and a basal (bottom) surface.

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

• Regeneration: High capacity for renewal and repair.

• Supported by connective tissue: Rests on a basement membrane.

Main locations: Skin surface (epidermis), lining of digestive tract, respiratory tract, urogenital tract, and glands.

Main Functions

• Protection

• Absorption

• Filtration

• Excretion

• Secretion

• Sensory reception

Classification of Epithelia

• By number of cell layers:

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

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

• By cell shape:

  • Squamous: Flattened and scale-like.

  • Cuboidal: Boxlike, as tall as they are wide.

  • Columnar: Tall and column shaped.

Types of Epithelial Tissue

• Simple Squamous Epithelium

  • Single layer of flattened cells; sparse cytoplasm.

  • Allows rapid diffusion or filtration.

  • Locations: Air sacs of lungs, lining of heart, blood vessels, lymphatic vessels.

  • Special types:

    • Endothelium: Lines blood vessels, lymphatic vessels, and heart.

    • Mesothelium: Forms serous membranes in ventral body cavity.

• Simple Cuboidal Epithelium

  • Single layer of cube-shaped cells.

  • Functions in secretion and absorption.

  • Locations: Kidney tubules, ducts and secretory portions of small glands, ovary surface.

• Simple Columnar Epithelium

  • Single layer of tall, closely packed cells.

  • Some have microvilli (increase surface area), some have cilia (move substances).

  • May contain mucus-secreting goblet cells.

  • Functions in absorption and secretion of mucus, enzymes, and other substances.

  • Locations: Digestive tract, gallbladder, excretory ducts, small bronchi, uterine tubes.

• Pseudostratified Columnar Epithelium

  • Single layer of cells of differing heights, giving a false impression of stratification.

  • May be ciliated; often contains goblet cells.

  • Functions in secretion (especially mucus) and propulsion of mucus by ciliary action.

  • Locations: Upper respiratory tract, ducts of large glands, parts of male reproductive tract.

• Stratified Squamous Epithelium

  • Thick membrane composed of several cell layers; basal cells are cuboidal or columnar, surface cells are squamous.

  • Protects underlying tissues in areas subject to abrasion.

  • Locations: Nonkeratinized type forms moist linings of esophagus, mouth, vagina; keratinized type forms epidermis of skin.

• Stratified Cuboidal and Columnar Epithelium

  • Rare in the body.

  • Locations: Ducts of some large glands (sweat, mammary, salivary), male urethra.

• Transitional Epithelium

  • Resembles both stratified squamous and stratified cuboidal.

  • Stretches readily and permits distension of urinary organ by contained urine.

  • Locations: Lines ureters, bladder, and part of urethra.

Glandular Epithelia

• Gland: One or more cells that make and secrete an aqueous fluid (secretion).

• Endocrine glands: Ductless; secrete hormones directly into blood or interstitial fluid.

• Exocrine glands: Secrete products onto body surfaces or into body cavities via ducts (e.g., sweat, oil, salivary glands).

• Unicellular exocrine glands: Mucous cells and goblet cells; secrete mucin.

• Multicellular exocrine glands: Classified by mode of secretion:

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

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

  • Apocrine: Accumulate products, apex of cell pinches off (controversial in humans; mammary glands are a possible example).

Connective Tissue

Overview

Connective tissue is the most abundant and widely distributed tissue type in the body. It provides support, protection, insulation, and storage of energy reserves, and transports substances (e.g., blood).

• Types: Connective tissue proper, cartilage, bone, blood.

• Functions: Binding and support, protection, insulation, energy storage, transportation.

Muscle Tissue

Overview

Muscle tissue is specialized for contraction, producing movement of body parts and substances.

• Skeletal muscle: Attached to bones; voluntary movement.

• Cardiac muscle: Found only in the heart; involuntary.

• Smooth muscle: Found in walls of hollow organs (except heart); involuntary.

Nervous Tissue

Overview

Nervous tissue is the main component of the nervous system (brain, spinal cord, nerves) and is responsible for regulating and controlling body functions.

• Neurons: Specialized cells that generate and conduct nerve impulses.

• Supporting cells (neuroglia): Support, insulate, and protect neurons.

Body Membranes

Major Categories

Body membranes are thin sheets of tissue that cover surfaces, line body cavities, and divide organs. Most are composed of an epithelial layer and an underlying connective tissue layer.

• Cutaneous membrane: The skin; a dry membrane composed of stratified squamous epithelium (epidermis) attached to a thick layer of connective tissue (dermis).

• Mucous membranes (mucosae): Line body cavities that open to the exterior (digestive, respiratory, urogenital tracts); may secrete mucus.

• Serous membranes (serosae): Line closed ventral body cavities and cover organs; composed of simple squamous epithelium (mesothelium) resting on areolar connective tissue; secrete serous fluid for lubrication.

Examples of serous membranes: Pleurae (lungs), pericardium (heart), peritoneum (abdominopelvic organs).

Tissue Repair

Steps in Tissue Repair

Tissue repair occurs when barriers are compromised, involving both inflammatory and immune responses. Repair can occur by regeneration (restoring original function) or fibrosis (scar formation).

1. Inflammation: Release of inflammatory chemicals causes dilation of blood vessels and increased permeability; clotting occurs.

2. Organization: Blood clot is replaced by granulation tissue (new capillary-enriched tissue); epithelium regenerates; fibroblasts produce collagen fibers to bridge the gap.

3. Regeneration and fibrosis: Scab detaches; fibrous tissue matures; epithelium thickens and resembles adjacent tissue, resulting in a fully regenerated epithelium with underlying scar tissue (if present).

Regenerative Capacity of Tissues

• High regenerative capacity: Epithelial tissues, bone, areolar connective tissue, dense irregular connective tissue, blood-forming tissue.

• Moderate regenerative capacity: Smooth muscle, dense regular connective tissue.

• Low or no functional regenerative capacity: Cardiac muscle, nervous tissue in brain and spinal cord (some limited regeneration possible).

Summary Table: Principal Types of Body Tissues

Tissue Type	Main Function	Location
Epithelial	Protection, absorption, filtration, secretion	Skin, lining of GI tract, glands
Connective	Support, protection, binding	Bones, tendons, fat, blood
Muscle	Movement	Skeletal muscles, heart, walls of hollow organs
Nervous	Internal communication	Brain, spinal cord, nerves

Additional info: Some details, such as the full classification of glands and the regenerative capacity of tissues, were expanded for academic completeness and clarity.