Chapter 4: Tissues – Structure, Function, and Repair

Tissues of the Human Body

Overview of the Four Principal Types of Body Tissues

The human body is composed of four principal types of tissues, each with distinct structures and functions. Understanding these tissues is fundamental to the study of anatomy and physiology.

• Epithelial Tissue: Forms boundaries, covers surfaces, lines cavities, and forms glands.

• Connective Tissue: Supports, protects, binds other tissues together; includes bone, tendons, fat, and blood.

• Muscle Tissue: Contracts to produce movement; includes skeletal, cardiac, and smooth muscle.

• Nervous Tissue: Internal communication; found in the brain, spinal cord, and nerves.

Epithelial Tissue

Characteristics and Locations

Epithelial tissue consists of closely packed cells that cover body surfaces and line body cavities. It serves as a protective barrier and is involved in absorption, secretion, and sensory reception.

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

• High regenerative capacity: Epithelial cells readily divide to replace lost cells.

• Two main forms:

  • Covering and lining epithelium: On external and internal surfaces (e.g., skin, lining of organs).

  • Glandular epithelium: Forms glands that secrete substances (e.g., salivary glands).

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

Structural and Functional Differences of Epithelial Tissue Types

Epithelial tissues are classified by the number of cell layers and the shape of cells.

• Simple epithelium: Single layer of cells; specialized for absorption, secretion, and filtration.

• Stratified epithelium: Multiple layers; provides protection in areas of high abrasion.

Types of Epithelial Cells

• Simple Squamous Epithelium

  • Single layer of flattened cells; cytoplasm is sparse.

  • Allows rapid diffusion and filtration.

  • Locations: Kidney glomeruli, air sacs of lungs.

  • Special types:

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

    • Mesothelium: Forms serous membranes in the ventral body cavity.

• Simple Cuboidal Epithelium

  • Single layer of cube-shaped cells.

  • Functions in secretion and absorption.

  • Locations: Walls of smallest ducts of glands, kidney tubules.

• Simple Columnar Epithelium

  • Single layer of tall, closely packed cells.

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

  • Contains mucus-secreting goblet cells.

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

  • Locations: Digestive tract, gallbladder, some glands, bronchi, uterine tubes.

• Pseudostratified Columnar Epithelium

  • Appears multi-layered but is actually a single layer.

  • Often ciliated; contains goblet cells.

  • Functions in secretion and movement of mucus via cilia.

  • Locations: Upper respiratory tract, ducts of large glands, tubules in testes.

• Stratified Squamous Epithelium

  • Multiple layers; most widespread stratified epithelium.

  • Protects underlying tissues in areas subject to abrasion.

  • Locations: Skin (keratinized), moist linings of mouth, esophagus, vagina (nonkeratinized).

• Stratified Cuboidal and Columnar Epithelium

  • Rare; found in sweat and mammary glands, pharynx, male urethra.

• Transitional Epithelium

  • Cells change shape when stretched.

  • Allows increased volume in urinary organs (e.g., bladder).

Glandular Epithelia

Glands are cells or groups of cells that produce and secrete aqueous fluids called secretions. Glands are classified as endocrine or exocrine based on their method of secretion.

• Endocrine glands: Secrete hormones directly into the blood; ductless.

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

Types of Exocrine Glands

• Unicellular glands: Mucous cells and goblet cells; secrete mucin to form mucus.

• Multicellular glands: Classified by structure and method of secretion.

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

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

  • Apocrine glands: Apex of cell ruptures; rare in humans (mammary glands are closest example).

Connective Tissue

Characteristics and Functions

Connective tissue is the most abundant and widely distributed tissue type. It supports, protects, insulates, stores energy, and transports substances throughout the body.

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

• Blood: Atypical connective tissue; consists of cells surrounded by plasma matrix.

Muscle Tissue

Characteristics and Types

Muscle tissue is specialized for contraction and movement. It is highly vascularized and responsible for most types of body movement.

• Skeletal muscle: Attached to bones; voluntary movement.

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

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

Nervous Tissue

Characteristics and Functions

Nervous tissue is the main component of the nervous system, which regulates and controls body functions.

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

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

Body Membranes

Major Categories of Body Membranes

Body membranes are composed of at least two primary tissue types: an epithelium bound to underlying connective tissue. They serve as linings and coverings throughout the body.

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

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

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

Membrane Type	Location	Epithelium	Function
Cutaneous	Skin	Stratified squamous	Protection
Mucous	Digestive, respiratory, urogenital tracts	Varies (often columnar)	Secretion, absorption
Serous	Closed ventral body cavities	Simple squamous (mesothelium)	Lubrication

Additional info: Serous membranes are named according to location: pleurae (lungs), pericardium (heart), peritoneum (abdominopelvic cavity).

Tissue Repair

Steps in Tissue Repair

Tissue repair occurs when barriers are compromised, involving inflammatory and immune responses. Repair can occur by regeneration or fibrosis.

• Regeneration: Same kind of tissue replaces destroyed tissue; original function is restored.

• Fibrosis: Connective tissue replaces destroyed tissue; original function is lost, resulting in scar formation.

Phases of Tissue Repair

1. Inflammation: Chemicals released cause dilation and increased permeability of blood vessels; clotting occurs.

2. Organization: Blood clot replaced by granulation tissue; epithelium regenerates; fibroblasts produce collagen fibers to bridge gap.

3. Regeneration and Fibrosis: Scab detaches; tissue matures; epithelium thickens and resembles adjacent tissue; results in fully regenerated epithelium with underlying scar tissue.

Regenerative Capacity of Tissues

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

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

• Low/no functional capacity: Cardiac muscle, nervous tissue in brain and spinal cord (some cell division occurs; research ongoing).