Chapter 4: Tissues – Structure, Function, and Classification in Human Anatomy & Physiology

Overview of Human Tissue Types

Introduction to Tissues

Tissues are groups of cells working together to perform specific functions in the body. The study of tissues is called histology. There are four basic tissue types in the human body: epithelial, connective, muscle, and nervous tissue.

• Epithelial tissue: Forms boundaries, protects, secretes, absorbs, and filters substances. Found in skin, lining of digestive tract, and glands.

• Connective tissue: Supports, protects, and binds other tissues together. Examples include bones, tendons, and fat.

• Muscle tissue: Contracts to produce movement. Includes skeletal, cardiac, and smooth muscle.

• Nervous tissue: Enables internal communication. Found in the brain, spinal cord, and nerves.

Example: The skin contains epithelial tissue (epidermis), connective tissue (dermis), muscle tissue (arrector pili muscles), and nervous tissue (sensory receptors).

Epithelial Tissue

General Features

Epithelial tissue covers body surfaces, lines cavities, and forms glands. It is characterized by closely packed cells with minimal extracellular material.

• Polarity: Has an apical (top) surface and a basal (bottom) surface attached to underlying connective tissue.

• Specializations: Apical surfaces may have microvilli (increase surface area) or cilia (movement).

• Basement membrane: Thin layer anchoring epithelium to connective tissue, composed of basal lamina (from epithelial cells) and reticular lamina (from connective tissue).

• Avascular but innervated: No blood vessels, but contains nerves. Nutrients diffuse from underlying tissues.

• High regenerative capacity: Rapidly replaces damaged cells.

Types of Epithelial Tissue

• Simple epithelium: One cell layer thick; specialized for absorption, secretion, and filtration.

• Stratified epithelium: Multiple layers; provides protection.

• Pseudostratified epithelium: Appears multilayered but is a single layer with nuclei at different heights.

• Transitional epithelium: Changes shape in response to stretching (e.g., urinary bladder).

Cell Shapes

• Squamous: Flat, scale-like cells; nucleus flattened.

• Cuboidal: Cube-shaped cells; round, central nucleus.

• Columnar: Tall, column-shaped cells; oval nucleus near basal surface.

Examples and Locations

• Simple squamous: Lining of blood vessels, alveoli of lungs; allows diffusion.

• Stratified squamous: Skin (keratinized), mouth/esophagus (non-keratinized); protects against abrasion.

• Simple cuboidal: Kidney tubules, glands; secretion and absorption.

• Simple columnar: Digestive tract lining; absorption and secretion.

• Pseudostratified columnar: Trachea; secretion and movement of mucus.

• Transitional: Urinary bladder; stretches to accommodate urine.

Glandular Epithelium

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

• Endocrine glands: Secrete hormones directly into blood (e.g., thyroid, pancreas).

• Unicellular glands: Goblet cells (produce mucus).

• Multicellular glands: Sweat glands, salivary glands.

Cell Junctions in Tissues

Types of Cell Junctions

• Tight junctions: Seal adjacent cells to prevent leakage of molecules between them.

• Desmosomes: Anchor cells together, providing mechanical strength; cadherin proteins interlock like a zipper.

• Gap junctions: Allow passage of ions and small molecules between cells; important for communication (e.g., cardiac muscle).

Connective Tissue

General Features

Connective tissue supports, binds, and protects other tissues. It is characterized by cells scattered within an abundant extracellular matrix.

• Matrix: Composed of ground substance and protein fibers; determines tissue properties.

• Vascularity: Varies from avascular (cartilage) to highly vascular (bone).

• Regeneration: Varies from poor (cartilage) to good (bone, blood).

Connective Tissue Fibers

• Collagen fibers: Strong, flexible, thick; resist tension.

• Elastic fibers: Thin, flexible, allow stretch and recoil.

• Reticular fibers: Thin, branched, form delicate support networks.

Connective Tissue Cell Types

• "-blast" cells: Mitotic, secretory; produce matrix (e.g., fibroblast).

• "-clast" cells: Break down matrix (e.g., osteoclast).

• "-cyte" cells: Mature, maintain tissue (e.g., chondrocyte).

Classification of Connective Tissues

Tissue Class	Subclasses	Examples
Connective Tissue Proper	Loose (areolar, adipose, reticular) Dense (regular, irregular, elastic)	Fat, tendons, ligaments
Cartilage	Hyaline, elastic, fibrocartilage	Trachea, ear, intervertebral discs
Bone	Compact, spongy	Skeleton
Blood	Blood cells, plasma	Blood vessels

Examples and Functions

• Areolar tissue: Loose, supports and binds other tissues; found under epithelia.

• Adipose tissue: Stores fat, insulates, cushions organs.

• Dense regular tissue: Parallel collagen fibers; tendons and ligaments.

• Dense irregular tissue: Irregular collagen fibers; dermis of skin.

• Elastic tissue: Allows stretch; walls of arteries.

• Cartilage: Hyaline (ends of bones), elastic (ear), fibrocartilage (intervertebral discs).

• Bone: Rigid support, protection, mineral storage.

• Blood: Transports nutrients, gases, wastes.

Muscle Tissue

General Features

Muscle tissue is specialized for contraction, producing movement. It consists of tightly packed, excitable, and highly vascular cells.

• Skeletal muscle: Attached to bones; voluntary movement.

• Cardiac muscle: Found in heart; involuntary, rhythmic contractions.

• Smooth muscle: Walls of hollow organs; involuntary movement.

Nervous Tissue

General Features

Nervous tissue is responsible for communication and control. It consists of large neurons and smaller supporting neuroglia cells.

• Neurons: Transmit electrical signals.

• Neuroglia: Support, protect, and nourish neurons.

Membranes

Types of Membranes

• Cutaneous membrane: Skin; dry, keratinized stratified squamous epithelium over dense irregular connective tissue.

• Mucous membrane: Lines body cavities open to exterior (digestive, respiratory, urinary, reproductive tracts); wet, variable epithelium over areolar connective tissue.

• Serous membrane: Lines closed ventral body cavities; simple squamous epithelium (mesothelium) over areolar connective tissue; produces serous fluid.

Tissue Repair Mechanisms

Regeneration vs. Fibrosis

• Regeneration: Replacement of damaged tissue with the same tissue type; restores function. Occurs in tissues with good blood supply and high mitotic activity (e.g., epithelium, bone).

• Fibrosis: Replacement with scar tissue (dense connective tissue); function may be lost. Occurs in tissues with poor blood supply or low mitotic activity (e.g., nervous tissue, cardiac muscle).

Stages of Tissue Repair

1. Inflammation: Blood vessels become permeable; clotting occurs; scab forms.

2. Organization: Granulation tissue restores blood supply; fibroblasts produce collagen; macrophages remove debris.

3. Regeneration and fibrosis: Epithelium regenerates; underlying area may become scar tissue.

Additional info: The first image shows compact bone tissue, characterized by concentric rings (osteons) and central canals. The second image is a diagram summarizing the four major tissue types and their locations/functions in the human body.