Ch. 4: Tissue Level of Organization

Overview of Tissue Types

The human body is composed of four basic tissue types, each with distinct structures and functions essential for maintaining homeostasis and supporting life.

• Epithelial Tissue: Covers body surfaces, lines cavities, and forms glands. Functions include protection, absorption, secretion, and sensation.

• Connective Tissue: Provides support, binds tissues together, stores energy, and participates in immune responses.

• Muscular Tissue: Responsible for movement through contraction; includes skeletal, cardiac, and smooth muscle.

• Nervous Tissue: Initiates and transmits electrical impulses for communication, coordination, and control.

Cellular Junctions

Cellular junctions are specialized structures that connect adjacent cells, providing structural integrity and facilitating communication.

• Tight Junctions: Seal adjacent cells to prevent leakage of molecules; commonly found in epithelial tissues lining organs.

• Desmosomes: Anchor cells together, providing mechanical strength; prevalent in tissues subject to stress, such as skin and cardiac muscle.

• Gap Junctions: Allow direct communication between cells via channels; important in cardiac and smooth muscle for coordinated contraction.

Classification of Epithelial Tissues

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

• Number of Layers:

  • Simple Epithelium: Single layer; functions in absorption, secretion, and filtration.

  • Stratified Epithelium: Multiple layers; provides protection against abrasion.

  • Pseudostratified Epithelium: Appears layered due to cell nuclei at different levels, but all cells touch the basement membrane.

• Cell Shape:

  • Squamous: Flat, scale-like cells; found in areas of diffusion (e.g., alveoli, capillaries).

  • Cuboidal: Cube-shaped cells; common in glands and kidney tubules.

  • Columnar: Tall, column-like cells; line digestive tract and have absorptive/secretory functions.

Example: Simple squamous epithelium lines blood vessels (endothelium); stratified squamous epithelium forms the skin's outer layer.

Specialized Epithelial Features

Certain epithelial tissues have specialized structures and functions.

• Transitional Epithelium: Found in the urinary bladder; cells change shape to allow stretching.

• Keratinized vs. Non-Keratinized Stratified Squamous Epithelium:

  • Keratinized: Contains keratin protein; forms dry, protective surfaces (e.g., skin).

  • Non-Keratinized: Lacks keratin; lines moist surfaces (e.g., mouth, esophagus).

• Cilia: Hair-like projections that move substances over cell surfaces (e.g., respiratory tract).

• Microvilli: Finger-like extensions that increase surface area for absorption (e.g., small intestine).

• Goblet Cells: Unicellular glands that secrete mucus for lubrication and protection.

Glandular Epithelial Tissue

Glandular epithelium forms glands that secrete substances into ducts or directly into the bloodstream.

• Endocrine Glands: Release hormones directly into the blood; ductless (e.g., thyroid gland).

• Exocrine Glands: Secrete products into ducts leading to body surfaces or cavities (e.g., sweat glands).

• Types of Secretion:

  • Holocrine: Entire cell disintegrates to release its product (e.g., sebaceous glands).

  • Apocrine: Part of the cell pinches off to release product (e.g., mammary glands).

  • Merocrine: Secretion by exocytosis without cell damage (e.g., salivary glands).

Connective Tissue Types

Connective tissue is the most abundant and diverse tissue type, classified as connective tissue proper and specialized connective tissue.

• Connective Tissue Proper:

  • Loose Connective Tissue: Fewer fibers, more ground substance; includes areolar, adipose, and reticular tissues.

  • Dense Connective Tissue: More fibers, less ground substance; includes dense regular (tendons, ligaments) and dense irregular (dermis).

• Specialized Connective Tissue:

  • Cartilage: Flexible, avascular tissue; types include hyaline, elastic, and fibrocartilage.

  • Bone: Rigid, mineralized tissue; supports and protects organs.

  • Blood: Fluid tissue; transports nutrients, gases, and wastes.

Example: Adipose tissue stores fat; hyaline cartilage forms the nose and trachea.

Components of Generalized Connective Tissue

Connective tissue consists of cells, fibers, ground substance, and the extracellular matrix (ECM).

• Ground Substance: Amorphous material filling spaces between cells and fibers; composed of water, proteoglycans, and glycoproteins.

• Fibers:

  • Collagen Fibers: Strong, flexible, resist stretching; most abundant.

  • Elastic Fibers: Stretch and recoil; found in skin, lungs, and blood vessels.

  • Reticular Fibers: Form supportive networks in soft organs (e.g., lymph nodes).

• Extracellular Matrix (ECM): Combination of ground substance and fibers; provides structural support.

• Cells: Various types with specialized functions.

Cells of Connective Tissue

Connective tissue contains several cell types, each with unique roles.

• Fibroblasts: Produce fibers and ground substance.

• Macrophages: Phagocytic cells that engulf pathogens and debris.

• Leukocytes: White blood cells involved in immune responses.

• Mast Cells: Release histamine and other mediators during inflammation.

• Adipocytes: Store fat for energy and insulation.

Extracellular Matrix of Bone and Blood

The ECM varies among connective tissues, influencing their properties and functions.

• Bone ECM: Mineralized with calcium phosphate, providing rigidity and strength.

• Blood ECM: Fluid (plasma), allowing transport of cells and dissolved substances.

• Other Connective Tissues: ECM composition determines flexibility, strength, and support.

Muscle Tissue Types

Muscle tissue enables movement and is classified into three types based on structure and function.

• Skeletal Muscle: Striated, voluntary; attached to bones for body movement.

• Cardiac Muscle: Striated, involuntary; forms the heart wall and pumps blood.

• Smooth Muscle: Non-striated, involuntary; found in walls of hollow organs (e.g., intestines, blood vessels).

Nervous Tissue

Nervous tissue is specialized for communication and control.

• Neurons: Excitable cells that transmit electrical signals.

• Neuroglia: Support, protect, and nourish neurons; do not conduct impulses.

Tissue Repair and Healing

Tissues vary in their ability to repair and regenerate after injury.

• Epithelial Tissue: High regenerative capacity due to rapid cell division.

• Connective Tissue: Variable repair capacity; bone heals well, cartilage heals slowly due to avascularity.

• Muscle Tissue: Limited regeneration; skeletal muscle can repair to some extent, cardiac and smooth muscle have poor regenerative ability.

• Nervous Tissue: Very limited repair in the central nervous system; peripheral nerves can regenerate under certain conditions.

Example: Cartilage injuries heal slowly because cartilage lacks blood vessels, limiting nutrient delivery and cell migration.