Tissues: Structure and Function

Introduction to Tissues

Tissues are groups of cells and their associated intercellular material that work together to perform specific functions. The study of tissues is known as histology. Multiple tissues with the same function combine to form organs.

• Four main types of tissues in the body:

  1. Epithelial tissue: Covers body surfaces, lines hollow organs, body cavities, and ducts; forms glands.

  2. Connective tissue: Protects, supports, and binds organs; stores energy as fat; provides immunity.

  3. Muscle tissue: Contracts to generate movement and body heat.

  4. Nervous tissue: Conducts electrical impulses and detects changes in the body.

Cell Junctions

• Tight junction (fluid-sealed): Prevents fluid from entering between cells.

• Desmosome (spot welding): Holds cells together.

• Gap junction (communication): Allows chemicals to move between cells.

Epithelial Tissue

General Characteristics

• Tightly packed cells: Minimal intercellular space.

• Avascular: No blood vessels; nutrients diffuse from underlying connective tissue.

• Free surface (apical): Faces body cavity, lumen, or external environment.

• Basement membrane: Collagen and glycoproteins attach epithelium to connective tissue, providing structural support.

  • Helps epithelial cells attach/anchor to connective tissue below.

  • Acts as a barrier, restricting movement of proteins and large molecules.

  • Provides a surface for cell migration during tissue repair.

• Regeneration: Rapid cell division; e.g., intestinal lining replaced every 2–3 days.

• Nerve supply: Provides information about internal and external environments.

Functions of Epithelial Tissue

• Protection: Covers and lines surfaces exposed to the environment.

• Absorption and secretion: Forms glands, lines kidney tubules, digestive tract.

• Permeability: Regulates what can pass through the tissue.

Specializations

• Cilia: Move substances (e.g., in respiratory tract).

• Goblet cells: Unicellular glands for secretion (produce mucus).

• Microvilli: Increase surface area for absorption and secretion.

Classification of Epithelial Tissue

• By shape:

  • Squamous: Flat, width greater than height.

  • Cuboidal: Cube-shaped, approximately equal dimensions.

  • Columnar: Taller than wide.

• By layers:

  • Simple: Single layer; regions of secretion or absorption (e.g., intestines, lungs).

  • Stratified: Two or more layers; areas exposed to mechanical or chemical stress (e.g., skin, mouth lining).

  • Pseudostratified: Appears layered, but all cells touch the basement membrane.

  • Transitional: Combination of differently shaped cells that can change shape (e.g., bladder).

Note: In stratified tissues, the shape of the cells in the outermost layer determines the tissue's name.

Specific Epithelial Tissues

• Simple squamous:

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

  • Mesothelium: Lines ventral body cavities.

  • Function: Diffusion, filtration, secretion.

• Simple cuboidal:

  • Location: Ducts, glands, kidney tubules, thyroid gland.

  • Function: Secretion and absorption.

• Simple columnar:

  • Location: Gastrointestinal tract, fallopian tubes, larger ducts.

  • Function: Secretion and absorption; may have goblet cells (mucus production) and microvilli (increase surface area).

  • Importance in digestive tract: Mucus protects lining, lubricates passage, neutralizes stomach acids, and traps pathogens.

• Stratified squamous:

  • Location: Skin, mouth, esophagus, vagina, rectum.

  • Function: Protection; outer layers may be keratinized for water resistance and durability.

• Stratified cuboidal:

  • Location: Sweat gland ducts.

  • Function: Protection.

• Stratified columnar:

  • Location: Salivary gland ducts, other large ducts.

  • Function: Protection and secretion.

• Transitional:

  • Location: Ureters, bladder.

  • Function: Allows stretching and expansion.

• Pseudostratified:

  • Location: Respiratory tract, male reproductive tract.

  • Function: Secretion and movement of mucus (often ciliated, with goblet cells).

Glandular Epithelium

• Unicellular glands: Goblet cells (produce mucin/mucus).

• Multicellular exocrine glands:

  • Holocrine: Cell fills with product and bursts (e.g., sebaceous glands).

  • Merocrine: Product released by exocytosis (e.g., sweat, salivary glands).

  • Apocrine: Apical portion of cytoplasm is shed with product (e.g., mammary glands).

Connective Tissue

General Characteristics

• Functions: Binds, supports, protects organs, stores energy, transports fluids, defends against microorganisms.

• Good blood supply: Most connective tissues are vascular except cartilage.

• Widely scattered cells in a matrix: Matrix can be watery (plasma) or hard (bone).

Importance of matrix: The matrix's consistency (watery or hard) is crucial for the tissue's function. Watery matrix allows flexibility and transport; hard matrix provides strength and support.

Components of Connective Tissue Matrix

• Ground substance: Hyaluronan and proteins; helps form ground substance for support.

• Protein fibers:

  • Collagen fibers: Tough, flexible, resist force in one direction (tendons, ligaments).

  • Elastic fibers: Stretch and return to original length (elastic ligaments).

  • Reticular fibers: Network, resist force in many directions (sheaths around organs).

Cells in Connective Tissue

• Fibroblasts: Always present; secrete hyaluronan and proteins, maintain tissue structure.

• Macrophages: Large phagocytic cells; engulf damaged cells/pathogens, mobilize immune response.

• Mast cells: White blood cells; contain histamine and heparin, stimulate inflammation.

• Adipocytes: Fat cells; store energy, provide insulation and shock absorption.

Other Features

• Nerve supply: Most connective tissues are innervated.

• No free surface: Unlike epithelial tissue, connective tissue is not exposed to the external environment.

Summary Table: Epithelial Tissue Types

Key Terms and Definitions

• Histology: Study of tissues.

• Basement membrane: Layer attaching epithelium to connective tissue.

• Apical surface: Free surface of epithelial cell facing lumen or environment.

• Matrix: Extracellular material in connective tissue (ground substance + fibers).

• Fibroblast: Cell that produces fibers and ground substance in connective tissue.

• Macrophage: Phagocytic cell that engulfs pathogens and debris.

• Adipocyte: Fat cell.

• Goblet cell: Unicellular gland that secretes mucus.

Additional info:

• Some tissues, such as cartilage, are avascular and heal slowly due to lack of blood supply.

• Keratinized stratified squamous epithelium is found in the skin and provides extra protection against abrasion and dehydration.

• Transitional epithelium is specialized for organs that need to stretch, such as the bladder.