Tissue: The Living Fabric

Introduction to Tissues

Tissues are groups of cells similar in structure that perform common or related functions. The study of tissues is called histology. There are four basic tissue types in the human body: epithelial, connective, muscle, and nervous tissue. Each type has specialized roles essential for maintaining homeostasis.

• 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.

Microscopy of Human Tissue

Preparation and Staining

To study tissues under a microscope, they must be:

• Fixed: Preserved with a solvent.

• Sectioned: Cut into thin slices for light or electron transmission.

• Stained: Enhanced with dyes (colored for light microscopy, heavy metals for electron microscopy) to increase contrast.

Artifacts may occur, causing distortions from the living state.

Epithelial Tissue

Definition and Functions

Epithelial tissue (epithelium) is a sheet of cells that covers body surfaces or lines cavities. It exists in two main forms:

• Covering and lining epithelia: On external and internal surfaces (e.g., skin, lining of digestive tract).

• Glandular epithelia: Secretory tissue in glands (e.g., salivary glands).

Main functions include protection, absorption, filtration, excretion, secretion, and sensory reception.

Special Characteristics of Epithelial Tissues

• Polarity: Cells have an apical (top) surface and a basal (bottom) surface. The apical surface may have microvilli or cilia; the basal surface attaches to the basal lamina.

• Specialized contacts: Cells are closely joined by tight junctions and desmosomes to form continuous sheets.

• Supported by connective tissue: The basement membrane (basal and reticular lamina) reinforces the epithelium.

• Avascular but innervated: No blood vessels, but supplied by nerves; nutrients diffuse from underlying connective tissue.

• Regeneration: High capacity for renewal, especially in areas subject to friction or damage.

Classification of Epithelia

Epithelia are classified by the number of cell layers and cell shape:

• Layers: Simple (one layer) or stratified (multiple layers).

• Shapes: Squamous (flat), cuboidal (cube-like), columnar (tall).

Types of Epithelial Tissue

• Simple squamous: Single layer of flat cells; allows diffusion/filtration (e.g., air sacs of lungs, lining of blood vessels).

• Simple cuboidal: Single layer of cube-shaped cells; secretion and absorption (e.g., kidney tubules, small glands).

• Simple columnar: Single layer of tall cells; absorption, secretion of mucus/enzymes (e.g., digestive tract, uterine tubes).

• Pseudostratified columnar: Appears stratified but is single-layered; secretion and movement of mucus (e.g., trachea).

• Stratified squamous: Multiple layers; protects underlying tissues (e.g., skin, mouth, esophagus).

• Transitional: Resembles both stratified squamous and cuboidal; stretches (e.g., bladder).

Glandular Epithelia

A gland consists of one or more cells that make and secrete an aqueous fluid (secretion). Glands are classified by:

• Site of product release: Endocrine (hormones, ductless, into blood) or exocrine (onto body surfaces or into cavities, via ducts).

• Number of cells: Unicellular (e.g., goblet cells) or multicellular (e.g., salivary glands).

Types of Multicellular Exocrine Glands

Classified by structure (simple/compound, tubular/alveolar) and mode of secretion:

Covering and Lining Membranes

• Cutaneous membrane: The skin; covers the body surface.

• Mucous membranes (mucosae): Line body cavities open to the exterior (e.g., digestive, respiratory tracts).

• Serous membranes: Line closed ventral body cavities (e.g., pleura, pericardium, peritoneum).

Connective Tissue

Overview and Functions

Connective tissue is the most abundant and widely distributed tissue type. Its major functions include binding and support, protection, insulation, storing reserve fuel, and transporting substances (blood).

• Four main classes: Connective tissue proper, cartilage, bone, blood.

Comparison of Classes of Connective Tissues

Common Characteristics of Connective Tissue

• All arise from mesenchyme (embryonic tissue).

• Varying degrees of vascularity (cartilage is avascular, bone is highly vascularized).

• Cells are suspended in an extracellular matrix (ECM) of fibers and ground substance.

Structural Elements of Connective Tissue

• Ground substance: Unstructured material filling space between cells; contains interstitial fluid, cell adhesion proteins, proteoglycans.

• Fibers: Collagen (strongest), elastic (stretch/recoil), reticular (short, branched, supportive).

• Cells: "Blast" cells (immature, secrete matrix), "cyte" cells (mature, maintain matrix), fat cells, white blood cells, mast cells, macrophages.

Types of Connective Tissue Proper

• Loose connective tissue: Areolar (supports, binds, holds fluid), adipose (stores fat, insulates), reticular (supports blood cells in lymphoid organs).

• Dense connective tissue: Dense regular (tendons, ligaments), dense irregular (dermis, joint capsules), elastic (arteries, some ligaments).

Muscle Tissue

Overview

Muscle tissue is highly vascularized and responsible for movement. Muscle cells contain myofilaments (actin and myosin) for contraction. There are three types:

• Skeletal muscle: Voluntary, striated, multinucleate, attached to bones.

• Cardiac muscle: Involuntary, striated, one nucleus, intercalated discs, found in heart.

• Smooth muscle: Involuntary, non-striated, spindle-shaped, one nucleus, found in walls of hollow organs.

Nervous Tissue

Overview

Nervous tissue is the main component of the nervous system (brain, spinal cord, nerves). It regulates and controls body functions. Two main cell types:

• Neurons: Generate and conduct nerve impulses.

• Supporting cells: Support, insulate, and protect neurons.

Tissue Repair

Steps in Tissue Repair

When tissue is damaged, repair occurs in three steps:

1. Inflammation: Release of chemicals, dilation of blood vessels, increased permeability, clotting.

2. Organization: Blood clot replaced by granulation tissue, epithelium regenerates, fibroblasts bridge gap.

3. Regeneration and fibrosis: Scab detaches, tissue matures, scar tissue may form.

Repair can occur by regeneration (restores original function) or fibrosis (scar tissue, function lost).

Regenerative Capacity of Different Tissues

• High: Epithelial, bone, areolar, dense irregular, blood-forming tissue.

• Moderate: Smooth muscle, dense regular connective tissue.

• Low/None: Cardiac muscle, nervous tissue in brain/spinal cord.

Clinical Relevance

• Scar tissue in organs (especially heart) can impair function, block movement, or cause organ failure.

• Adhesions may cause organs to stick together, further impairing function.