Tissue: The Living Fabric

Introduction to Tissues

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

Epithelial Tissue

Overview and Functions

Epithelial tissue (epithelium) is a sheet of cells that covers body surfaces or lines body cavities. It exists in two main forms: covering and lining epithelia (e.g., skin, lining of organs) and glandular epithelia (secretory tissue in glands). Main functions include protection, absorption, filtration, excretion, secretion, and sensory reception.

Special Characteristics of Epithelial Tissue

• Polarity: Cells have an apical (top) and basal (bottom) surface, each with distinct structures and functions. The apical surface may have microvilli; the basal surface attaches to the basal lamina.

• Specialized Contacts: Cells are closely joined by tight junctions and desmosomes, forming continuous sheets.

• Supported by Connective Tissues: The basement membrane (basal and reticular lamina) reinforces the epithelium and resists stretching and tearing.

• Avascular but Innervated: Epithelia lack blood vessels but are supplied by nerve fibers; nutrients diffuse from underlying connective tissue.

• Regeneration: High regenerative capacity, especially in areas exposed to friction or hostile environments.

Classification of Epithelia

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

• Simple epithelia: Single cell layer (for absorption, secretion, filtration).

• Stratified epithelia: Two or more layers (for protection).

Cell shapes include:

• Squamous: Flattened and scale-like

• Cuboidal: Box-like, cube-shaped

• Columnar: Tall, column-like

Types of Simple Epithelia

• Simple Squamous Epithelium: Single layer of flat cells; allows rapid diffusion (e.g., alveoli in lungs, kidney glomeruli, lining of blood vessels—endothelium, and serous membranes—mesothelium).

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

• Simple Columnar Epithelium: Single layer of tall cells; may have microvilli or cilia, and goblet cells. Functions in absorption and secretion (e.g., digestive tract, gallbladder, bronchi, uterine tubes).

• Pseudostratified Columnar Epithelium: Appears multilayered but is a single layer; often ciliated and contains goblet cells. Functions in secretion and movement of mucus (e.g., upper respiratory tract, ducts of large glands).

Types of Stratified Epithelia

• Stratified Squamous Epithelium: Multiple layers; protects against abrasion. Keratinized type forms the skin; nonkeratinized type lines moist cavities (e.g., mouth, esophagus).

• Stratified Cuboidal Epithelium: Rare; typically two layers; found in sweat, mammary, and salivary glands.

• Stratified Columnar Epithelium: Also rare; found in pharynx, male urethra, and some glandular ducts; apical layer is columnar.

• Transitional Epithelium: Basal cells are cuboidal or columnar; surface cells change shape to allow stretching (e.g., bladder, ureters, urethra).

Glandular Epithelia

A gland consists of one or more cells that secrete an aqueous fluid (secretion). Glands are classified by site of product release (endocrine or exocrine) and number of cells (unicellular or multicellular).

• Endocrine glands: Ductless; secrete hormones into interstitial fluid, which enters the blood or lymph.

• Exocrine glands: Secrete products into ducts or onto surfaces (e.g., sweat, oil, salivary glands); can be unicellular (goblet cells) or multicellular.

• Unicellular exocrine glands: Goblet cells and mucous cells; produce mucin, which forms mucus.

• Multicellular exocrine glands: Composed of a duct and secretory unit; classified by duct structure (simple or compound) and secretory unit shape (tubular, alveolar, or tubuloalveolar).

• Modes of secretion: Merocrine (by exocytosis), holocrine (cell ruptures), and apocrine (apex ruptures; controversial in humans).

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, energy storage, and transport of substances (e.g., blood). The four main classes are: connective tissue proper, cartilage, bone, and blood.

Common Characteristics of Connective Tissue

• All arise from mesenchyme (embryonic tissue).

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

• Composed of cells and an extensive extracellular matrix (ECM) (protein-sugar mesh).

Structural Elements of Connective Tissue

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

• Fibers: Collagen (strongest), elastic (stretch and recoil), reticular (short, fine, branched).

• Cells: "Blast" cells (immature, secrete matrix; e.g., fibroblasts, chondroblasts, osteoblasts), "cyte" cells (mature, maintain matrix), fat cells, white blood cells, mast cells, macrophages.

Types of Connective Tissue Proper

Loose Connective Tissues

• Areolar: Most widely distributed; supports and binds tissues; holds body fluids; defends against infection; stores nutrients.

• Adipose: Stores nutrients; provides insulation and shock absorption; cells are adipocytes.

• Reticular: Resembles areolar but with reticular fibers; supports free blood cells in lymph nodes, spleen, bone marrow.

Dense Connective Tissues

• Dense Regular: Parallel collagen fibers; high tensile strength; found in tendons and ligaments.

• Dense Irregular: Thicker, irregularly arranged collagen fibers; resists tension from many directions; found in dermis, joint capsules.

• Elastic: Contains many elastic fibers; allows tissue to recoil after stretching; found in walls of large arteries.

Types of Cartilage

• Hyaline Cartilage: Most abundant; supports and reinforces; found at ends of long bones, nose, trachea, larynx, ribs.

• Elastic Cartilage: Similar to hyaline but with more elastic fibers; maintains shape and flexibility; found in ear and epiglottis.

• Fibrocartilage: Strong, resists compression; found in intervertebral discs, knee menisci, pubic symphysis.

Bone (Osseous Tissue)

Bone supports and protects body structures, stores fat, and synthesizes blood cells. It has a hard, calcified matrix with collagen fibers. Osteoblasts produce matrix; osteocytes maintain it. Bone is richly vascularized and organized into osteons.

Blood

Blood is a fluid connective tissue consisting of cells (red and white blood cells, platelets) suspended in plasma. It functions in the transport of gases, nutrients, wastes, and other substances.

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, cardiac, and smooth muscle.

Skeletal Muscle

Attached to bones; voluntary control; cells are long, multinucleated, and striated.

Cardiac Muscle

Found only in the heart; involuntary; striated with one nucleus per cell; cells branch and connect at intercalated discs.

Smooth Muscle

Found in walls of hollow organs (except heart); involuntary; spindle-shaped cells with one nucleus; no striations.

Nervous Tissue

Overview

Nervous tissue is the main component of the nervous system (brain, spinal cord, nerves). It regulates and controls body functions. It consists of neurons (generate and conduct impulses) and glial cells (support, insulate, protect neurons).

Covering and Lining Membranes

Overview

Membranes are composed of at least two primary tissue types: an epithelium bound to underlying connective tissue. The three main types are cutaneous, mucous, and serous membranes.

Cutaneous Membrane

The skin; consists of keratinized stratified squamous epithelium attached to a thick layer of connective tissue. It is a dry membrane.

Mucous Membranes

Line body cavities open to the exterior (digestive, respiratory, urogenital tracts); moist membranes; epithelial sheet over lamina propria; may secrete mucus.

Serous Membranes

Line closed ventral body cavities; composed of simple squamous epithelium (mesothelium) on areolar connective tissue. Parietal serosae line cavity walls; visceral serosae cover organs. The cavity between layers contains serous fluid. Examples: pleurae (lungs), pericardium (heart), peritoneum (abdomen).

Tissue Repair

Overview

When tissue barriers are breached, inflammatory and immune responses are activated. Repair occurs by regeneration (same tissue replaces lost tissue) or fibrosis (connective tissue replaces lost tissue, forming scar tissue).

Steps in Tissue Repair

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

2. Organization: Blood clot replaced by granulation tissue; epithelium regenerates; fibroblasts produce collagen; debris is phagocytized.

3. Regeneration and Fibrosis: Scab detaches; fibrous tissue matures; epithelium thickens; scar tissue may remain.

Regenerative Capacity of Tissues

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

• Moderate: Smooth muscle, dense regular connective tissue

• Poor/None: Cardiac muscle, nervous tissue in brain and spinal cord

Developmental Aspects of Tissues

Tissues function well through youth and middle age with proper nutrition and minimal injury. With aging, epithelia thin, repair is less efficient, and tissues such as bone, muscle, and nervous tissue atrophy. DNA mutations increase cancer risk.