Tissue: The Living Fabric

Introduction to Tissues

Tissues are groups of cells that are similar in structure and perform a common or related function. 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: Covers surfaces

• Connective Tissue: Supports and binds other tissues

• Muscle Tissue: Produces movement

• Nervous Tissue: Controls and communicates

Epithelial Tissue

Structural and Functional Characteristics

• Polarity: Epithelial cells have an apical (upper, free) surface and a basal (lower, attached) surface.

• Specialized Contacts: Cells are tightly joined by junctions (tight junctions, desmosomes) to form continuous sheets.

• Supported by Connective Tissue: The basement membrane consists of basal lamina and reticular lamina.

• Avascular but Innervated: No blood vessels, but contains nerve fibers.

• Regeneration: High capacity for renewal and repair.

Classification of Epithelial Tissue

• By Layers:

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

  • Stratified Epithelium: Two or more layers; functions in protection.

• By Cell Shape:

  • Squamous: Flattened, scale-like cells

  • Cuboidal: Box-like, cube-shaped cells

  • Columnar: Tall, column-shaped cells

Types of Epithelia

• Simple Squamous Epithelium: Thin, flat cells; found in areas of diffusion/filtration (e.g., alveoli, blood vessels). Includes endothelium (lining blood vessels) and mesothelium (lining serous membranes).

• Simple Cuboidal Epithelium: Cube-shaped cells; functions in secretion and absorption; found in gland ducts and kidney tubules.

• Simple Columnar Epithelium: Tall cells; absorption and secretion; often with microvilli, mucus-producing cells, and cilia; found in digestive tract.

• Pseudostratified Columnar Epithelium: Appears stratified due to varying nucleus height, but all cells touch basement membrane; often ciliated; found in respiratory tract.

• Stratified Squamous Epithelium: Multiple layers; protection; found in skin (keratinized) and moist linings (non-keratinized) like mouth, esophagus.

• Stratified Cuboidal Epithelium: Rare; two layers; found in ducts of larger glands (sweat, mammary).

• Stratified Columnar Epithelium: Limited distribution; found in pharynx, male urethra, some ducts.

• Transitional Epithelium: Dome-shaped apical cells; stretches; lines urinary organs (bladder, ureters).

Glandular Epithelium

• Glands: One or more cells that secrete a product.

• Endocrine Glands: Ductless; secrete hormones into blood; mostly multicellular.

• Exocrine Glands: Secrete onto body surfaces or into cavities; includes mucous, sweat, oil, salivary glands.

• Unicellular Exocrine Glands: Mucous and goblet cells; secrete mucin via exocytosis.

• Multicellular Exocrine Glands: More complex; have ducts and secretory units.

  • Simple Glands: Unbranched duct

  • Compound Glands: Branched duct

• Modes of Secretion:

  • Merocrine: Secrete via exocytosis (e.g., sweat, saliva)

  • Holocrine: Cell ruptures to release product (e.g., oil glands)

  • Apocrine: Apex of cell pinches off (controversial in humans)

Connective Tissue

Characteristics

• Common Origin: All connective tissue arises from mesenchyme (embryonic tissue).

• Degrees of Vascularity: Varies from avascular (cartilage) to richly vascularized (bone, adipose).

• Extracellular Matrix: Large nonliving matrix separates living cells; allows tissue to bear weight and withstand tension.

Structural Elements

• Ground Substance: Unstructured material filling space between cells; composed of interstitial fluid, cell adhesion proteins, proteoglycans.

• Fibers:

  • Collagen: Thick, strong, most abundant

  • Elastic: Long, thin, stretch and recoil

  • Reticular: Short, fine, branching network

• Cells: Immature (-blast) and mature (-cyte) forms; fibroblasts, chondroblasts, osteoblasts; also fat cells, white blood cells, mast cells, macrophages.

Types of Connective Tissue

• Connective Tissue Proper: All mature CT except bone, cartilage, blood.

  • Loose Connective Tissue:

    • Areolar: Gel-like matrix, all fiber types, wraps/cushions organs, most widespread.

    • Adipose: Fat storage, packed adipocytes, shock absorber, insulator, energy site.

    • Reticular: Only reticular fibers, supports blood cells in lymph nodes, spleen, bone marrow.

  • Dense Connective Tissue:

    • Dense Regular: Parallel collagen fibers, resistance to tension in one direction; tendons, ligaments.

    • Dense Irregular: Irregular collagen bundles, strength in multiple directions; dermis, joint capsules.

    • Elastic: Mainly elastic fibers; walls of large arteries (e.g., aorta).

• Cartilage: Tough, flexible, avascular, 80% water, chondrocytes.

  • Hyaline: Most abundant, glossy matrix, covers ends of long bones, embryonic skeleton, nose, ribs.

  • Elastic: Visible elastic fibers, external ear, epiglottis.

  • Fibrocartilage: Alternating chondrocytes and collagen fibers, compressible, intervertebral discs, menisci.

• Bone (Osseous Tissue): Supports, protects, stores fat, synthesizes blood cells; rigid matrix with calcium salts and collagen; osteoblasts (produce matrix), osteocytes (mature cells); well supplied with nerves and blood vessels.

• Blood: Fluid matrix; red blood cells, white blood cells, platelets; transports nutrients, wastes, gases.

Muscle Tissue

Types and Functions

• Skeletal Muscle: Attached to skeleton, long cylindrical cells, multinucleated, voluntary, striated.

• Cardiac Muscle: Heart wall, striated, single nucleus per cell, intercalated discs, involuntary.

• Smooth Muscle: Walls of hollow organs, long tapered cells, single nucleus, involuntary, non-striated.

Nervous Tissue

General Characteristics

• Neurons: Specialized cells that transmit electrical impulses.

• Neuroglia: Supporting cells; found in CNS and PNS.

Membranes

Epithelial Membranes

• Composed of epithelium plus underlying connective tissue.

• Cutaneous Membrane: Skin; only dry membrane.

• Mucous Membrane: Lines cavities open to exterior; secretes mucus for moisture and protection.

• Serous Membrane: Lines cavities not open to exterior; always in pairs (visceral and parietal layers) with serous fluid between; examples: peritoneum, pleurae, pericardium.

Connective Tissue Membranes

• Synovial Membrane: Loose areolar tissue, no epithelium; surrounds joints; produces synovial fluid.

Summary Table: Types of Tissues and Their Functions

Additional info:

• Mesenchyme is the embryonic tissue from which all connective tissues derive.

• Basement membrane is essential for anchoring epithelium to underlying connective tissue.

• Serous membranes reduce friction between organs by secreting serous fluid.

• Muscle tissue types differ in structure, control (voluntary/involuntary), and location.