Tissues

Major Tissue Types

Tissues are groups of cells with similar structure and function. The four major tissue types in the human body are:

• Epithelial Tissue

• Connective Tissue

• Muscle Tissue

• Nervous Tissue

Epithelial Tissues

General Characteristics

Epithelial tissues cover body surfaces, line cavities, and form glands. They are characterized by:

• Tightly packed cells with minimal extracellular matrix

• Avascularity (lack of blood vessels)

• Free (apical) surface exposed to the lumen or exterior

• Basement membrane anchoring the epithelium to underlying connective tissue

• High regenerative capacity

• Innervation (presence of nerves)

Classifying Epithelial Tissues

Epithelial tissues are classified by cell shape and number of layers:

• By Shape: Squamous (flat), Cuboidal (cube-shaped), Columnar (tall)

• By Layers: Simple (one layer), Stratified (multiple layers)

Types of Epithelial Tissues

• Simple Squamous Epithelium

  • Location: Alveoli of lungs, lining of blood vessels (endothelium), serous membranes (mesothelium)

  • Function: Rapid diffusion, filtration, and secretion

• Simple Cuboidal Epithelium

  • Location: Kidney tubules, ducts of glands

  • Function: Secretion and absorption

• Simple Columnar Epithelium

  • Location: Lining of digestive tract, gallbladder

  • Function: Absorption, secretion of mucus and enzymes

  • Specializations: Goblet cells (mucus secretion), microvilli (increase surface area)

• Pseudostratified Columnar Epithelium

  • Location: Lining of trachea, upper respiratory tract

  • Function: Secretion (especially mucus), propulsion by cilia

  • Specialization: Cilia

  

• Stratified Squamous Epithelium

  • Location: Epidermis of skin (keratinized), lining of mouth, esophagus, vagina (non-keratinized)

  • Function: Protection against abrasion

  • Keratinized: Surface cells contain keratin for waterproofing

• Transitional Epithelium

  • Location: Urinary bladder, ureters

  • Function: Allows stretching and distension

Glands

Glands are composed of epithelial cells specialized for secretion. They are classified as:

• Exocrine glands: Secrete products into ducts (e.g., sweat, salivary glands)

• Endocrine glands: Secrete hormones directly into the bloodstream

Methods of Secretion:

• Merocrine: Secrete by exocytosis (e.g., sweat glands)

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

• Holocrine: Entire cell ruptures (e.g., sebaceous glands)

Connective Tissues

General Characteristics

Connective tissues support, bind, and protect other tissues and organs. They are characterized by:

• Vascularity (varying degrees)

• Scattered cells (e.g., fibroblasts, chondrocytes, osteocytes, macrophages, mast cells)

• Extracellular matrix composed of ground substance and fibers (collagen, elastic, reticular)

Ground substance contains hyaluronic acid, providing support and hydration.

Types of Connective Tissues

• Loose Connective Tissues

  • Areolar: Under epithelia; cushions organs; contains fibroblasts, collagen, and elastic fibers

  • Adipose: Under skin, around organs; stores fat; adipocytes; energy storage and insulation

  • Reticular: Lymphoid organs; supports cells; reticular fibers

• Dense Connective Tissues

  • Dense Regular: Tendons, ligaments; parallel collagen fibers; resists tension

  • Dense Irregular: Dermis; irregular collagen fibers; withstands tension in many directions

  • Elastic: Walls of large arteries; elastic fibers; allows recoil

  

• Cartilage

  • General: Avascular, cells in lacunae, surrounded by perichondrium

  • Hyaline Cartilage: Ends of bones, nose, trachea; supports and reinforces; chondrocytes in matrix

  • Fibrocartilage: Intervertebral discs, pubic symphysis; absorbs shock; thick collagen fibers

  • Elastic Cartilage: External ear, epiglottis; maintains shape, flexible; elastic fibers

• Osseous Tissue (Bone)

  • Compact Bone: Shaft of long bones; supports, protects, stores minerals; osteocytes in lacunae

  • Spongy Bone: Ends of long bones; produces blood cells; trabeculae structure

  

• Fluid Connective Tissue

  • Blood: Within blood vessels; transports gases, nutrients, wastes; red and white blood cells in plasma

  • Lymph: Within lymphatic vessels; immune response; lymphocytes in fluid matrix

Muscle Tissue

General Function

Muscle tissue is specialized for contraction, producing movement. There are three types:

• Skeletal Muscle: Attached to bones; voluntary movement; long, striated, multinucleate fibers

• Cardiac Muscle: Heart wall; involuntary; striated, branched, intercalated discs

• Smooth Muscle: Walls of hollow organs; involuntary; non-striated, spindle-shaped cells

Nervous Tissue

Structure and Function

Nervous tissue is found in the brain, spinal cord, and nerves. It is specialized for communication via electrical impulses.

• Neurons: Conduct nerve impulses

• Glial cells: Support, protect, and nourish neurons

Membranes

Body membranes are composed of epithelial and connective tissues, serving as coverings or linings.

• Mucous Membrane: Lines body cavities open to the exterior (e.g., digestive, respiratory tracts); secretes mucus

• Serous Membrane: Lines closed body cavities (e.g., peritoneum, pleura); secretes serous fluid

• Synovial Membrane: Lines joint cavities; secretes synovial fluid for lubrication

• Cutaneous Membrane: The skin; protects body surface

Tissue Repair

Repair Cells

• Parenchymal cells: Functional cells of a tissue

• Stromal cells: Supportive connective tissue cells

Steps of Tissue Repair

1. Inflammation: Damaged cells release signals; increased blood flow; degranulation of mast cells

2. Granulation Tissue Formation: New capillaries and fibroblasts form a temporary matrix

3. Maturation: Regeneration (replacement with original tissue) or fibrosis (replacement with scar tissue)

• Factors Influencing Repair: Vascularity, age, metabolic rate