Tissues: Structure and Function in Anatomy & Physiology

Tissues: Structure and Function

Introduction to Tissues

Tissues are groups of similar cells that perform specific functions in the body. The study of tissues is known as histology. There are four major types of tissue in the human body: epithelial, connective, muscle, and nervous tissue.

• Epithelial tissue: Covers exposed surfaces, lines internal passageways, and forms glands.

• Connective tissue: Fills internal spaces, supports other tissues, transports materials, and stores energy.

• Muscle tissue: Specialized for contraction, enabling movement.

• Nervous tissue: Carries electrical signals from one part of the body to another.

Epithelial Tissue

General Characteristics

• Composed of layers of cells covering internal or external surfaces.

• Forms glands that produce fluid secretions.

Functions of Epithelial Tissue

• Provides physical protection.

• Controls permeability (regulates entry and exit of substances).

• Provides sensation (contains sensory nerves).

• Produces specialized secretions (glandular epithelium).

Characteristics of Epithelia

• Polarity (apical and basal surfaces).

• Cellularity (cells bound by cell junctions).

• Attachment to a basement membrane.

• Avascularity (no direct blood supply).

• Regeneration (high rate of cell division).

Specializations of Epithelial Cells

• Move fluids over the epithelium (protection).

• Move fluids through the epithelium (permeability).

• Produce secretions (protection and messaging).

Intercellular Connections

• Cell adhesion molecules (CAMs): Transmembrane proteins that connect cells.

• Proteoglycans: Act as intercellular cement (contain glycosaminoglycans such as hyaluronan).

• Cell junctions:

  • Gap junctions: Allow rapid communication; permit ions and small molecules to pass (e.g., cardiac muscle contractions).

  • Tight junctions: Prevent passage of water and solutes; keep enzymes, acids, and wastes in the lumen of the digestive tract.

  • Desmosomes: Spot and hemidesmosomes; tie cells together and attach cells to the basement membrane.

Attachment to the Basement Membrane

• Basal lamina: Closest to the epithelium.

• Reticular lamina: Deeper portion, provides strength.

Epithelial Maintenance and Repair

• Continual division of stem cells located near the basement membrane.

Classification of Epithelia

Epithelia are classified by cell shape and number of layers.

• By shape:

  • Squamous: Thin and flat.

  • Cuboidal: Square-shaped.

  • Columnar: Tall, slender rectangles.

• By layers:

  • Simple: Single layer of cells.

  • Stratified: Multiple layers of cells.

Types of Epithelia and Their Functions

• Simple squamous epithelium: Absorption and diffusion; forms inner lining of heart and blood vessels.

• Stratified squamous epithelium: Protects against mechanical stresses (e.g., skin).

• Simple cuboidal epithelium: Secretion and absorption (e.g., kidney tubules).

• Transitional epithelium: Tolerates repeated cycles of stretching (e.g., urinary bladder).

• Simple columnar epithelium: Absorption and secretion (e.g., stomach, intestines).

• Pseudostratified columnar epithelium: Typically have cilia (e.g., nasal cavity, trachea).

Glandular Epithelium

• Endocrine glands: Release hormones into the bloodstream; no ducts.

• Exocrine glands: Discharge secretions through ducts onto epithelial surfaces.

Modes of Secretion

• Merocrine secretion: Released by secretory vesicles (exocytosis); e.g., merocrine sweat glands.

• Apocrine secretion: Released by shedding cytoplasm; e.g., mammary glands.

• Holocrine secretion: Released by cells bursting; e.g., sebaceous glands.

Types of Secretions Produced by Exocrine Glands

• Serous glands: Watery secretions.

• Mucous glands: Secrete mucins.

• Mixed exocrine glands: Both serous and mucous.

Connective Tissue

Components of Connective Tissues

• Specialized cells

• Extracellular protein fibers

• Fluid called ground substance

Functions of Connective Tissues

• Establishing a structural framework for the body

• Transporting fluids and dissolved materials

• Protecting delicate organs

• Supporting, surrounding, and interconnecting other types of tissue

• Storing energy reserves, especially triglycerides

• Defending the body from invading microorganisms

Categories of Connective Tissues

• Connective tissue proper: Connect and protect (e.g., tendons, adipose tissue).

• Fluid connective tissues: Transport (e.g., blood, lymph).

• Supporting connective tissues: Structural strength (e.g., cartilage, bone).

Cells of Connective Tissue Proper

• Fibroblasts: Most abundant; secrete proteins and hyaluronan.

• Fibrocytes: Second most abundant; maintain connective tissue fibers.

• Adipocytes: Fat cells; store energy.

• Mesenchymal cells: Stem cells that respond to injury or infection.

• Melanocytes: Synthesize and store melanin.

• Macrophages: Engulf pathogens and damaged cells; can be fixed or free.

• Mast cells: Stimulate inflammation; release histamine and heparin.

• Lymphocytes: Migrate throughout the body; produce antibodies.

• Microphages: Phagocytic blood cells (e.g., neutrophils, eosinophils).

Connective Tissue Fibers

• Collagen fibers: Most common; strong and flexible; resist force in one direction (e.g., tendons, ligaments).

• Reticular fibers: Form a network of interwoven fibers; strong and flexible; resist force in many directions (e.g., liver, spleen).

• Elastic fibers: Contain elastin; branched and wavy; return to original length after stretching (e.g., elastic ligaments of vertebrae).

Types of Connective Tissue Proper

• Loose connective tissues: "Packing materials"; fill spaces between organs, cushion cells, and support epithelia.

  • Areolar tissue: Open framework; holds capillary beds (e.g., under skin).

  • Adipose tissue: Contains many adipocytes; stores fat; provides insulation and energy reserve.

  • Reticular tissue: Provides support; found in liver, kidney, spleen, lymph nodes, and bone marrow.

• Dense connective tissues: Tightly packed, high numbers of collagen fibers.

  • Dense regular: Parallel collagen fibers (e.g., tendons, ligaments).

  • Dense irregular: Interwoven networks of collagen fibers (e.g., dermis of skin).

  • Elastic tissue: Made of elastic fibers (e.g., elastic ligaments of spinal vertebrae).

Fluid Connective Tissues

• Blood: Contains plasma (matrix), red blood cells (erythrocytes), white blood cells (leukocytes), and platelets.

• Lymph: Forms as interstitial fluid that enters lymphatic vessels; monitored by immune system.

Supporting Connective Tissues

• Cartilage: Matrix is a firm gel containing chondroitin sulfates; cells are chondrocytes in lacunae.

  • Hyaline cartilage: Most common; tough and somewhat flexible; reduces friction between bones.

  • Elastic cartilage: Supportive but bends easily (e.g., external ear).

  • Fibrocartilage: Very durable and tough; resists compression (e.g., intervertebral discs).

• Bone (osseous tissue): For weight support; matrix is calcified (calcium salts); cells are osteocytes in lacunae; periosteum covers bone.

Growth and Repair

• Cartilage growth:

  • Interstitial growth: Enlarges cartilage from within.

  • Appositional growth: Growth at outer surface of cartilage.

• Bone growth: Involves osteocytes, canaliculi, and periosteum.

Tissue Membranes

• Physical barriers that line or cover body surfaces; consist of an epithelium supported by connective tissue.

Four Types of Tissue Membranes

• Mucous membranes (mucosae): Line passageways with external connections (digestive, respiratory, urinary, reproductive tracts); epithelial surface must be moist.

• Serous membranes: Line cavities not open to the outside; thin but strong; secrete serous fluid to reduce friction.

  • Peritoneum: Lines peritoneal cavity; covers abdominal organs.

  • Pleura: Covers lungs.

  • Pericardium: Covers heart.

• Cutaneous membrane: Skin; covers the body; thick, waterproof, and usually dry.

• Synovial membrane: Line synovial joint cavities; produce synovial fluid for lubrication.

Muscle Tissue

• Specialized for contraction; produces movement.

• Three types:

  • Skeletal muscle: Large muscles responsible for body movement; cells are long, thin, called muscle fibers; striated voluntary muscle.

  • Cardiac muscle: Found only in the heart; cells form branching networks connected at intercalated discs; regulated by pacemaker cells; striated involuntary muscle.

  • Smooth muscle: Found in walls of hollow, contracting organs; cells are small and spindle-shaped; can divide and regenerate; nonstriated involuntary muscle.

Nervous Tissue

• Specialized for conducting electrical impulses; concentrated in the brain and spinal cord.

• Two types of cells:

  • Neurons: Nerve cells that perform electrical communication.

  • Neuroglia: Supporting cells; repair and supply nutrients to neurons.

• Parts of a neuron:

  • Cell body: Contains the nucleus and nucleolus.

  • Dendrites: Short branches extending from the cell body; receive incoming signals.

  • Axon (nerve fiber): Long, thin extension of the cell body; carries outgoing electrical signals.