Chapter 4 The Tissue Level of Organization

Tissues Overview

• Tissues are collections of specialized cells performing specific functions.

• Organs are formed by tissues in combination.

• Histology is the study of tissues.

Four Types of Tissue

• Epithelial: Covers surfaces, lines passageways, forms glands.

• Connective: Fills spaces, supports tissues, transports, stores energy.

• Muscle: Specialized for contraction (skeletal, heart, hollow organs).

• Nervous: Carries electrical signals.

Epithelial Tissue

• Includes epithelia (layers of cells) and glands (secreting structures).

• Functions: Protection, permeability control, provide sensation, produce secretions.

• Characteristics: Polarity, cellularity, attachment (basement membrane), avascularity, regeneration.

Specializations

• Move fluids, transport fluids, produce secretions.

• Polarity:

  • Apical surface (microvilli, cilia).

  • Basolateral surface.

Integrity

• Intercellular connections, attachment to basement membrane, maintenance and repair.

• Intercellular connections:

  • Cell adhesion molecules (CAMs).

  • Proteoglycans (intercellular cement containing glycosaminoglycans like hyaluronan).

• Cell junctions:

  • Gap junctions: Rapid communication via connexons, allow small molecules/ions to pass, coordinate contractions.

  • Tight junctions: Prevent passage of water/solutes, keep enzymes/acids/wastes in lumen.

  • Desmosomes: CAMs and proteoglycans link membranes; spot desmosomes tie cells, hemidesmosomes attach to basement membrane.

Attachment

• Basal lamina (closest to epithelium).

• Reticular lamina (deeper, provides strength).

Maintenance and Repair

• Continual stem cell division near basement membrane.

Classification of Epithelia

• Shape: Squamous (flat), Cuboidal (square), Columnar (tall).

• Layers: Simple (single), Stratified (multiple).

Types

• Squamous:

  • Simple: Absorption/diffusion; mesothelium (body cavities), endothelium (heart/vessels).

  • Stratified: Protection against stress; keratin adds strength.

• Cuboidal:

  • Simple: Secretion/absorption; glands, kidney tubules.

  • Stratified: Rare; ducts of sweat/mammary glands.

• Transitional:

  • Tolerates stretching; appearance changes; urinary bladder.

• Columnar:

  • Simple: Absorption/secretion; stomach, intestines.

  • Pseudostratified: Cilia; nasal cavity, trachea, bronchi.

  • Stratified: Rare; protection in pharynx, anus, urethra.

Glandular Epithelia

• Glands are epithelial cell collections producing secretions.

  • Endocrine: Hormones into bloodstream; no ducts.

  • Exocrine: Secretions through ducts onto surfaces.

Gland Structure

• Unicellular: Goblet cells (mucin to form mucus).

• Multicellular: Classified by duct structure (simple/compound), secretory portion shape (tubular/alveolar), and branching.

Secretion Methods

• Merocrine:

  • Exocytosis (e.g., sweat glands).

• Apocrine:

  • Shedding cytoplasm (e.g., mammary glands).

• Holocrine:

  • Cell bursting (e.g., sebaceous glands).

Secretion Types

• Serous glands: Watery secretions.

• Mucous glands: Secrete mucins.

• Mixed exocrine glands: Both serous and mucous.

Connective Tissue

• Components:

  • Specialized cells.

  • Extracellular protein fibers.

  • Ground substance.

• Matrix: Fibers and ground substance; determines function.

Functions

• Structural framework, transport, protection, support, energy storage (triglycerides), defense.

Categories

• Connective tissue proper: Connect and protect.

• Fluid connective tissues: Transport.

• Supporting connective tissues: Structural strength.

Connective Tissue Proper

• Loose connective tissue:

  • More ground substance, fewer fibers (e.g., adipose tissue).

• Dense connective tissue:

  • More fibers, less ground substance (e.g., tendons).

Cells:

• Fibroblasts: Abundant, secrete proteins & hyaluronan.

• Fibrocytes: Maintain fibers.

• Adipocytes: Fat storage.

• Mesenchymal cells: Stem cells.

• Melanocytes: Melanin.

• Macrophages: Phagocytic.

• Mast cells: Inflammation (histamine, heparin).

• Lymphocytes: Antibodies.

• Microphages: Phagocytic blood cells.

Fibers

• Collagen: Strong, flexible, resist force in one direction; tendons, ligaments.

• Reticular: Stroma network, resist forces in many directions; organ sheaths.

• Elastic: Elastin, branched, wavy, return to original length; elastic ligaments.

Ground Substance

• Viscous, fills spaces, slows pathogens.

Types of Loose Connective Tissues in Adults:

• Areolar tissue:

  • Least specialized, open framework, viscous ground substance, elastic fibers, holds capillary beds, subcutaneous

• Adipose tissue:

  • Adipocytes storing fat (white fat common, brown fat with mitochondria for warmth).

• Reticular tissue:

  • Supportive stroma, liver, kidney, spleen, lymph nodes, bone marrow.

Dense Connective Tissues

• Dense regular: Parallel collagen fibers; tendons (muscle to bone), ligaments (bone to bone), aponeuroses (tendinous sheets).

• Dense irregular: Interwoven collagen; dermis strength, perichondrium/periosteum sheaths, organ capsules.

• Elastic tissue: Elastic fibers; elastic ligaments of vertebrae.

Fasciae

• Connective tissue layers supporting organs.

  • Superficial, deep, subserous.

Fluid Connective Tissues

• Blood:

  • Plasma matrix, formed elements (red/white blood cells, platelets).

• Lymph:

  • Interstitial fluid in lymphatic vessels, monitored by immune system.

Supporting Connective Tissues

• Cartilage: Shock absorption/protection; chondroitin sulfate matrix; chondrocytes in lacunae.

  • Avascular; perichondrium (fibrous and cellular layers).

Types of Cartilage

• Hyaline: Common, flexible, reduces friction; synovial joints, rib tips, sternum, trachea.

• Elastic: Supportive, bends easily; external ear, epiglottis.

• Fibrocartilage: Durable, tough, limits movement; joints, pubic bones, vertebrae.

Cartilage Growth

• Interstitial (from within).

• Appositional (at outer surface).

  • Bone (osseous tissue):

• Weight support, calcified, resists shattering.

• Osteocytes in lacunae around central canals with canaliculi.

• Covered by periosteum (fibrous and cellular layers).

Tissue Membranes

• Physical barriers; epithelium supported by connective tissue.

  • Mucous: Line external passageways; digestive, respiratory, urinary, reproductive tracts; lamina propria.

  • Serous: Line closed cavities; parietal/visceral portions; peritoneum, pleura, pericardium.

  • Cutaneous: Skin, waterproof.

  • Synovial: Line joint cavities; synovial fluid lubrication; no true epithelium.

Muscle Tissue

• Specialized for contraction.

  • Skeletal: Body movement; striated voluntary muscle.

  • Cardiac: Heart; striated involuntary muscle.

  • Smooth: Hollow organs; nonstriated involuntary muscle.

Nervous Tissue

• Electrical impulses; brain and spinal cord.

  • Neurons: conduct impulses (cell body, dendrites, axon).

  • Neuroglia: supporting cells.

Tissue Injuries and Repair

• Inflammation: Triggered by trauma/infection; prostaglandins, proteins, potassium ions released.

Process

• Lysosomes release enzymes (necrosis).

• Necrotic tissues/debris accumulate (abscess).

Regeneration Varies

• Epithelia, connective tissues, smooth muscle regenerate well.

• Skeletal, cardiac, nervous tissues regenerate poorly.

  • Cardiac muscle replaced by fibrosis

Aging Tissues

• Regeneration decreases with age.

Effects of aging

• Thinner epithelia, fragile connective tissues, increased bruising, brittle bones.

Aging and cancer incidence

• Cancer rates increase with age, often due to chemical exposure.