Chapter 4: Tissue - The Living Fabric (ANP Study Notes)

Chapter 4: Tissue - The Living Fabric

The Four Types of Tissue

Tissues are groups of cells and cell products that perform specific functions. Histology is the study of tissues. The four main types of tissues combine to form organs and are classified by their structure and function:

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

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

• Muscle tissue: Contracts to produce movement (e.g., biceps, heart).

• Neural tissue: Carries information via electrical signals.

Characteristics and Functions of Epithelial Tissue

Epithelial tissue exists as layers covering surfaces (epithelia) or as clusters forming glands. It serves several essential functions:

• Physical Protection: Forms barriers against mechanical and chemical damage.

• Controls Permeability: Regulates substances passing through the epithelium.

• Produces Specialized Secretions: Secretions provide protection, lubrication, and communication.

• Provides Sensation: Contains sensory nerves for environmental detection.

Key characteristics include:

• Polarity: Cells have distinct apical (free) and basal surfaces. Apical surfaces may have microvilli (for absorption/secretion) or cilia (for moving fluid).

• Cellularity: Epithelial tissues are tightly packed with minimal extracellular material.

• Attachment: Cells are attached to each other and to the basement membrane via cell adhesion molecules (CAMs) and specialized junctions (gap, tight, desmosomes).

• Avascularity: Epithelia lack blood vessels; nutrients diffuse from adjacent connective tissue.

• Regeneration: High rate of cell replacement via mitosis of stem cells near the basement membrane.

Classification of Epithelial Tissue

Epithelia are classified by cell shape and number of layers:

• Cell Shapes: Squamous (thin, flat), Cuboidal (cube-shaped), Columnar (tall, slender).

• Layers: Simple (one layer, for absorption/secretion), Stratified (multiple layers, for protection).

Types of epithelia:

• Simple squamous: Diffusion, permeability, friction reduction. Mesothelium lines body cavities; Endothelium lines heart and blood vessels.

• Stratified squamous: Protection from abrasion. Keratinized (skin), non-keratinized (mouth, esophagus, vagina).

• Simple cuboidal: Secretion and absorption (glands, ducts, kidney).

• Stratified cuboidal: Protection, secretion, absorption (rare; mammary/sweat gland ducts).

• Simple columnar: Secretion and absorption (stomach, intestines).

• Stratified columnar: Protection (rare; pharynx, urethra, anus).

• Pseudostratified columnar: Secretion, cilia movement (nasal cavity, trachea, bronchi).

• Transitional: Stretching/recoiling (urinary bladder, ureters).

Glandular Epithelia

Glands are collections of epithelial cells producing secretions. They are classified as:

• Exocrine glands: Discharge secretions into ducts to epithelial surfaces.

• Endocrine glands: Release hormones directly into the blood (no ducts).

Exocrine glands are further classified by structure and secretion type:

Structure	Examples
Unicellular	Mucous cells (respiratory tract), Goblet cells (digestive tract)
Multicellular	Simple (single duct), Compound (multiple ducts); Tubular or alveolar/acinar shapes

Secretion Type	Description
Serous	Thin, watery, enzyme-rich
Mucous	Thick, mucin-rich
Mixed	Both serous and mucous

Modes of secretion:

• Merocrine: Exocytosis (e.g., sweat glands).

• Apocrine: Shedding of cytoplasm (e.g., mammary glands).

• Holocrine: Cell bursts (e.g., sebaceous glands).

Characteristics and Functions of Connective Tissue

Connective tissue consists of specialized cells and an extracellular matrix (protein fibers and ground substance). Functions include:

• Structural framework, strength, and stability

• Support and interconnection of tissues

• Protection and maintenance of organ positions

• Transport of fluids and dissolved materials

• Defense against microorganisms

• Energy storage

Cell types in connective tissue proper:

• Fibroblasts: Secrete matrix and fibers

• Fibrocytes: Maintain fibers

• Adipocytes: Store fat

• Mesenchymal cells: Stem cells for repair

• Melanocytes: Store melanin pigment

• Mast cells: Stimulate inflammation

• Macrophages: Phagocytose pathogens

• Microphages: Phagocytic white blood cells (neutrophils, eosinophils)

• Lymphocytes: Immune defense, antibody production

Connective tissue fibers:

• Collagen: Strong, flexible, resist force in one direction (skin, tendons, ligaments)

• Reticular: Network, support, resist force in many directions (organs, sheaths)

• Elastic: Stretchy, return to original length (elastic ligaments, arteries)

Ground substance is clear, viscous, and fills spaces between cells and fibers, slowing pathogen movement.

Classification of Connective Tissue in Adults

Type	Function	Examples
Connective tissue proper	Connects and protects	Loose (areolar, adipose, reticular), Dense (regular, irregular, elastic)
Fluid connective tissue	Transport	Blood, lymph
Supporting connective tissue	Structural strength	Cartilage, bone

Connective Tissue Proper and the Fasciae

Embryonic connective tissue (mesenchyme, mucous connective tissue) is only found in embryos. Adult connective tissue proper is divided into:

• Loose connective tissue: More ground substance, fewer fibers. Types: Areolar (packing material), Adipose (fat storage, insulation), Reticular (support for organs).

• Dense connective tissue: More fibers, less ground substance. Types: Regular (parallel collagen, tendons/ligaments), Irregular (interwoven fibers, skin/organs), Elastic (abundant elastic fibers, arteries/vertebrae).

Fasciae are connective tissue layers supporting organs:

• Superficial fascia: Adipose and areolar tissue, separates skin from underlying tissues.

• Deep fascia: Dense regular CT, forms strong internal framework.

• Subserous fascia: Areolar CT, separates deep fascia from serous membranes.

Fluid Connective Tissues

Major extracellular fluids include interstitial fluid (between cells), plasma (blood), and lymph (lymphatic vessels). Fluid connective tissues:

• Blood: Watery matrix (plasma), formed elements (erythrocytes, leukocytes, platelets).

• Lymph: Monitored by immune system, formed from interstitial fluid, returned to venous system.

Supporting Connective Tissues

Cartilage and bone provide structural support:

• Cartilage: Gel-like matrix (chondroitin sulfates), chondrocytes in lacunae, avascular, surrounded by perichondrium. Types: Hyaline (joints, ribs, trachea), Elastic (ear, epiglottis), Fibrocartilage (menisci, discs).

• Bone: Calcified matrix (calcium salts), collagen fibers, osteocytes in lacunae, periosteum surrounds bone. Osteon is the functional unit; central canal contains vessels/nerves; lamellae are matrix layers.

Membranes

Membranes are physical barriers composed of epithelium and connective tissue. Four types:

• Mucous membranes: Line cavities open to exterior (digestive, respiratory, urinary, reproductive); epithelium + lamina propria; moist for absorption/excretion.

• Serous membranes: Line cavities not open to exterior (thoracic, abdominal); simple squamous epithelium + areolar CT; produce serous fluid. Layers: Visceral (covers organs), Parietal (lines cavity walls). Locations: Pleura (lungs), Pericardium (heart), Peritoneum (abdomen).

• Cutaneous membrane: Skin; thick, water-resistant, dry.

• Synovial membranes: Line joint cavities; lack true epithelium; produce synovial fluid for lubrication.

Muscle Tissue

Muscle tissue consists of elongated cells specialized for contraction and movement. Three types:

• Skeletal muscle: Large, multinucleated, striated, voluntary control; moves skeleton.

• Cardiac muscle: Branching, uninucleated, striated, involuntary; found only in heart; connected at intercalated discs.

• Smooth muscle: Small, spindle-shaped, uninucleated, non-striated, involuntary; found in walls of hollow organs (blood vessels, bladder, digestive tract).

Nervous Tissue

Nervous tissue is concentrated in the brain, spinal cord, and nerves. It senses the environment, conducts electrical impulses, and processes information. Two main cell types:

• Neurons: Functional cells; regions include cell body (nucleus), dendrites (receive signals), axon (sends signals; action potential).

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

The axon hillock is the connection between the axon and neuron body.

Tissue Response to Injury

Tissues restore homeostasis after injury in two stages:

1. Inflammation: Caused by trauma or infection. Damaged cells release chemicals, activating mast cells to release histamine, heparin, and prostaglandins. Results in redness, warmth, swelling (edema), pain, and tissue death (necrosis). Phagocytes clean up debris.

2. Regeneration: Fibroblasts lay down collagen (scar tissue), new cells migrate or are produced by mesenchymal stem cells. Epithelia, most connective tissues, and smooth muscle regenerate well; cartilage, skeletal muscle, cardiac muscle, and nervous tissue regenerate poorly.

Effect of Aging on Tissues

Tissue repair slows with age due to decreased metabolism, hormonal changes, and reduced activity. Effects include:

• Thinning epithelia and connective tissues

• Increased bruising, joint pain, bone brittleness

• Mental deterioration

• Cardiovascular disease (leading cause of death)

• Increased cancer rates (second leading cause of death)

Additional info: Classification tables and details on cell junctions, fasciae, and tissue regeneration were expanded for academic completeness.