The Tissue Level of Organization

Introduction to Tissues

Tissues are collections of specialized cells and cell products that perform specific functions. The study of tissues is known as histology. Tissues combine to form organs, which then interact within organ systems to maintain the functions of the human body.

• Tissues: Groups of similar cells working together to perform a specific function.

• Organs: Structures composed of multiple tissue types working together.

• Organ Systems: Groups of organs that perform related functions.

Four Major Types of Tissue

Overview of Tissue Types

The human body contains four primary tissue types, each with distinct roles:

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

• Connective Tissue: Fills internal spaces, provides structural support, transports materials, and stores energy.

• Muscle Tissue: Specialized for contraction, producing movement.

• Nervous Tissue: Carries electrical signals throughout the body.

Epithelial Tissue

Functions and Characteristics

Epithelial tissue includes layers of cells that cover surfaces and line cavities, as well as glands that produce secretions. Its main functions are:

• Physical protection

• Control of permeability

• Providing sensation

• Producing specialized secretions

Key characteristics include:

• Polarity: Distinct apical (top) and basal (bottom) surfaces

• Cellularity: Cells are closely bound by cell junctions

• Attachment: Bound to a basement membrane

• Avascularity: Lacks blood vessels

• Regeneration: High capacity for renewal via stem cells

Specializations of Epithelial Cells

• Move fluids over the epithelium (protection)

• Move fluids through the epithelium (permeability)

• Produce secretions (protection and messaging)

Microvilli increase surface area for absorption/secretion, while cilia move substances across the surface.

Maintaining Epithelial Integrity

• Intercellular Connections: Cell adhesion molecules (CAMs), proteoglycans, and specialized junctions (gap junctions, tight junctions, desmosomes)

• Attachment to Basement Membrane: Basal lamina (closest to epithelium) and reticular lamina (provides strength)

• Maintenance and Repair: Stem cells near the basement membrane continually divide to replace lost cells

Classification of Epithelia

Epithelia are classified by cell shape and number of layers:

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

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

Examples of Epithelial Types

• Simple Squamous: Absorption and diffusion (e.g., alveoli of lungs, lining of blood vessels)

• Stratified Squamous: Protection against abrasion (e.g., skin, mouth lining)

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

• Stratified Cuboidal: Protection, secretion (rare; e.g., sweat gland ducts)

• Transitional: Stretches (e.g., urinary bladder)

• Simple Columnar: Absorption and secretion (e.g., digestive tract)

• Pseudostratified Columnar: Protection, secretion, movement of mucus (e.g., trachea)

• Stratified Columnar: Protection (rare; e.g., salivary gland ducts)

Glandular Epithelia

• Endocrine Glands: Release hormones into the bloodstream (no ducts)

• Exocrine Glands: Release secretions onto epithelial surfaces through ducts

Exocrine glands are classified by duct structure (simple or compound), shape (tubular or alveolar), and method of secretion (merocrine, apocrine, holocrine).

Connective Tissue

Functions and Components

Connective tissues provide a structural framework, transport fluids, protect organs, support and connect other tissues, store energy, and defend against pathogens. All connective tissues share three basic components:

• Specialized cells

• Extracellular protein fibers

• Ground substance (fluid)

The matrix consists of fibers and ground substance, determining the tissue's properties.

Categories of Connective Tissue

• Connective Tissue Proper: Connects and protects (e.g., adipose, tendons)

• Fluid Connective Tissues: Transport (e.g., blood, lymph)

• Supporting Connective Tissues: Structural strength (e.g., cartilage, bone)

Connective Tissue Proper

• Loose Connective Tissue: More ground substance, fewer fibers (e.g., areolar, adipose, reticular)

• Dense Connective Tissue: More fibers, less ground substance (e.g., dense regular, dense irregular, elastic)

Cells of Connective Tissue Proper

• Fibroblasts: Produce fibers and ground substance

• Fibrocytes: Maintain fibers

• Adipocytes: Store fat

• Mesenchymal Cells: Stem cells for repair

• Macrophages: Phagocytize pathogens and debris

• Mast Cells: Release histamine and heparin (inflammation)

• Lymphocytes: Immune response

• Microphages: Phagocytic blood cells

• Melanocytes: Synthesize melanin pigment

Connective Tissue Fibers

• Collagen Fibers: Strong, resist tension (tendons, ligaments)

• Reticular Fibers: Form supportive networks (stroma of organs)

• Elastic Fibers: Stretch and return to original length (elastic ligaments)

Types of Loose Connective Tissue

• Areolar Tissue: Cushions organs, supports epithelia

• Adipose Tissue: Stores fat, insulates, cushions

• Reticular Tissue: Supports cells in organs like liver, spleen

Types of Dense Connective Tissue

• Dense Regular: Parallel collagen fibers (tendons, ligaments)

• Dense Irregular: Interwoven fibers (dermis, organ capsules)

• Elastic Tissue: Elastic fibers (vertebral ligaments)

Fasciae

Connective tissue layers that support and surround organs:

• Superficial Fascia: Separates skin from underlying tissues

• Deep Fascia: Dense connective tissue, forms strong framework

• Subserous Fascia: Between deep fascia and serous membranes

Fluid Connective Tissues: Blood and Lymph

Blood

Blood consists of a fluid matrix (plasma) and formed elements:

• Red Blood Cells (Erythrocytes): Transport oxygen and carbon dioxide

• White Blood Cells (Leukocytes): Defend against infection

• Platelets: Involved in blood clotting

Lymph

Lymph forms as interstitial fluid enters lymphatic vessels, is monitored by the immune system, and returns to the bloodstream near the heart.

Supporting Connective Tissues: Cartilage and Bone

Cartilage

• Matrix: Firm gel with chondroitin sulfates

• Cells: Chondrocytes in lacunae

• Avascular: Receives nutrients by diffusion

• Perichondrium: Outer fibrous and inner cellular layers

Types of cartilage:

• Hyaline Cartilage: Most common, reduces friction (joints, ribs, trachea)

• Elastic Cartilage: Flexible (external ear, epiglottis)

• Fibrocartilage: Durable, resists compression (intervertebral discs, pubic symphysis)

Bone (Osseous Tissue)

• Matrix is calcified with collagen fibers for strength and flexibility

• Osteocytes in lacunae, arranged around central canals

• Covered by periosteum (fibrous and cellular layers)

Tissue Membranes

Types of Membranes

• Mucous Membranes: Line passageways with external openings (digestive, respiratory tracts)

• Serous Membranes: Line sealed internal cavities (pleura, pericardium, peritoneum)

• Cutaneous Membrane: Skin, covers body surface

• Synovial Membranes: Line joint cavities, produce synovial fluid

Muscle Tissue

Types of Muscle Tissue

• Skeletal Muscle: Voluntary, striated, moves skeleton

• Cardiac Muscle: Involuntary, striated, found in heart, connected by intercalated discs

• Smooth Muscle: Involuntary, non-striated, found in walls of hollow organs

Nervous Tissue

Structure and Function

Nervous tissue is specialized for conducting electrical impulses and is concentrated in the brain and spinal cord. It consists of:

• Neurons: Conduct electrical impulses

• Neuroglia: Support, protect, and nourish neurons

Parts of a neuron include the cell body, dendrites (receive signals), and axon (transmits signals).

Tissue Injuries and Repair

Inflammation and Regeneration

• Inflammation: Response to injury or infection, involving redness, heat, swelling, pain

• Regeneration: Replacement of damaged tissue; varies by tissue type

• Fibrosis: Replacement of normal tissue with scar tissue (mainly in tissues with poor regenerative capacity)

Aging and Tissues

Effects of Aging

• Slower tissue repair and maintenance

• Thinner epithelia, fragile connective tissues, increased risk of disease

• Increased cancer incidence with age