The Tissue Level of Organization: Study Notes

The Tissue Level of Organization

An Introduction to Tissues

Tissues are collections of specialized cells and cell products that perform specific functions. When combined, tissues form organs such as the heart or liver. The study of tissues is known as histology.

• Tissues are essential for the structure and function of organs.

• Histology provides insight into how tissues contribute to organ function and pathology.

4.1 Four Types of Tissue

Major Tissue Types and Their Roles

• 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; includes skeletal, cardiac, and smooth muscle.

• Nervous Tissue: Carries electrical signals throughout the body.

4.2 Epithelial Tissue

Types and Functions of Epithelial Tissue

• Epithelia: Layers of cells covering surfaces (internal or external).

• Glands: Structures that produce fluid secretions.

Functions of Epithelial Tissue:

• Physical protection

• Control of permeability

• Provision of sensation

• Production of specialized secretions

Characteristics of Epithelia:

• Polarity: Apical (top) and basal (bottom) surfaces

• Cellularity: Cells bound by cell junctions

• Attachment: Bound to a basement membrane

• Avascularity: Lacks blood vessels

• Regeneration: High mitotic rate for repair

Specializations:

• Movement of fluids over/through the epithelium

• Production of secretions

• Microvilli (increase absorption/secretion), cilia (move fluids)

Integrity of Epithelia

• Maintained by intercellular connections, attachment to the basement membrane, and epithelial maintenance/repair.

Types of Cell Junctions:

• Gap Junctions: Allow rapid communication; permit passage of ions and small molecules.

• Tight Junctions: Prevent passage of water/solutes; maintain compartmentalization.

• Desmosomes: Provide strong attachment; spot desmosomes tie cells together, hemidesmosomes attach cells to the basement membrane.

Basement Membrane:

• Basal Lamina: Closest to epithelium

• Reticular Lamina: Provides strength, deeper portion

Maintenance and Repair: Epithelial cells are replaced by stem cells near the basement membrane.

4.3 Classification of Epithelia

Structure-Function Relationships

• By Shape: Squamous (thin, flat), Cuboidal (square), Columnar (tall, slender)

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

Type	Structure	Function	Location
Simple Squamous	Single flat layer	Absorption, diffusion	Mesothelium, endothelium
Stratified Squamous	Multiple flat layers	Protection	Skin, mouth, esophagus
Simple Cuboidal	Single square layer	Secretion, absorption	Glands, kidney tubules
Stratified Cuboidal	Multiple square layers	Protection, secretion	Sweat/mammary ducts (rare)
Transitional	Multiple, variable shape	Stretching	Urinary bladder
Simple Columnar	Single tall layer	Absorption, secretion	Digestive tract
Pseudostratified Columnar	Appears layered, all cells touch basement	Protection, secretion, cilia movement	Respiratory tract
Stratified Columnar	Multiple tall layers	Protection	Pharynx, anus, urethra (rare)

Glandular Epithelia

• Endocrine Glands: Release hormones into bloodstream; ductless.

• Exocrine Glands: Discharge secretions through ducts onto surfaces.

Gland Structure:

• Unicellular: Goblet cells (secrete mucin)

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

Methods of Secretion:

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

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

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

Types of Exocrine Secretions: Serous (watery), mucous (mucins), mixed (both).

4.4 Connective Tissue

Functions and Categories

• Composed of specialized cells, extracellular protein fibers, and ground substance (fluid).

• Matrix: Combination of fibers and ground substance; determines tissue function.

Functions:

• Structural framework

• Transport of fluids/materials

• Protection of organs

• Support and interconnection

• Energy storage (triglycerides)

• Defense against microorganisms

Categories:

• Connective tissue proper (connect/protect)

• Fluid connective tissues (transport)

• Supporting connective tissues (structural strength)

4.5 Connective Tissue Proper

Types, Cells, and Fibers

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

• Dense Connective Tissue: More fibers, less ground substance (e.g., tendons)

Cell Types:

• Fibroblasts: Most abundant; secrete proteins/hyaluronan

• Fibrocytes: Maintain fibers

• Adipocytes: Store fat

• Mesenchymal Cells: Stem cells for repair

• Melanocytes: Produce melanin

• Macrophages: Phagocytosis; fixed or free

• Mast Cells: Stimulate inflammation (histamine, heparin)

• Lymphocytes: Immune response; can become plasma cells

• Microphages: Phagocytic blood cells (neutrophils, eosinophils)

Fibers:

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

• Reticular: Network, resist forces in many directions (sheaths around organs)

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

Ground Substance: Clear, viscous, fills spaces, slows pathogens.

Types of Loose Connective Tissue

• Areolar: Least specialized, holds capillaries, under skin

• Adipose: Fat storage, insulation, energy reserve

• Reticular: Supportive stroma for organs (liver, spleen, lymph nodes)

Types of Dense Connective Tissue:

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

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

• Elastic: Abundant elastic fibers (vertebral ligaments)

Fasciae

• Superficial Fascia: Separates skin from underlying tissues

• Deep Fascia: Dense regular connective tissue sheets

• Subserous Fascia: Between deep fascia and serous membranes

4.6 Blood and Lymph

Fluid Connective Tissues

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

• Lymph: Interstitial fluid entering lymphatic vessels, monitored by immune system, returned to veins

4.7 Supporting Connective Tissues

Cartilage and Bone

• Cartilage: Shock absorption, protection; matrix is firm gel with chondroitin sulfates; cells are chondrocytes in lacunae; avascular

• Perichondrium: Outer fibrous and inner cellular layers

Types of Cartilage:

• Hyaline: Most common, tough/flexible, reduces friction (joints, ribs, trachea)

• Elastic: Flexible, found in ear/epiglottis

• Fibrocartilage: Durable, limits movement, prevents bone contact (intervertebral discs, pubic symphysis)

Growth: Interstitial (within), appositional (surface)

Bone (Osseous Tissue): Weight support, calcified matrix, osteocytes in lacunae, organized around central canals, covered by periosteum.

4.8 Tissue Membranes

Types and Functions

• Mucous Membranes: Line passages with external connections; moist; contain lamina propria (areolar tissue)

• Serous Membranes: Line cavities not open to outside; parietal (cavity), visceral (organ); secrete serous fluid

• Cutaneous Membrane: Skin; thick, waterproof, dry

• Synovial Membranes: Line joint cavities; produce synovial fluid; lack true epithelium

4.9 Muscle Tissue Overview

Types and Features

• Skeletal Muscle: Long fibers, striated, voluntary, do not divide (new fibers from myosatellite cells)

• Cardiac Muscle: Branching cells, intercalated discs, striated, involuntary, regulated by pacemaker cells

• Smooth Muscle: Small, spindle-shaped, nonstriated, involuntary, can divide/regenerate

4.10 Nervous Tissue Overview

Structure and Role

• Specialized for electrical impulse conduction; concentrated in brain and spinal cord

• Neurons: Conduct impulses; have cell body, dendrites (receive signals), axon (transmits signals)

• Neuroglia: Support, repair, and supply nutrients to neurons

4.11 Tissue Injuries and Repair

Response to Injury

• Inflammation: Triggered by trauma or infection; damaged cells release signaling molecules; mast cells activated

• Necrosis: Tissue destruction; pus accumulates; abscess forms if pus is trapped

• Regeneration: Epithelia, connective tissues (except cartilage), and smooth muscle regenerate well; skeletal/cardiac muscle and nervous tissue regenerate poorly

• Fibrosis: Replacement of damaged cardiac muscle with fibrous tissue

4.12 Aging, Regeneration, and Cancer

Aging Effects on Tissues

• Regeneration slows with age due to decreased repair, hormonal changes, and reduced activity

• Results in thinner epithelia, fragile connective tissues, increased bruising, brittle bones, cardiovascular disease, and mental deterioration