Chapter 4: Histology – Structure and Function of Tissues

Histology: The Study of Tissues

Introduction to Tissues

Histology is the study of the normal structure of tissues, which are groups of structurally and functionally related cells and their external environment that together perform common functions. All tissues share two basic components: a population of cells related in structure and function, and the surrounding material called the Extracellular Matrix (ECM).

Primary Tissue Types

• Epithelial Tissues: Sheets of tightly packed cells with little ECM; cover and line body surfaces and cavities, and form parts of glands.

• Connective Tissues: Connect all other tissues together; cells are scattered through the ECM; bind, support, protect, and allow transport of substances.

• Muscular Tissues: Cells contract and generate force; little ECM.

• Nervous Tissues: Cells (neurons) generate, send, and receive messages; includes supporting cells with unique ECM.

Extracellular Matrix (ECM)

Components and Functions

The ECM is composed of substances surrounding the cells in a tissue, providing strength, directing cells, regulating development, and holding cells in position. It consists of two main components: Ground Substance and Protein Fibers.

• Ground Substance: Gel-like substance containing extracellular fluid (ECF), water, ions, nutrients, and three families of macromolecules:

  • Glycosaminoglycans (GAGs): Negatively charged polysaccharide chains (e.g., hyaluronic acid) that attract positive ions and water.

  • Proteoglycans: GAGs bonded to a protein core; form aggregates that make ECM firmer and act as diffusion barriers.

  • Glycoproteins: Cell-adhesion molecules (CAMs) that bind cell surface proteins and fibers, maintaining tissue architecture.

• Protein Fibers: Provide tensile strength; three types:

  • Collagen Fibers: Resistant to tension and pressure; make up 20-25% of body protein.

  • Elastic Fibers: Made of elastin protein, allow tissues to stretch.

  • Reticular Fibers: Thinner collagen fibers forming supportive scaffolds and webs in organs.

Marfan Syndrome

Marfan Syndrome is a genetic disorder caused by defects in the gene coding for the glycoprotein fibrillin-1. Defective fibrillin-1 results in improperly anchored elastic fibers in the ECM, leading to symptoms such as tall stature, long limbs and fingers, skeletal abnormalities, joint dislocations, heart valve and eye lens issues. The most lethal complication is dilation and rupture of the aorta.

Cell Junctions

Types of Cell Junctions

Cell junctions are connections between neighboring cells in a tissue, linked by integral proteins. They include:

• Tight (Occluding) Junctions: Integral "locking" proteins prevent passage of macromolecules, making spaces between cells impermeable.

• Desmosomes: Integral "linker" proteins absorb and distribute physical force, increasing tissue resistance to mechanical stress.

• Gap Junctions: Protein channels allow small substances to pass freely between cells.

Epithelial Tissues

Functions of Epithelial Tissue

Epithelial tissue acts as a barrier on every external and internal body surface. Its functions include:

• Protection: Shields underlying tissue from injury; produces keratin and undergoes rapid mitosis.

• Immune Defenses: Contains immune cells scattered throughout.

• Secretion: Forms glands producing substances like sweat, oil, and hormones.

• Transport: Selectively permeable barriers allow certain substances to pass.

• Sensation: Supplied with nerves to detect environmental changes.

Components and Classification of Epithelia

The ECM of epithelia is found in the basement membrane, anchoring the tissue to underlying connective tissue. It consists of:

• Basal Lamina: ECM of epithelial tissue; collagen fibers and ground substance.

• Reticular Lamina: Produced by connective tissue; reticular fibers and ground substance.

Epithelial cells have an apical surface (facing extracellular space), basal surface (contacting deeper cells or basal lamina), and lateral surfaces.

Classification of Epithelia

• Number of Cell Layers:

  • Simple Epithelia: Single layer of cells.

  • Stratified Epithelia: More than one layer.

  • Pseudostratified Epithelia: Single layer appearing multilayered.

• Shape of Cells:

  • Squamous: Flattened cells.

  • Cuboidal: Short, cube-shaped cells.

  • Columnar: Tall, elongated cells.

Covering and Lining Epithelia

These epithelia form membranes on inner and outer body surfaces. Simple epithelia are one cell layer thick and facilitate diffusion or transport, while stratified epithelia are thicker and provide protection.

Simple Epithelia

• Simple Squamous: Single layer of flat cells; rapid diffusion; found in lungs, serous membranes, blood vessels.

• Simple Cuboidal: Single layer of cube-shaped cells; diffusion and secretion; found in kidney tubules and glands.

• Simple Columnar: Single layer of tall cells; absorption, secretion, some with microvilli or cilia; found in intestine, uterine tube, kidney tubules.

• Pseudostratified Columnar: Appears stratified; most are ciliated; goblet cells secrete mucus; found in respiratory passages.

Transport Across Simple Epithelia

• Paracellular Transport: Substances leak between cells.

• Transcellular Transport: Substances move through cells, entering via the phospholipid bilayer, diffusing through cytosol, and exiting through the other surface.

Stratified Epithelia

• Stratified Squamous: Nonkeratinized type found in mouth, pharynx, esophagus, anus, vagina.

• Stratified Cuboidal: Rare; two layers; lines sweat gland ducts.

• Stratified Columnar: Rare; columnar in apical layers, cuboidal in basal; found in salivary gland ducts, male urethra, conjunctiva.

• Transitional: Cuboidal in basal, dome-shaped apical cells when relaxed; squamous when stretched; found in urinary tract.

Carcinogens and Carcinomas

Carcinogens are agents that induce DNA changes and may cause cancer. Carcinomas are cancers of epithelial tissue, such as basal cell carcinoma. The basement membrane can slow the spread; if not penetrated, the cancer is called carcinoma in situ.

Summary Tables of Epithelial Tissues

The following tables summarize the main types of simple and stratified epithelia, their components, functions, and locations.

Type	Components	Function	Location
Simple Squamous	Single layer of flat cells	Barrier, rapid diffusion	Air sacs of lungs, serous membranes, blood vessels
Simple Cuboidal	Single layer of cube-shaped cells	Absorption, secretion	Kidney tubules, glands
Simple Columnar	Single layer of tall cells	Absorption, secretion, movement	Digestive tract, uterine tube, kidney tubules
Pseudostratified Columnar	Single layer, appears stratified	Secretion, movement	Respiratory passages, nasal cavity

Type	Components	Function	Location
Keratinized Stratified Squamous	Multiple layers, apical cells dead	Protection from mechanical stress	Epidermis of skin
Nonkeratinized Stratified Squamous	Multiple layers, apical cells alive	Protection from mechanical stress	Mouth, pharynx, esophagus, anus, vagina
Stratified Cuboidal	Two or more layers of cuboidal cells	Some absorption and secretion	Ducts of sweat glands
Stratified Columnar	Two or more layers, apical columnar	Protection, some absorption and secretion	Ducts of salivary glands, male urethra, conjunctiva
Transitional	Multiple layers, apical dome-shaped	Protection, distensibility	Urinary bladder, ureters, urethra

Glandular Epithelia

Types of Glands

• Exocrine Glands: Release secretions to the apical surface via ducts; local actions only.

• Endocrine Glands: Secrete hormones directly into the blood; lack ducts; reach distant targets.

Exocrine glands vary from single-celled (e.g., goblet cells) to multicellular glands with branching ducts and secretory units.

Connective Tissues

Functions and Classification

Connective tissues bind, support, protect, and transport substances. They are classified as:

• Connective Tissue Proper: Includes loose, dense, reticular, and adipose tissues.

• Specialized Connective Tissues: Includes cartilage, bone, and blood.

Cells of Connective Tissue Proper

• Fibroblasts: Produce protein fibers and ECM elements.

• Adipocytes: Fat cells with lipid inclusions.

• Mast Cells: Immune cells with granules containing inflammatory mediators.

• Phagocytes: Immune cells that engulf foreign substances and dead cells.

Types of Connective Tissue Proper

• Loose (Areolar) Connective Tissue: Ground substance with all three fiber types; supports and houses blood vessels.

• Dense Irregular Connective Tissue: Collagen fibers arranged haphazardly; resists tension from all directions.

• Dense Regular Collagenous Connective Tissue: Parallel collagen fibers; resists tension in one direction.

• Dense Regular Elastic Connective Tissue: Mostly parallel elastic fibers; allows organs to stretch.

• Reticular Tissue: Numerous reticular fibers; forms supportive networks and traps foreign cells.

• Adipose Tissue: Fat tissue; functions in insulation, protection, and energy reserve.

Adipose Tissue and Obesity

• White Adipose: Accumulates in various body regions; main function is energy storage.

• Brown Adipose: Abundant in infants; main function is heat production.

• Obesity: Excess adipose tissue; can be hypertrophic (cell size increases) or hypercellular (cell number increases).

Specialized Connective Tissues

Cartilage

• Hyaline Cartilage: Fine collagen bundles; covers ends of bones, nose, respiratory tract.

• Fibrocartilage: Bundles of collagen; found in intervertebral discs and articular discs.

• Elastic Cartilage: Filled with elastic fibers; found in external ear and larynx.

Bone

Bone supports the body, protects organs, provides muscle attachment, stores calcium, and houses bone marrow. Its ECM is 35% organic (collagen and osteoid) and 65% inorganic (calcium phosphate crystals).

• Osteoblasts: Synthesize organic ECM and promote calcium deposition.

• Osteocytes: Mature osteoblasts; maintain bone.

• Osteoclasts: Break down bone ECM.

Blood

Blood is a fluid connective tissue with plasma as ECM. Cells include erythrocytes (oxygen transport), leukocytes (immunity), and platelets (clotting).

Muscle Tissues

Types of Muscle Tissue

• Skeletal Muscle: Attached to skeleton; voluntary movement; multinucleate cells.

• Cardiac Muscle: Found in heart; involuntary; branched cells with intercalated discs.

• Smooth Muscle: Found in hollow organs, blood vessels; involuntary; flattened cells with single nucleus.

Nervous Tissue

Components and Functions

Nervous tissue makes up the brain, spinal cord, and nerves. Its ECM is mostly ground substance with unique proteoglycans. Cells include:

• Neurons: Generate, conduct, and receive nerve impulses; consist of cell body, axon, and dendrites; amitotic.

• Neuroglial Cells: Support neurons, anchor blood vessels, monitor ECF, speed up impulse transmission, circulate fluid; can divide by mitosis.

Tissues in Organs

Organs are composed of two or more tissues that combine structurally and functionally. For example, skeletal muscle contains muscle tissue and dense irregular connective tissue, while the trachea contains six tissue types, each serving a specific role.

Membranes

Types of Membranes

• Serous Membranes: Line body cavities; consist of mesothelium, basement membrane, and loose connective tissue; produce serous fluid.

• Synovial Membranes: Line joint cavities; made of synoviocytes and connective tissue; secrete synovial fluid.

• Mucous Membranes: Line passages opening to the outside; consist of epithelium, basement membrane, lamina propria, and sometimes smooth muscle; contain goblet cells.

• Cutaneous Membrane: Refers to the skin.

Tissue Repair

Regeneration and Fibrosis

Tissue repair involves regeneration (replacement with same cell type) or fibrosis (collagen fills defect, forming scar tissue). The capacity for repair depends on the tissue's ability to undergo mitosis.

• Epithelial Tissues: Typically regenerate.

• Connective Tissues: Mostly regenerate; cartilage often heals by fibrosis.

• Smooth Muscle: Usually regenerates.

• Cardiac and Skeletal Muscle: Generally heal by fibrosis; limited regeneration via satellite cells.

• Nervous Tissue: Neurons generally do not regenerate.

Nutrition and blood supply are important for tissue repair; protein and vitamin C are needed for collagen synthesis, and adequate blood flow is essential for healing.