The Tissue Level of Organization

Chapter 4: The Tissue Level of Organization Overview

• This chapter is based on Principles of Anatomy and Physiology by Gerard Tortora and Bryan Derrickson, Sixteenth Edition.

• The chapter's primary objectives are:   - To understand various types of tissues and their origins.   - To discuss the mechanisms of cell adhesion.   - To compare the four primary tissue types: epithelial, connective, muscular, and nervous.   - To examine the structure and function of biological membranes.   - To explain cellular and tissue repair processes.

Definition and Types of Tissues

• Tissue: A group of cells that usually have a common embryonic origin and function together to carry out specialized activities.

• The Four Basic Tissue Types:   

  - Epithelial Tissue: Covers body surfaces and lines hollow organs, body cavities, and ducts; it also forms glands.   

  - Connective Tissue: Protects and supports the body and its organs. Various types bind organs together, store energy reserves as fat, and help provide the body with immunity to disease-causing organisms.   

  - Muscular Tissue: Composed of cells specialized for contraction and generation of force. In the process, muscular tissue generates heat that warms the body.   

  - Nervous Tissue: Detects changes in a variety of conditions inside and outside the body and responds by generating electrical signals called nerve impulses (action potentials) that activate muscular contractions and glandular secretions.

Comparison Between Epithelial and Connective Tissues

• Epithelial Tissue:   - Many cells are tightly packed together.   - Little to no extracellular matrix (ECM).   - Always has a free surface (apical surface).   - Avascular (lacks blood vessels).   - Visualized at $500 \times$ magnification as dense clusters of cells.

• Connective Tissue:   - Large amount of extracellular matrix separates relatively few, scattered cells.   - Highly vascular (with exceptions like cartilage and tendons).   - Does not usually occur on free surfaces.

General Features of Epithelial Tissue

• Cells are arranged in continuous sheets, in either single or multiple layers.

• Cells are densely packed with many cell junctions.

• Polarity:  

   - Apical (free) surface: Faces the body surface, a body cavity, the lumen of an internal organ, or a tubular duct.

  - Lateral surfaces: Face adjacent cells on either side; contain junctions.

   - Basal surface: Opposite the apical surface; adheres to extracellular materials.

• Basement Membrane: A thin extracellular layer consisting of two layers:  

   - Basal lamina: Closer to and secreted by the epithelial cells.  

   - Reticular lamina: Closer to the underlying connective tissue.

• Innervation and Vascularity: Epithelial tissue has a nerve supply but is avascular (vessels are located in the adjacent connective tissue; exchange occurs via diffusion).

• Regeneration: Has a high rate of cell division (mitosis) because it is repeatedly subjected to physical stress and injury.

Classification of Epithelial Tissue

Epithelia are classified based on cell shape and the number of layers.

Classification by Layers

• Simple: A single layer of cells.

• Pseudostratified: Appears to have multiple layers because cell nuclei are at different levels, but all cells rest on the basement membrane.

• Stratified: Two or more layers of cells.

Classification by Shape

• Squamous: Thin, flat cells.

• Cuboidal: Cube-shaped; about as tall as they are wide.

• Columnar: Tall and cylindrical like columns.

• Transitional (Urothelium): Cells that change shape from squamous to cuboidal depending on the degree of stretch.

Detailed Survey of Surface Epithelia

A. Simple Squamous Epithelium

• Description: Single layer of flat cells resembling a tiled floor; centrally located nucleus.

• Location: Lines the cardiovascular (CV) and lymphatic system (endothelium); forms the epithelial layer of serous membranes (mesothelium). Also found in air sacs (alveoli) of lungs, glomerular capsules of kidneys, inner surface of the tympanic membrane (eardrum), and cornea.

• Function: Filtration, diffusion, and secretion.

B. Simple Cuboidal Epithelium

• Description: Single layer of cube-shaped cells; round, central nucleus.

• Location: Covers surface of ovaries; lines anterior surface of eye lens capsule; forms pigmented epithelium of the retina; lines kidney tubules and small ducts of many glands.

• Function: Secretion and absorption.

C. Nonciliated Simple Columnar Epithelium

• Description: Single layer of nonciliated column-like cells with oval nuclei near the base. Contains microvilli at the apical surface and goblet cells (which secrete mucus).

• Location: Lines gastrointestinal (GI) tract (from stomach to anus), ducts of many glands, and gallbladder.

• Function: Secretion and absorption; mucus lubricates linings and prevents destruction of the stomach lining by acidic juices.

D. Ciliated Simple Columnar Epithelium

• Description: Single layer of ciliated column-like cells; oval nuclei near base; goblet cells interspersed.

• Location: Lines some bronchioles, uterine (fallopian) tubes, uterus, paranasal sinuses, central canal of spinal cord, and ventricles of the brain.

• Function: Cilia move mucus and foreign particles toward the throat; cilia also move oocytes through the fallopian tubes to the uterus.

E. Pseudostratified Columnar Epithelium

• Nonciliated: Lines the epididymis, larger ducts of glands, and parts of the male urethra. Functions in absorption and secretion.

• Ciliated: Lines airways of most of the upper respiratory tract. Functions in secreting mucus (goblet cells) and sweeping it away with cilia.

F. Stratified Squamous Epithelium

• Description: Multiple layers; apical cells are flat; deep layers vary from cuboidal to columnar. Basal cells divide and push upward; as they move toward the surface, they dehydrate and harden.

• Keratinized: Contains the protein keratin; forms the superficial layer of the skin (epidermis).

• Nonkeratinized: Does not contain large amounts of keratin; lines wet surfaces (mouth, esophagus, pharynx, vagina, epiglottis) and covers the tongue.

• Function: Protection against abrasion, water loss, UV radiation, and foreign invasion; first line of defense.

G. Stratified Cuboidal Epithelium

• Description: Two or more layers; apical cells are cube-shaped. Rare.

• Location: Ducts of adult sweat glands and esophageal glands; part of male urethra.

• Function: Protection; limited secretion and absorption.

H. Stratified Columnar Epithelium

• Description: Shortened, irregularly shaped basal layers; only apical layer is columnar. Uncommon.

• Location: Lines part of urethra; large excretory ducts of some glands (esophageal); anal mucous membrane; conjunctiva of the eye.

• Function: Protection and secretion.

I. Urothelium (Transitional Epithelium)

• Description: Variable appearance. Relaxed tissue looks like stratified cuboidal; stretched tissue looks like stratified squamous.

• Location: Lines urinary bladder, ureters, and portions of the urethra.

• Function: Allows urinary organs to stretch while maintaining a protective lining.

Glandular Epithelium

• Gland: A single cell or a mass of epithelial cells specialized for secretion.

• Endocrine Glands:  

   - Description: Ductless. Secretions (hormones) enter interstitial fluid and diffuse into the bloodstream.   

  - Examples: Pituitary, pineal, thyroid, parathyroid, adrenal glands, pancreas, ovaries, testes, thymus.   

  - Function: Maintain homeostasis by regulating metabolic activities.

• Exocrine Glands:  

   - Description: Secrete products into ducts that empty onto surfaces (skin or lumen of hollow organs).

    - Examples: Sweat, oil, earwax, salivary glands, and digestive enzymes from the pancreas.   

  - Function: Produce substances like sweat (to lower temperature), oil, or saliva.

Structural Classification of Exocrine Glands

• Unicellular: Single cells (e.g., goblet cells).

• Multicellular: Composed of many cells forming a distinctive structure.   - Simple Glands: Simple tubular, branched tubular, coiled tubular, acinar, and branched acinar.   - Compound Glands: Compound tubular, compound acinar, and compound tubuloacinar.

Functional Classification of Exocrine Glands

• Merocrine/Eccrine: Secretions are released from the cell in secretory vesicles via exocytosis (e.g., salivary glands).

• Apocrine: Secretory product accumulates at the apical surface; that portion of the cell pinches off to release the secretion (e.g., mammary glands).

• Holocrine: Secretory product accumulates in the cytosol; the cell matures, ruptures, and dies, becoming the secretory product itself (e.g., sebaceous/oil glands). Cell division replaces the lost cell.

Connective Tissue: Cells and Matrix

• Basic Elements: Cells and Extracellular Matrix (ECM).

• Cell Types:   

  - Fibroblasts: Large, flat cells with branching processes; most numerous. They secrete fibers and ground substance.   

  - Macrophages: Develop from monocytes; carry out phagocytosis of bacteria and debris.   

  - Plasma Cells (Plasmocytes): Develop from B lymphocytes; secrete antibodies.   

  - Mast Cells: Produce histamine; involve in inflammatory responses and killing bacteria.  

   - Adipocytes: Fat cells; store triglycerides.   

  - Leukocytes (White Blood Cells): Eosinophils (parasitic/allergic responses) and Neutrophils (infection sites).

• Extracellular Matrix (ECM): Composed of ground substance and fibers.   

  - Ground Substance: Support medium for exchange of substances. Contains water and organic molecules: hyaluronic acid, chondroitin sulfate, and glucosamine.   

  • Fibers:     

  - Collagen Fibers: Strong, flexible bundles of collagen protein; the most abundant protein in the body.     

  - Elastic Fibers: Stretchable and strong; made of elastin and fibrillin. Found in skin, blood vessels, and lungs.     

  - Reticular Fibers: Made of collagen and glycoproteins; provide support in blood vessel walls and form networks around fat and muscle cells.

Classification of Connective Tissue

Embryonic Hookup

• Mesenchyme: The tissue from which all other connective tissues eventually arise.

• Mucous Connective Tissue: Found in the umbilical cord.

Mature Connective Tissue

1. Loose Connective Tissue: Areolar, Adipose, Reticular.

2. Dense Connective Tissue: Dense regular, Dense irregular, Elastic.

3. Cartilage (Supporting):     

   - Hyaline Cartilage: Bluish-white, shiny; contains chondrocytes in lacunae. Weakest type. Found at ends of long bones, ribs, nose, larynx, trachea.    

    - Fibrocartilage: Thick bundles of collagen; lacks perichondrium. Strongest type. Found in pubic symphysis, intervertebral discs, and menisci.     

   - Elastic Cartilage: Threadlike network of elastic fibers; perichondrium present. Found in epiglottis, external ear (auricle), and auditory tubes.

4. Bone Tissue (Supporting):   

     - Compact Bone: Consists of osteons (Haversian systems), lamellae, lacunae, osteocytes, canaliculi, and central canals.     

   - Spongy Bone: Consists of columns called trabeculae; spaces are filled with red bone marrow.

5. Liquid Connective Tissue:   

     - Blood: Consists of blood plasma and formed elements (red blood cells/erythrocytes, white blood cells/leukocytes, platelets/thrombocytes).

Muscular Tissue

• Skeletal Muscle Tissue: Long, cylindrical, striated fibers. Multinucleated (nuclei at periphery). Voluntary. Attached to bones by tendons.

• Cardiac Muscle Tissue: Branched, striated fibers with one central nucleus. Attach via intercalated discs (containing desmosomes and gap junctions). Involuntary. Found in the heart wall.

• Smooth Muscle Tissue: Nonstriated, spindle-shaped cells with one central nucleus. Involuntary. Found in walls of hollow structures (blood vessels, airways, GI tract, bladder). Some have gap junctions for unified contraction; others (like in the iris) contract individually.

Excitable Cells and Tissue Repair

• Excitable Cells: Neurons and muscle fibers. They exhibit electrical excitability due to specific voltage-gated channels in their plasma membranes.

• Tissue Repair: The process of replacing worn-out, damaged, or dead cells.  

   - Epithelial cells: Replaced by division of stem cells.

    - Connective tissue: Limited repair capacity.  

   - Muscle tissue: Limited repair.

    - Nervous tissue: Some can perform limited repair; others cannot.

• Fibrosis: The formation of scar tissue.

Types of Tissue

• Tissue: A group of cells that usually have a common embryonic origin and function together to carry out specialized activities.

• The Four Basic Tissue Types:

  • Epithelial Tissue: Covers body surfaces and lines hollow organs, body cavities, and ducts; it also forms glands.

  • Connective Tissue: Protects and supports the body and its organs. Various types bind organs together, store energy reserves as fat, and help provide the body with immunity to disease-causing organisms.

  • Muscular Tissue: Composed of cells specialized for contraction and generation of force. Generates heat that warms the body.

  • Nervous Tissue: Detects changes in a variety of conditions inside and outside the body and responds by generating electrical signals called nerve impulses (action potentials).

Types of Epithelial Tissue

• Classifications by Layers:

  • Simple: A single layer of cells.

  • Pseudostratified: Appears to have multiple layers but all cells rest on the basement membrane.

  • Stratified: Two or more layers of cells.

• Classifications by Shape:

  • Squamous: Thin, flat cells.

  • Cuboidal: Cube-shaped; about as tall as wide.

  • Columnar: Tall and cylindrical like columns.

  • Transitional: Cells change shape depending on the degree of stretch.

• Locations in the Body:

  • Found in surfaces such as the skin, lining of GI tract, respiratory tract, and various glands.

• Examples:

  • Simple Squamous: Air sacs in lungs.

  • Simple Cuboidal: Kidney tubules.

  • Simple Columnar: Lining of the stomach.

Glands

• Endocrine Glands:

  • Ductless, secrete hormones directly into the bloodstream; examples include the pituitary and thyroid glands.

• Exocrine Glands:

  • Secrete products into ducts; examples include sweat, salivary glands, and digestive enzymes from the pancreas.

Types of Connective Tissue

• Locations in the Body:

  • Found throughout the body, including bones, cartilage, blood, and adipose tissue.

• Examples:

  • Loose Connective Tissue: Areolar tissue.

  • Dense Connective Tissue: Tendons and ligaments.

  • Cartilage: Hyaline and fibrocartilage.

  • Bone: Compact and spongy.

Types of Muscle Tissue

• Skeletal Muscle Tissue: Long, striated fibers, multinucleated, voluntary control, attached to bones.

• Cardiac Muscle Tissue: Branched fibers, involuntary, found in heart walls, contains intercalated discs.

• Smooth Muscle Tissue: Non-striated fibers, involuntary, found in walls of hollow organs like blood vessels and the GI tract.

Excitable Cells

• Neurons and muscle fibers are excitable and exhibit electrical excitability due to specific voltage-gated channels in their plasma membranes.

Tissue Repair

• The process of replacing worn-out, damaged, or dead cells varies across tissue types:

  • Epithelial cells: High rate of regeneration.

  • Connective tissue: Limited capacity for repair.

  • Muscle tissue: Limited repair capabilities.

  • Nervous tissue: Some can perform limited repair, while others cannot.

• Fibrosis: The formation of scar tissue in injured tissues.