Tissues: The Living Fabric – Structure and Function in Anatomy & Physiology

Tissues: The Living Fabric

Introduction to Tissues

Tissues are groups of cells with similar structure and common function. In the human body, there are four primary tissue types: epithelial, connective, muscle, and nervous. Each type plays a distinct role in maintaining the body's structure and function.

• Epithelial Tissue: Covers surfaces, lines cavities, and forms glands.

• Connective Tissue: Supports, protects, and binds other tissues.

• Muscle Tissue: Responsible for movement.

• Nervous Tissue: Initiates and transmits electrical impulses.

Epithelial Tissue

Characteristics of Epithelial Tissue

• Cellularity: Composed of closely packed cells with minimal extracellular matrix.

• Cell-Cell Contacts: Includes specialized junctions such as desmosomes and tight junctions for adhesion and communication.

• Polarity: Exhibits an apical (free) surface and a basal (attached) surface; the apical surface may be ciliated.

• Avascularity: Lacks blood vessels; nutrients diffuse from underlying tissues.

• Basement Membrane: Acellular structure anchoring epithelium, composed of:

  • Basal lamina: Secreted by epithelial cells.

  • Reticular lamina: Produced by underlying connective tissue.

Classification of Epithelia

Epithelia are classified by the number of cell layers and the shape of the cells at the apical surface.

• Simple Epithelium: Single cell layer; functions in absorption, secretion, and filtration.

• Stratified Epithelium: Multiple layers; provides protection.

• Pseudostratified Epithelium: Appears layered but all cells touch the basement membrane.

• Transitional Epithelium: Multiple layers; cells can change shape (e.g., urinary bladder).

Types of Epithelial Tissue

• Simple Squamous: Single layer of flat cells; allows diffusion and filtration (e.g., alveoli, Bowman's capsule).

• Simple Cuboidal: Single layer of cube-shaped cells; secretion and absorption (e.g., kidney tubules).

• Simple Columnar: Single layer of tall cells; absorption and secretion, often with microvilli or cilia (e.g., digestive tract lining).

• Pseudostratified Columnar: Single layer, nuclei at different levels; often ciliated with goblet cells (e.g., respiratory tract).

• Stratified Squamous: Multiple layers; protection against abrasion (e.g., skin, mouth, esophagus). Keratinized in skin.

• Stratified Cuboidal: Two layers; found in sweat and mammary glands.

• Stratified Columnar: Rare; found in some glandular ducts and junctions.

• Transitional: Several layers; surface cells change shape with organ distension (e.g., urinary bladder).

Specialized Epithelia and Membranes

• Endothelium: Simple squamous lining of circulatory system; reduces friction.

• Mucous Membranes: Line body cavities open to exterior; moist, absorptive, and secretory (e.g., digestive, respiratory tracts).

• Cutaneous Membrane: Skin; keratinized, stratified squamous epithelium on connective tissue.

• Serous Membranes: Line closed body cavities; secrete serous fluid (e.g., pleura, pericardium).

Glandular Epithelium

• Endocrine Glands: Ductless; secrete hormones into blood.

• Exocrine Glands: Secrete products via ducts to surfaces.

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

  • Multicellular: Composed of secretory unit and duct; classified as simple (unbranched) or compound (branched), and by shape (tubular, alveolar, or tubuloalveolar).

  • Modes of Secretion:

    • Merocrine: Secrete by exocytosis (e.g., sweat, salivary glands).

    • Holocrine: Entire cell ruptures (e.g., sebaceous glands).

    • Apocrine: Apex of cell pinches off (e.g., mammary glands).

Connective Tissue

Properties of Connective Tissue

• Origin: Derived from mesenchyme (mesodermal origin).

• Vascularity: Varies from avascular (cartilage) to highly vascular (bone).

• Matrix: Non-living extracellular matrix is a major component, consisting of:

  • Ground Substance: Fills extracellular space; contains fibers.

  • Fibers:

    • Collagen: Strong, provides tensile strength.

    • Elastic: Stretch and recoil (yellow fibers).

    • Reticular: Fine, branching fibers; support soft tissues.

• Cells: Four main types of precursor cells secrete matrix:

  • Fibroblast: For connective tissue proper (mature: fibrocyte).

  • Chondroblast: For cartilage (mature: chondrocyte).

  • Osteoblast: For bone (mature: osteocyte).

  • Hemocytoblast: For blood.

Types of Connective Tissue Proper

• Loose Connective Tissue:

  • Areolar: Gel-like matrix; supports and binds other tissues; reservoir for water and salts; contains mast cells and fibroblasts.

  • Adipose: Fat storage; insulates and cushions organs; highly vascularized.

  • Reticular: Network of reticular fibers; forms stroma of lymphoid organs.

• Dense Connective Tissue:

  • Dense Regular: Parallel collagen fibers; resists tension in one direction (e.g., tendons, ligaments).

  • Dense Irregular: Irregularly arranged fibers; resists tension in multiple directions (e.g., dermis, joint capsules).

  • Elastic: High proportion of elastic fibers; allows recoil (e.g., vocal cords, some ligaments).

Specialized Connective Tissues

• Cartilage: Avascular, flexible, and resilient; three types:

  • Hyaline: Most common; glassy matrix; found in nose, trachea, ends of long bones.

  • Fibrocartilage: Strong, resists compression; found in intervertebral discs.

  • Elastic: Contains more elastic fibers; found in ear (pinna), epiglottis.

• Bone: Rigid, supports and protects; matrix contains collagen fibers and calcium salts (hydroxyapatite); highly vascularized.

• Blood: Fluid matrix (plasma) with formed elements (red and white blood cells, platelets); transports gases, nutrients, wastes.

Muscle Tissue

Types of Muscle Tissue

• Skeletal Muscle: Voluntary, striated, multinucleated; moves bones.

• Cardiac Muscle: Involuntary, striated, branched cells joined by intercalated discs; found in heart.

• Smooth Muscle: Involuntary, non-striated, spindle-shaped cells; found in walls of hollow organs (e.g., digestive tract, blood vessels).

Nervous Tissue

Structure and Function

• Neurons: Conduct electrical impulses; main functional cells of the nervous system.

• Neuroglia (Supporting Cells): Support, protect, and insulate neurons.

Tissue Repair

Processes of Tissue Repair

• Inflammatory Response: Generalized, non-specific response to injury.

• Immune Response: Specific, targeted response to pathogens.

• Regeneration: Replacement of damaged tissue with original tissue type.

• Fibrosis: Formation of scar tissue (fibrous connective tissue).

Integumentary System: Structure and Function

Overview

The skin (cutaneous membrane) is the largest organ of the body, providing protection, temperature regulation, sensation, and waste removal. It consists of two main layers: the epidermis (outer, ectodermal origin) and the dermis (inner, mesodermal origin). Below the dermis lies the hypodermis (subcutaneous tissue), composed of loose connective tissue and adipose tissue.

Epidermis

• Cell Types:

  • Keratinocytes: Most abundant; produce keratin for protection.

  • Melanocytes: Produce melanin pigment.

  • Merkel Cells: Sensory receptors for touch.

  • Langerhans Cells: Macrophages; immune defense.

• Epidermal Layers (from deep to superficial):

  1. Stratum basale (germinativum): Deepest; mitotically active; contains melanocytes and Merkel cells.

  2. Stratum spinosum: Several layers; contains Langerhans cells.

  3. Stratum granulosum: Keratinization begins; cells contain keratohyalin granules.

  4. Stratum lucidum: Present only in thick skin; clear, dead cells.

  5. Stratum corneum: Outermost; dead, keratinized cells; provides barrier function.

Dermis

• Two Layers:

  • Papillary Layer: Loose connective tissue; contains capillaries, nerve endings, and Meissner's corpuscles (touch receptors); forms dermal papillae (fingerprints).

  • Reticular Layer: Dense, irregular connective tissue; contains collagen and elastic fibers for strength and flexibility.

Skin Color

• Melanin: Yellow to brown pigment; produced by melanocytes; responsible for skin color and protection from UV radiation.

• Carotene: Yellow to orange pigment; accumulates in stratum corneum and fat.

• Hemoglobin: Red pigment in blood; visible in fair skin due to low melanin.

Accessory Structures

• Hair: Produced by follicles; protective and insulatory; color determined by melanin types and trichosiderin (red hair).

• Hair Follicle: Extends into dermis; contains hair bulb and dermal papilla; associated with sebaceous glands and arrector pili muscles.

• Nails: Modified epidermis; composed of hard keratin; consists of free edge, body, and root; growth occurs at nail matrix.

Glands

• Sudoriferous (Sweat) Glands:

  • Eccrine: Widely distributed; secrete watery sweat for thermoregulation.

  • Apocrine: Found in axillary and genital areas; secrete thicker, milky sweat; become active at puberty.

• Sebaceous Glands: Secrete sebum (oil) into hair follicles; lubricate and waterproof skin and hair.

Clinical Considerations

• Burns:

  • First-degree: Only epidermis affected; redness and pain.

  • Second-degree: Epidermis and part of dermis; blisters and pain.

  • Third-degree: Full-thickness; destroys epidermis and dermis; may require grafting.

• Skin Cancer:

  • Basal Cell Carcinoma: Most common, least malignant.

  • Squamous Cell Carcinoma: Arises from keratinocytes; can metastasize.

  • Malignant Melanoma: Cancer of melanocytes; highly metastatic.

Summary Table: Types of Epithelial Tissue

Type	Structure	Location	Function
Simple Squamous	Single layer, flat cells	Alveoli, Bowman's capsule, endothelium	Diffusion, filtration
Simple Cuboidal	Single layer, cube-shaped	Kidney tubules, glands	Secretion, absorption
Simple Columnar	Single layer, tall cells	Digestive tract lining	Absorption, secretion
Pseudostratified Columnar	Single layer, nuclei at different levels	Respiratory tract	Secretion, movement of mucus
Stratified Squamous	Multiple layers, flat surface cells	Skin, mouth, esophagus	Protection
Transitional	Multiple layers, variable shape	Urinary bladder	Stretch, distension

Key Formulas and Concepts

• Osmosis and Diffusion (relevant to epithelial function): $J = -D \frac{dC}{dx}$ Where $J$ is the flux, $D$ is the diffusion coefficient, and $\frac{dC}{dx}$ is the concentration gradient.

• Fick's Law of Diffusion (applies to simple squamous epithelium): $\text{Rate of diffusion} = \frac{(\text{Surface area}) \times (\text{Concentration difference})}{\text{Thickness}}$

Additional info: Some explanations and examples have been expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.