Tissues: Structure, Function, and Response in Human Anatomy & Physiology

Introduction to Tissues

Tissues are groups of similar cells that perform specific structural and functional roles in the body. Understanding tissue types, their components, and their responses to injury is fundamental in anatomy and physiology.

Overview of Four Tissue Types

Tissue Type	Characteristics	General Functions
Epithelial	Cells closely packed, form sheets; avascular; high regeneration	Protection, secretion, absorption, excretion, sensation
Connective	Cells scattered in extracellular matrix; vascular (except cartilage)	Support, binding, protection, storage, transport
Muscular	Elongated cells (fibers); contractile proteins	Movement, posture, heat production
Nervous	Neurons and supporting glial cells; excitable	Communication, control, integration

The Extracellular Matrix (ECM)

The extracellular matrix (ECM) surrounds cells and can be gel-like, fluid, or solid. It is made by tissue cells and serves to:

• Provide tissues with structural support

• Direct and hold cells in place

• Regulate development and cell behavior

The ECM consists of two main components:

1. Ground Substance: Interstitial fluid (IF), ions, nutrients, and sticky proteins

   • Glycosaminoglycans (GAGs)

   • Proteoglycans

   • Glycoproteins (CAMs)

2. Protein Fibers: Made by fibroblasts

   • Collagen fibers: strong, resist tension

   • Elastic fibers: stretch and recoil

   • Reticular fibers: form supportive networks

Epithelial Tissues

Characteristics and Functions

• Covering and lining surfaces; act as barriers

• Form glands

Functions of Epithelial Tissue:

1. Provide physical protection

2. Immune defenses

3. Secretion

4. Absorption

5. Sensation

Components and Classification

• Cellularity: tightly packed cells

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

• Attachment: connected to connective tissue via basement membrane

• Avascular: no blood vessels

• Extensive innervation

• Regeneration: rapid cell turnover

Classified by number of layers and cell shape:

• Simple: one layer

• Stratified: multiple layers

• Pseudostratified: appears layered but is not

• Transitional: changes shape with stretching

Types of Epithelial Tissue

1. Simple Squamous: One layer, thin, found in linings (mesothelium, endothelium); good for diffusion

2. Simple Cuboidal: One layer, large spherical nucleus, found in glands

3. Simple Columnar: Tall cells, often with goblet cells, found in digestive tract lining

4. Pseudostratified Ciliated Columnar: Appears stratified, contains cilia and goblet cells, found in respiratory tract

5. Stratified Squamous: Most widespread, can be keratinized (skin) or nonkeratinized (mouth, esophagus)

6. Transitional: Stretches and recoils, found in urinary bladder

Cell Junctions

• Tight junctions: seal cells together

• Desmosomes: anchor cells

• Gap junctions: allow communication

Glandular Epithelium

Glands are classified by their mode of secretion:

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

• Apocrine: Secrete by pinching off part of the cell (e.g., mammary glands)

• Holocrine: Entire cell disintegrates to release product (e.g., sebaceous glands)

Connective Tissue

General Features

• Most diverse, abundant, and widely distributed tissue

• Originates from embryonic mesenchyme

• Contains cells, ground substance, and fibers

Functions of Connective Tissue

• Support and structural framework

• Binding and connecting tissues

• Protection

• Storage (e.g., fat)

• Transport (e.g., blood)

• Immune protection

Cells Associated with Connective Tissue Proper

• Fibroblasts: secrete fibers and ground substance

• Adipocytes: store fat

• Mast cells: mediate inflammation

• Phagocytes/macrophages: engulf debris and pathogens

• Leukocytes: immune defense (neutrophils, eosinophils)

Types of Connective Tissue Proper

Loose Connective Tissue

• Areolar: Most abundant, loosely arranged fibers, supports and surrounds organs

• Reticular: Network of reticular fibers, forms internal framework of lymphoid organs

• Adipose: Fat storage, insulation, energy reserve

Dense Connective Tissue

• Dense Regular: Parallel collagen fibers, found in tendons and ligaments

• Dense Irregular: Randomly arranged fibers, found in dermis

• Elastic: More elastic fibers, found in walls of large arteries

Specialized Connective Tissues

Cartilage

• Cells: chondroblasts/chondrocytes in lacunae

• ECM: collagen fibers, proteoglycans, GAGs

• Types:

  • Hyaline: glassy, most abundant, precursor to bone

  • Fibrocartilage: thick bundles of collagen, found in intervertebral discs

  • Elastic: more elastic fibers, found in ear

• Avascular, surrounded by perichondrium

Bone (Osseous Tissue)

• Cells: osteoblasts (build), osteoclasts (break down), osteocytes (maintain)

• Matrix: collagen fibers + calcium phosphate salts (osteoid)

• Functions: support, protection, mineral storage, blood cell formation

• Types: compact (dense), spongy (cancellous)

Fluid Connective Tissues

• Blood: RBCs, WBCs, platelets in plasma; transport, immunity, clotting

• Lymph: Fluid with lymphocytes, returns to blood via lymphatic system

Muscular Tissue

Specialized for contraction and movement. Muscle cells (fibers or myocytes) contain myofilaments (actin, myosin) for contraction.

Type	Location	Appearance	Voluntary/Involuntary	Nuclei	Specializations
Skeletal	Attached to bones	Striated, long fibers	Voluntary	Multinucleate	Rapid contraction, fatigue
Cardiac	Heart	Striated, branched	Involuntary	1-2 nuclei	Intercalated discs, rhythmic contraction
Smooth	Walls of hollow organs	Non-striated, spindle-shaped	Involuntary	Single nucleus	Slow, sustained contraction

All muscle types require calcium ions (Ca2+) for contraction.

Nervous Tissue

Forms the brain, spinal cord, and nerves. Specialized for communication via electrical impulses.

• Neurons: Conduct nerve impulses (action potentials); have dendrites (input), cell body (soma), and axon (output)

• Neuroglia (glial cells): Support, protect, and nourish neurons; can undergo mitosis

Membranes

• Mucous membranes (mucosa): Line body cavities open to exterior

• Serous membranes: Line closed cavities; have visceral and parietal layers

• Cutaneous membrane: Skin (epidermis and dermis)

• Synovial membranes: Line joint cavities

Tissue Response to Injury

Inflammatory Response

• Triggered by injury or infection

• Involves release of chemicals (histamine, heparin, prostaglandins)

• Symptoms: redness, heat, swelling, pain

• Functions:

  • Isolate injury

  • Bring oxygen, nutrients, phagocytes, antibodies

  • Remove toxins and wastes

Sign/Symptom	Primary Cause
Redness	Vasodilation increases blood flow
Heat	Increased blood flow
Swelling	Increased permeability, fluid leaves vessels
Pain	Chemicals stimulate nerve endings

Tissue Repair

1. Regeneration: Replacement with original tissue (epithelial, connective, smooth muscle)

2. Fibrosis: Replacement with scar tissue (collagen)

Inflammatory Conditions

• Arthritis

• Asthma

• Colitis

Treatment: Rest, ice therapy, omega-3 fatty acids, anti-inflammatory drugs (NSAIDs)

Note: C-reactive protein (hs-CRP) assay measures inflammation.

Tissue Modification and Pathology

• Hypertrophy: Increase in cell size

• Atrophy: Decrease in cell size

• Hyperplasia: Increase in cell number

• Dysplasia: Abnormal cell growth

• Metaplasia: Change from one cell type to another

• Anaplasia: Loss of differentiation

• Carcinogen: Cancer-causing agent

• Carcinoma: Cancer of epithelial tissue

• Benign vs. Malignant: Benign tumors do not spread; malignant tumors metastasize