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

Introduction to Tissues

Tissues are groups of similar cells that perform specific structural and functional roles in the body. The study of tissues is called histology, and understanding tissue types is fundamental to anatomy and physiology.

Overview of Four Tissue Types

Tissue Type	Characteristics	General Functions
Epithelial	Cells closely packed, form sheets; avascular; high regeneration	Protection, absorption, secretion, 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	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 several functions:

• Provides tissues with structural support

• Directs and holds cells in place

• Regulates development and cell behavior

The ECM consists of two main components:

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

   • Glycosaminoglycans (GAGs)

   • Proteoglycans

   • Glycoproteins (CAMs)

2. Protein Fibers: Produced by fibroblasts; include collagen, elastic, and reticular fibers.

Epithelial Tissues

Characteristics and Functions

Epithelial tissues cover and line body surfaces, acting as barriers and forming glands. Key features include:

• Cellularity: closely packed cells

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

• Attachment: connected to underlying connective tissue via basement membrane

• Avascular: no blood vessels

• Innervation: rich nerve supply

• Regeneration: high mitotic rate

Functions of epithelial tissue:

1. Physical protection

2. Immune defenses

3. Secretion

4. Absorption

5. Sensation

Cell Junctions

• Tight junctions: Seal cells together

• Desmosomes: Provide mechanical strength

• Gap junctions: Allow communication between cells

Classification of Epithelia

Classified by number of layers and cell shape:

• Simple: One layer

• Stratified: Multiple layers

• Pseudostratified: Appears layered but is not

• Transitional: Changes shape as it stretches

Types of Epithelial Tissue

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

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

3. Simple Columnar: One layer, 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: Multiple layers, most widespread; can be keratinized (skin) or nonkeratinized (mouth, esophagus)

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

Glandular Epithelium

Glands are classified by mode of secretion:

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

• Apocrine: Part of cell pinched off (e.g., mammary glands)

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

Connective Tissue

General Features

Connective tissue is the most diverse and widely distributed tissue type. It originates from embryonic mesenchyme and is usually vascular (except cartilage).

Functions of Connective Tissue

• Support and structural framework

• Binding and connection of tissues

• Protection

• Storage of energy (fat)

• Transport (blood)

Cells of Connective Tissue Proper

• Fibroblasts: Produce 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 (produce matrix), chondrocytes (maintain matrix in lacunae)

• Types:

  • Hyaline: Most common, glassy appearance, precursor to bone

  • Fibrocartilage: Strongest, found in intervertebral discs

  • Elastic: Flexible, found in ear

• Properties: Avascular, surrounded by perichondrium, poor healing

Bone (Osseous Tissue)

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

• Matrix: Collagen fibers and mineral salts (calcium phosphate)

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

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

Fluid Connective Tissues

• Blood: Contains plasma, red and white blood cells, platelets; transports gases, nutrients, wastes

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

Muscular Tissue

Muscle tissue is specialized for contraction and movement. Muscle cells (fibers or myocytes) contain myofilaments (actin and myosin).

Type	Location	Appearance	Voluntary/Involuntary	Nuclei	Specializations
Skeletal	Attached to bones	Striated, long, cylindrical	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

Nervous tissue forms the brain, spinal cord, and nerves. It is specialized for communication via electrical impulses.

• Neurons: Conduct nerve impulses; consist of dendrites (input), cell body (soma), and axon (output)

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

Membranes

• Mucous membranes: Line body cavities open to exterior (e.g., digestive tract)

• 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

Tissues respond to injury to maintain homeostasis. The inflammatory response is immediate and involves:

• Vasodilation (increased blood flow)

• Increased permeability (swelling)

• Phagocyte migration

• Release of chemical mediators (histamine, prostaglandins)

Signs of inflammation include redness, heat, swelling, pain.

Tissue Repair

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

2. Fibrosis: Replacement with scar tissue

Inflammatory Conditions

• Arthritis

• Asthma

• Colitis

Treatment may include rest, ice therapy, omega-3 fatty acids, and anti-inflammatory drugs (NSAIDs).

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 invade; malignant tumors metastasize (spread)

Additional info: C-reactive protein (hs-CRP) assay is used to measure inflammation in the body, as CRP levels increase during acute inflammation.