Chapter 4: Tissue—The Living Fabric (Anatomy & Physiology Study Notes)

Introduction to Tissues

Tissues are groups of cells similar in structure that perform common or related functions, essential for maintaining homeostasis in the body. The study of tissues is known as histology. There are four basic tissue types in the human body: epithelial, connective, muscle, and nervous tissue.

• Epithelial tissue: Covers body surfaces and lines cavities.

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

• Muscle tissue: Responsible for movement.

• Nervous tissue: Enables internal communication.

Epithelial Tissue

Definition and Main Forms

Epithelial tissue (epithelium) is a sheet of cells that covers body surfaces or lines body cavities. It exists in two main forms:

• Covering and lining epithelia: Found on external and internal surfaces (e.g., skin).

• Glandular epithelia: Forms secretory tissue in glands (e.g., salivary glands).

Main functions include protection, absorption, filtration, excretion, secretion, and sensory reception.

Special Characteristics of Epithelial Tissue

Epithelial tissue is distinguished by five key characteristics:

• Polarity: Cells have distinct top (apical) and bottom (basal) surfaces.

• Specialized contacts: Cells are closely packed and bound by specialized junctions (tight junctions, desmosomes).

• Supported by connective tissues: All epithelial sheets rest on and are supported by connective tissue (basement membrane).

• Avascular, but innervated: No blood vessels; nutrients diffuse from underlying tissues. Nerve fibers are present.

• Regeneration: High capacity for renewal, especially when exposed to friction or hostile environments.

Polarity

• Apical surface: Exposed to exterior or cavity; may have microvilli (fingerlike projections for absorption).

• Basal surface: Attached to underlying tissue via the basal lamina, an adhesive sheet.

Specialized Contacts

• Tight junctions: Prevent substances from passing between cells.

• Desmosomes: Anchor cells together, providing mechanical strength.

Support by Connective Tissue

• Basement membrane: Composed of basal and reticular lamina; reinforces the epithelial sheet and defines boundaries.

Avascular but Innervated

• No blood vessels; nutrients diffuse from underlying connective tissue.

• Supplied by nerve fibers.

Regeneration

• Cells divide rapidly to replace lost or damaged cells.

Classification of Epithelia

Epithelial tissues are classified by the number of cell layers and cell shape:

• Number of layers:

  • Simple epithelia: Single layer; specialized for absorption, secretion, filtration.

  • Stratified epithelia: Multiple layers; specialized for protection.

• Cell shape:

  • Squamous: Flattened, scale-like.

  • Cuboidal: Box-like, cube-shaped.

  • Columnar: Tall, column-like.

In stratified epithelia, cell shape is named according to the apical layer.

Glandular Epithelia

Glands are one or more cells that make and secrete an aqueous fluid called a secretion. Glands are classified by:

• Site of product release:

  • Endocrine: Internally secreting (e.g., hormones).

  • Exocrine: Externally secreting (e.g., sweat).

• Number of cells:

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

  • Multicellular: Multiple cells (e.g., salivary glands).

Endocrine Glands

• Ductless; secrete hormones into interstitial fluid, which enters the blood or lymph.

• Hormones act on specific target organs.

Exocrine Glands

• Secrete products onto body surfaces or into cavities (e.g., sweat, oil, mucous, salivary glands).

• Unicellular (goblet cells) or multicellular (salivary glands).

• Modes of secretion:

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

  • Holocrine: Accumulate products until rupture (e.g., sebaceous glands).

Connective Tissue

Overview and Functions

Connective tissue is the most abundant and widely distributed tissue type. Its major functions include binding and support, protection, insulation, storing reserve fuel, and transporting substances (e.g., blood).

• Four main classes:

  • Connective tissue proper

  • Cartilage

  • Bone

  • Blood

Common Characteristics

• Common embryonic origin: mesenchyme.

• Varying degrees of vascularity (cartilage is avascular; bone is highly vascularized).

• Cells are suspended in an extracellular matrix (ECM) composed of protein-sugar mesh.

Structural Elements

• Ground substance: Gel-like material; medium for diffusion of solutes.

• Fibers: Provide support; three types:

  • Collagen: Strongest, most abundant; high tensile strength.

  • Elastic: Stretch and recoil; made of elastin.

  • Reticular: Short, fine, highly branched; form networks for support.

• Cells: "Blast" cells (immature, secrete matrix) and "cyte" cells (mature, maintain matrix). Other cell types include fat cells, white blood cells, mast cells, and macrophages.

Types of Connective Tissue

Connective Tissue Proper

• Loose connective tissue:

  • Areolar: Most widely distributed; supports and binds other tissues; contains fibroblasts, macrophages, fat cells.

  • Adipose: White fat (nutrient storage, insulation, shock absorption); brown fat (heat production).

  • Reticular: Resembles areolar but with reticular fibers; supports blood cells in lymph nodes, spleen, bone marrow.

• Dense connective tissue:

  • Dense regular/irregular: High tensile strength; tendons and ligaments; mostly fibers, poorly vascularized.

  • Elastic: Found in ligaments and large arteries; allows stretch and recoil.

Cartilage

• Matrix secreted by chondroblasts/chondrocytes; found in lacunae.

• 80% water; tough yet flexible; avascular.

• Types:

  • Hyaline: Most abundant; tips of long bones, nose, trachea, larynx, ribs.

  • Elastic: More elastic fibers; ears, epiglottis.

  • Fibrocartilage: Strong; intervertebral discs, knee.

Bone (Osseous Tissue)

• Supports and protects; stores fat; synthesizes blood cells.

• Matrix: collagen fibers, inorganic calcium salts.

• Osteoblasts produce matrix; osteocytes maintain it (in lacunae).

• Richly vascularized.

Blood

• Most atypical connective tissue; cells in plasma matrix.

• Red blood cells (most common), white blood cells, platelets.

• Fibers are soluble proteins (precipitate during clotting).

• Functions in transport of nutrients, wastes, gases.

Comparison Table: Classes of Connective Tissue

Class	Main Function	Key Features	Examples
Connective Tissue Proper	Support, binding, storage	Loose and dense types; fibroblasts	Areolar, adipose, tendons, ligaments
Cartilage	Support, flexibility	Chondrocytes in lacunae; avascular	Hyaline, elastic, fibrocartilage
Bone	Support, protection	Osteocytes in lacunae; highly vascularized	Compact bone, spongy bone
Blood	Transport	Cells in plasma; soluble fibers	Red and white blood cells, platelets

Nervous Tissue

Nervous tissue is the main component of the nervous system (brain, spinal cord, nerves). It regulates and controls body functions and is composed of two specialized cell types:

• Neurons: Generate and conduct nerve impulses.

• Glia: Support, insulate, and protect neurons.

Muscle Tissue

Muscle tissue is highly vascularized and responsible for movement. Muscle cells contain myofilaments made of actin and myosin, which enable contraction. There are three types:

• Skeletal muscle: Voluntary movement; attached to bones.

• Cardiac muscle: Involuntary; found in the heart.

• Smooth muscle: Involuntary; found in walls of hollow organs.

Covering and Lining Membranes

Membranes are composed of at least two primary tissue types: an epithelial layer bound to underlying connective tissue proper. Types include:

• Cutaneous membranes: Skin.

• Mucous membranes: Line body cavities open to exterior (digestive, respiratory, urogenital tracts).

• Serous membranes: Line closed ventral body cavities (lungs, heart, abdomen).

Additional info: For more detailed histological images and clinical correlations, refer to the full textbook or laboratory manual.