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

Tissue: The Living Fabric

Introduction to Tissues

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

• Epithelial tissue: Covers surfaces and lines cavities.

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

• Muscle tissue: Contracts to produce movement.

• Nervous tissue: Initiates and transmits electrical impulses.

4.1 Tissue Preparation for Microscopy

Steps in Tissue Preparation

To study tissues under a microscope, samples must be:

• Fixed: Preserved with a solvent.

• Sectioned: Cut into thin slices for light or electron transmission.

• Stained: Enhanced contrast using dyes (light microscopy) or heavy metals (electron microscopy).

Transmission electron microscopy (TEM) shows internal sections, while scanning electron microscopy (SEM) shows surfaces.

4.2 Epithelial Tissue

General Characteristics

Epithelial tissue (epithelium) is a sheet of cells covering body surfaces or lining cavities. It has two main forms:

• Covering and lining epithelium: Forms the outer layer of skin, lines open cavities, and covers organs.

• Glandular epithelium: Forms glands (e.g., salivary glands).

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

Special Characteristics of Epithelial Tissue

• Polarity: Has apical (free) and basal (attached) surfaces with different structures and functions.

• Specialized contacts: Cells fit closely together, forming continuous sheets with tight junctions and desmosomes.

• Supported by connective tissue: All epithelial sheets rest on a basement membrane (basal and reticular lamina).

• Avascular but innervated: No blood vessels, but supplied by nerves; nutrients diffuse from underlying tissues.

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

Classification of Epithelial Tissue

Epithelia are classified by the number of cell layers and cell shape:

• Simple epithelium: Single cell layer (for absorption, secretion, filtration).

• Stratified epithelium: Two or more layers (for protection in high-abrasion areas).

Cell shapes:

• Squamous: Flattened and scale-like.

• Cuboidal: Box-like, as tall as wide.

• Columnar: Tall and column-shaped.

Types of Epithelial Tissue

• Simple Squamous Epithelium: Single layer of flat cells; allows rapid diffusion (e.g., air sacs of lungs, lining of blood vessels).

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

• Simple Columnar Epithelium: Single layer of tall cells; absorption and secretion, may have microvilli or cilia (e.g., digestive tract, uterine tubes).

• Pseudostratified Columnar Epithelium: Appears multilayered but is single-layered; secretion and movement of mucus (e.g., trachea).

• Stratified Squamous Epithelium: Multiple layers; protects underlying tissues (e.g., skin, mouth, esophagus).

• Transitional Epithelium: Resembles both stratified squamous and cuboidal; stretches to allow filling of urinary organs (e.g., bladder).

Glandular Epithelia

A gland consists of one or more cells that secrete an aqueous fluid (secretion). Glands are classified by:

• Site of product release: Endocrine (ductless, hormones) or exocrine (ducts, onto surfaces).

• Number of cells: Unicellular (e.g., goblet cells) or multicellular (e.g., salivary glands).

• Unicellular exocrine glands: Mucous and goblet cells; secrete mucin (forms mucus).

• Multicellular exocrine glands: Composed of a duct and secretory unit; classified by structure (simple/compound, tubular/alveolar) and mode of secretion (merocrine, holocrine, apocrine).

4.3 Connective Tissue

General Features and Functions

Connective tissue is the most abundant and widely distributed tissue type. Its major functions include binding and support, protection, insulation, energy storage, and transport (blood).

• Four main classes: Connective tissue proper, cartilage, bone, blood.

Comparison of Connective Tissue Classes

Tissue Class	Subclasses	Cells	Matrix Components	General Features
Connective Tissue Proper	Loose (areolar, adipose, reticular); Dense (regular, irregular, elastic)	Fibroblasts, fibrocytes, adipocytes, defense cells	Gel-like ground substance; collagen, reticular, elastic fibers	Binding, resisting tension, water reservoir, energy storage
Cartilage	Hyaline, elastic, fibrocartilage	Chondroblasts, chondrocytes	Gel-like ground substance; collagen, elastic fibers	Resists compression, supports, avascular
Bone	Compact, spongy	Osteoblasts, osteocytes	Calcified ground substance; collagen fibers	Support, resists compression/tension
Blood	---	RBCs, WBCs, platelets	Plasma (fluid); no fibers	Transport of gases, nutrients, wastes

Common Characteristics of Connective Tissue

• Extracellular matrix: Nonliving material (ground substance + fibers) separates cells, allowing tissue to bear weight and tension.

• Common origin: All arise from mesenchyme (embryonic tissue).

Structural Elements of Connective Tissue

• Ground substance: Unstructured material filling space between cells; contains interstitial fluid, cell adhesion proteins, and proteoglycans (e.g., chondroitin sulfate, hyaluronic acid).

• Fibers: Collagen (strongest), elastic (stretch/recoil), reticular (supportive networks).

• Cells: "-blast" (immature, matrix-secreting) and "-cyte" (mature, maintain matrix) forms; also adipocytes, leukocytes, mast cells, macrophages.

Types of Connective Tissue Proper

• Loose connective tissue: Areolar (packing, support), adipose (fat storage, insulation), reticular (internal support for lymphoid organs).

• Dense connective tissue: Regular (tendons, ligaments), irregular (dermis, organ capsules), elastic (arteries, vertebral ligaments).

Cartilage, Bone, and Blood

• Cartilage: Hyaline (joints, ribs, nose), elastic (ear, epiglottis), fibrocartilage (intervertebral discs, knee).

• Bone: Supports, protects, stores fat, synthesizes blood cells; highly vascularized.

• Blood: Fluid tissue; transports gases, nutrients, wastes.

4.4 Muscle Tissue

Types and Functions

Muscle tissue is highly vascularized and responsible for movement. Muscle cells contain myofilaments (actin and myosin) for contraction. Three types:

• Skeletal muscle: Voluntary, striated, multinucleate; moves bones.

• Cardiac muscle: Involuntary, striated, one nucleus, intercalated discs; found in heart.

• Smooth muscle: Involuntary, non-striated, spindle-shaped; found in walls of hollow organs.

4.5 Nervous Tissue

Structure and Function

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

• Neurons: Respond to stimuli and transmit electrical signals via dendrites and axons.

• Glial cells (neuroglia): Support, insulate, and protect neurons.

4.6 Membranes

Types of Membranes

Membranes are organs composed of at least two tissue types. Three main types:

• Cutaneous membrane: Skin; keratinized stratified squamous epithelium attached to connective tissue; dry membrane.

• Mucous membranes (mucosae): Line body cavities open to exterior (digestive, respiratory, urogenital tracts); moist, may secrete mucus.

• Serous membranes (serosae): Line closed ventral body cavities; simple squamous epithelium (mesothelium) on areolar connective tissue; secrete serous fluid.

4.7 Tissue Repair

Steps in Tissue Repair

1. Inflammation: Release of chemicals, dilation of blood vessels, increased permeability, clotting.

2. Organization: Blood clot replaced by granulation tissue, epithelium regenerates, fibroblasts produce collagen, debris removed.

3. Regeneration and fibrosis: Epithelium thickens, scar tissue forms, function may be restored or lost depending on tissue type.

Regenerative Capacity of Tissues

• High: Epithelial, bone, areolar connective, dense irregular connective, blood-forming tissue.

• Moderate: Smooth muscle, dense regular connective tissue.

• Poor/None: Cardiac muscle, nervous tissue (brain, spinal cord).

Developmental Aspects of Tissues

Embryonic Germ Layers

• Ectoderm: Forms nerve tissue.

• Mesoderm: Forms muscle and connective tissues.

• Endoderm: Forms epithelial tissues (all three layers contribute to epithelia).

Additional info: As the body ages, tissue repair becomes less efficient, epithelia thin, and risk of cancer increases due to DNA mutations.