Tissue: The Living Fabric

Introduction to Tissues

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

Microscopic Human Tissue Preparation

To study tissues under a microscope, they 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).

Artifacts may occur, causing distortions from the living state.

Epithelial Tissue

Overview and Functions

Epithelial tissue is a sheet of cells covering body surfaces or lining cavities. It exists in two main forms:

• Covering and lining epithelia: External and internal surfaces (e.g., skin).

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

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

Characteristics of Epithelial Tissue

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

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

• Supported by connective tissue: Basement membrane (basal and reticular lamina) provides structural support and boundary.

• Avascular, but innervated: No blood vessels; nourished by diffusion, but supplied by nerve fibers.

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

Classification of Epithelia

Epithelia are classified by two criteria:

• Number of cell layers: Simple (one layer), Stratified (two or more layers).

• Cell shape: Squamous (flattened), Cuboidal (cube-like), Columnar (tall).

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

Types of Epithelial Tissue

• Simple squamous epithelium: Rapid diffusion; found in kidneys, lungs, endothelium (vessels), mesothelium (serous membranes).

• Simple cuboidal epithelium: Secretion and absorption; forms walls of ducts and kidney tubules.

• Simple columnar epithelium: Absorption and secretion; found in digestive tract, gallbladder, bronchi, uterine tubes.

• Pseudostratified columnar epithelium: Appears multi-layered but is single-layered; ciliated; found in upper respiratory tract.

• Stratified squamous epithelium: Protection; found in skin (keratinized) and moist linings (nonkeratinized).

• Stratified cuboidal/columnar epithelium: Rare; found in sweat glands, mammary glands, pharynx, male urethra.

• Transitional epithelium: Lines urinary organs; cells change shape to allow stretching.

Glandular Epithelia

Glands are classified by site of product release and number of cells:

• Endocrine: Internally secreting (hormones).

• Exocrine: Externally secreting (sweat, saliva).

• Unicellular: Mucous and goblet cells.

• Multicellular: Salivary glands.

Multicellular exocrine glands are further classified by structure (simple vs. compound; tubular vs. alveolar) and mode of secretion:

• Merocrine: Secrete by exocytosis (sweat, pancreas).

• Holocrine: Accumulate products and rupture (sebaceous glands).

• Apocrine: Apex ruptures; controversial in humans.

Connective Tissue

Overview and Functions

Connective tissue is the most abundant and widely distributed tissue, serving functions such as binding, support, protection, insulation, storage, and transport. Four main classes are:

• Connective tissue proper

• Cartilage

• Bone

• Blood

Connective Tissue Proper

• Loose connective tissue: Areolar (fills spaces, supports organs), adipose (stores fat, insulates), reticular (supports soft organs).

• Dense connective tissue: Regular (parallel fibers, tendons/ligaments), irregular (random fibers, dermis), elastic (stretch/recoil, arteries).

Specialized Connective Tissue

• Cartilage: Flexible, avascular; types include hyaline (nose, ribs, joints), fibrocartilage (discs, knees), elastic (ear).

• Bone: Hard, mineralized; skeletal framework, protection, mineral storage.

• Blood: Fluid matrix; transports gases, nutrients, waste.

• Lymph: Fluid matrix; immune function, fluid balance.

Muscle Tissue

Overview and Types

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

• Skeletal muscle: Voluntary, striated, multinucleated, attached to bones.

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

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

Nervous Tissue

Structure and Function

Nervous tissue is the main component of the nervous system (brain, spinal cord, nerves), regulating and controlling body functions. It consists of:

• Neurons: Specialized cells generating and conducting nerve impulses.

• Supporting cells: Neuroglial cells that support, insulate, and protect neurons.

Body Membranes

Types of Membranes

Body membranes are composed of at least two primary tissue types: an epithelium bound to underlying connective tissue. Three main types:

• Cutaneous membrane: Skin; dry, outer covering.

• Mucous membrane (mucosae): Lines cavities open to exterior (respiratory, digestive, urinary, reproductive).

• Serous membrane (serosae): Lines closed body cavities (pleura, pericardium, peritoneum).

Mucous membranes are one of the three major categories of body membranes.

Tissue Repair

Steps of Tissue Repair

Tissue repair involves a sequence of steps to restore function and structure:

1. Inflammation: Injured tissue releases chemicals; blood vessels become leaky, clotting proteins form a scab.

2. Organization: Blood clot replaced by granulation tissue; damaged tissue cleaned by immune cells.

3. Regeneration and/or Fibrosis: Original tissue regrows (regeneration) or scar tissue forms (fibrosis) if regeneration isn’t possible.

This process restores tissue function and structure.

Summary Table: Types of Human Tissue