Tissue: The Living Fabric

Introduction to Tissues

Tissues are groups of cells similar in structure that perform common or related functions, forming the foundation for all organs and body structures. The study of tissues is called 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.

Microscopy of Human Tissue

Preparation and Staining

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).

Epithelial Tissue

General Characteristics

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

• Covering and lining epithelia: On 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.

Special Characteristics of Epithelial Tissues

• Polarity: Cells have an apical (top) and basal (bottom) surface, each with distinct structures and functions.

• Specialized contacts: Cells are closely joined by tight junctions and desmosomes.

• Supported by connective tissues: All epithelial sheets rest on a basement membrane composed of 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 subject to friction or hostile environments.

Classification of Epithelia

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

• Layers: Simple (one layer) or stratified (multiple layers).

• Shapes: Squamous (flat), cuboidal (cube-like), columnar (tall).

Types of Epithelial Tissue

• Simple squamous: Single layer of flat cells; allows diffusion/filtration (e.g., air sacs of lungs, lining of blood vessels).

• Simple cuboidal: Single layer of cube-shaped cells; secretion and absorption (e.g., kidney tubules, small glands).

• Simple columnar: Single layer of tall cells; absorption, secretion of mucus/enzymes (e.g., digestive tract, uterine tubes).

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

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

• Transitional: Resembles both stratified squamous and cuboidal; stretches (e.g., bladder, ureters).

Glandular Epithelia

Glands are one or more cells that make and secrete an aqueous fluid (secretion). They are classified by:

• Site of product release: Endocrine (into blood/lymph) or exocrine (onto body surface or into cavities).

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

Modes of Secretion

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

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

• Apocrine: Apex of cell pinches off (controversial in humans).

Covering and Lining Membranes

• Cutaneous membrane: The skin; covers the body surface.

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

• Serous membranes: Line closed ventral body cavities (e.g., pleura, pericardium, peritoneum).

Connective Tissue

General Characteristics

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 (blood).

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

Comparison of Classes of Connective Tissues

Common Characteristics of Connective Tissue

• All arise from mesenchyme (embryonic tissue).

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

• Cells are suspended in an extracellular matrix (ECM) of fibers and ground substance.

Structural Elements of Connective Tissue

• Ground substance: Unstructured material filling space between cells; contains interstitial fluid, cell adhesion proteins, and proteoglycans.

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

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

Types of Connective Tissue Proper

• Loose connective tissue: Areolar, adipose, reticular.

• Dense connective tissue: Dense regular, dense irregular, elastic.

Summary Table: Epithelial and Connective Tissue Types