Introduction to Tissues

Tissues are groups of similar cells and extracellular material that work together to perform specific functions. The study of tissues is called histology. There are four primary types of tissues in the human body, each with distinct structures and functions.

• Epithelial tissue

• Connective tissue

• Muscle tissue

• Nervous tissue

Extracellular Matrix

• Provides structural scaffolding and regulates cell activities.

• Composed of water, proteins, and polysaccharides in varying ratios.

• Constantly remodeled depending on tissue needs.

Epithelial Tissue

Characteristics of Epithelial Tissue

Epithelial tissue lines body surfaces and forms glands. It is composed of closely packed cells with minimal extracellular matrix.

• Cellularity: Almost entirely made of tightly packed cells.

• Polarity: Has an apical (top) surface exposed to the environment or body space, and a basal (bottom) surface attached to connective tissue.

• Attachment to Basement Membrane: Serves as a barrier between epithelium and underlying connective tissue.

• Avascularity: Lacks blood vessels; nutrients diffuse from underlying tissues.

• Extensive Innervation: Richly supplied with nerves to detect environmental changes.

• High Regeneration Capacity: Rapidly replaces lost or damaged cells.

Functions of Epithelial Tissue

• Physical Protection: Shields underlying tissues from dehydration, abrasion, and destruction.

• Selective Permeability: Regulates entry and exit of substances.

• Secretions: Some epithelial cells are specialized to secrete substances (e.g., enzymes, hormones).

• Sensations: Contains nerve endings to supply information to the nervous system.

Classification of Epithelial Tissue

Epithelia are classified by two criteria: the number of cell layers and the shape of cells at the apical surface.

• By Number of Cell Layers:

  • Simple epithelium: Single cell layer; all cells contact the basement membrane. Functions in filtration, absorption, or secretion.

  • Stratified epithelium: Two or more layers; only basal layer contacts basement membrane. Found in areas subject to mechanical stress.

  • Pseudostratified epithelium: Appears layered, but all cells contact the basement membrane; not all reach the apical surface.

• By Cell Shape:

  • Squamous cells: Flat and thin.

  • Cuboidal cells: Cube-shaped.

  • Columnar cells: Taller than they are wide.

  • Transitional cells: Change shape depending on stretch (e.g., in the urinary bladder).

Example: Simple squamous epithelium lines blood vessels; stratified squamous epithelium forms the outer layer of skin.

Glands

Glands are composed of epithelial tissue and are specialized for secretion. They can be unicellular or multicellular.

• Endocrine glands: Lack ducts; secrete hormones directly into the blood (e.g., thyroid gland).

• Exocrine glands: Have ducts; secrete products onto epithelial surfaces (e.g., sweat, salivary glands).

Multicellular Exocrine Glands

• Acini: Clusters of cells that produce secretions.

• Ducts: Transport secretions to the epithelial surface.

• Fibrous capsule: Surrounds the gland, with septa dividing it into lobes.

Classification by Method of Secretion

Connective Tissue (CT)

Functions of Connective Tissue

• Supports, protects, and binds organs

• Most diverse, abundant, and widely distributed tissue type

• Vascularity varies (e.g., bone is highly vascular, cartilage is avascular)

• Wide range of regenerative ability

Characteristics of Connective Tissue

All connective tissues share three basic components:

• Cells: Specific to each CT type; can be resident (stationary) or wandering (mobile).

• Protein fibers: Provide support and strength.

• Ground substance: Non-cellular material produced by CT cells; forms the extracellular matrix with protein fibers.

Resident Cells

• Fibroblasts: Most abundant; produce fibers and ground substance.

• Adipocytes: Fat cells; store energy.

• Mesenchymal cells: Embryonic stem cells for repair.

• Fixed macrophages: Phagocytize damaged cells/pathogens; stimulate immune response.

Wandering Cells

• Mast cells: Secrete heparin (inhibits blood clotting) and histamine (dilates blood vessels).

• Plasma cells: Produce antibodies.

• Free macrophages: Mobile phagocytes.

• Other leukocytes: Neutrophils (phagocytize bacteria), T-lymphocytes (attack foreign materials).

Protein Fibers

• Collagen fibers: Unbranched, strong, and flexible; found in tendons and ligaments.

• Reticular fibers: Thinner than collagen; form supportive networks in organs like lymph nodes.

• Elastic fibers: Contain elastin; stretch and recoil easily; found in skin and arteries.

Ground Substance

• Produced by CT cells; can be viscous (blood), semisolid (cartilage), or solid (bone).

• Glycosaminoglycans (GAGs): Large, negatively charged molecules that attract water.

• Proteoglycans: GAGs linked to proteins.

• Glycoproteins: Proteins with attached carbohydrates; bind CT cells and fibers to ground substance.

Functions of Connective Tissue

• Physical protection

• Support and structural framework

• Binding of structures

• Storage (e.g., fat, minerals)

• Transport (e.g., blood)

• Immune protection

Classification of Connective Tissue

CT Proper

• Loose CT: Fewer cells and fibers, more ground substance; supports structures and organs. Types:

  1. Areolar

  2. Adipose

  3. Reticular

Example: Areolar CT is found beneath the skin; adipose CT stores energy; reticular CT forms the framework of lymphoid organs.