Tissue, The Living Fabric

Cellular Organization: Unicellular organisms function independently; multicellular organisms aggregate cells into tissues to work cooperatively.
Division of Labor: Specialized structural modifications optimize physiological efficiency across complex systems.
Definition: A tissue is a group of structurally similar cells performing a common, related, or specialized function.
Primary Tissue Classes: Epithelial, connective, muscle, and nervous tissues.

General Characteristics: Continuous cellular sheets covering external surfaces, lining internal cavities, and forming glands.
Classification Principles:
- Layering: Simple (single cell layer) or Stratified (two or more cell layers).
- Cell Shape: Squamous (flat), Cuboidal (cube-like), Columnar (tall/rectangular).
Key Properties:
- Polarity: Features an apical surface (exposed to exterior or lumen) and a basal surface (bound to basal lamina).
- Junctions: Held tightly by tight junctions and desmosomes.
- Basement Membrane: Formed by basal lamina and reticular lamina to reinforce the epithelium.
- Avascular but Innervated: Lacks blood vessels (nutrients diffuse from underlying connective tissue) but contains nerve fibers.
- Regeneration: High mitotic capacity for rapid replacement.

Simple Squamous: Single layer of flattened cells; facilitates rapid diffusion/filtration; located in alveoli, kidney glomeruli, and blood vessel linings.
Simple Cuboidal: Single layer of cube-like cells; functions in active secretion and absorption; located in kidney tubules and gland ducts.
Simple Columnar: Single layer of tall cells (often with goblet cells or cilia); functions in absorption and secretion; lines most of the GI tract (non-ciliated) and uterine tubes (ciliated).

Pseudostratified Columnar: Single layer of varying cell heights giving a layered appearance; secretes and propels mucus; lines trachea and upper respiratory tract.
Stratified Squamous: Multiple cell layers protecting against mechanical friction and abrasion.
- Keratinized: Surface cells are dead and filled with keratin; forms the epidermis of skin.
- Non-keratinized: Surface cells remain living; lines mouth, esophagus, and vagina.
Transitional: Apical cell shape changes with tissue distention (dome to flattened); lines ureters, urinary bladder, and urethra.

Overview: Most abundant tissue type; derived from embryonic mesenchyme; composed of cells suspended in a non-living extracellular matrix.
Loose Connective Tissue:
- Areolar: Gel-like matrix with collagen, elastic, and reticular fibers; cushions organs and assists immune defense; located beneath epithelia.
- Adipose: Densely packed adipocytes with sparse matrix; stores reserve fuel, insulates, and cushions organs; located subcutaneously and around kidneys.
- Reticular: Reticular fiber network; provides supportive framework (stroma) in lymphoid organs (lymph nodes, spleen, bone marrow).
Dense Connective Tissue:
- Dense Regular: Parallel collagen bundles; withstands single-direction tensile stress; forms tendons and ligaments.
- Dense Irregular: Irregularly arranged collagen fibers; resists multi-directional tension; forms dermis and organ capsules.
- Elastic: High concentration of elastin fibers; allows tissue recoil; located in walls of large arteries and bronchial tubes.

Cartilage: Avascular, resilient tissue with chondrocytes residing in lacunae.
- Hyaline: Amorphous matrix; provides flexible support and cushioning; covers long bone ends, nose, trachea, and costal cartilages.
- Elastic: Rich in elastic fibers; maintains shape while granting flexibility; external ear and epiglottis.
- Fibrocartilage: Matrix dominated by thick collagen bundles; absorbs compressive shock; intervertebral discs and knee menisci.
Bone (Osseous Tissue): Hard, calcified matrix with collagen fibers and osteocytes in lacunae; highly vascularized; supports, protects, stores minerals, and hosts hematopoiesis.
Blood: Fluid connective tissue consisting of plasma matrix and formed elements (red cells, white cells, platelets); transports gases, nutrients, wastes, and hormones.

Structural Components:
- Neurons: Impulse-conducting cells featuring a cell body (soma), dendrites (receptive processes), and an axon (impulse generator/conductor).
- Neuroglia: Supporting non-conducting cells (astrocytes, oligodendrocytes, microglia, ependymal cells) that nourish, protect, and insulate neurons.
Locations: Brain, spinal cord, and peripheral nerves.

Skeletal Muscle: Voluntary control; striated, long multinucleated cylindrical cells with peripheral nuclei; attached to bones via tendons.
Cardiac Muscle: Involuntary control; striated, branching cells with 1–2 central nuclei and intercalated discs containing gap junctions; located in myocardium.
Smooth Muscle: Involuntary control; non-striated, spindle-shaped cells with a single central nucleus; located in walls of hollow organs and blood vessels.