Histology and Tissue Types Vocabulary

Histology: The study of tissues and their microscopic structure.
Tissue: Specialized cells working together for a common function.
Primary Tissue Types:
- Epithelial: Covers surfaces, lines cavities, and forms boundaries.
- Connective: Supports, protects, binds, and insulates.
- Muscle: Produces movement via contraction.
- Nervous: Enables internal communication and control.

Definition: Closely packed cell sheets covering surfaces or lining cavities.
Categories by Location:
- Covering & Lining: Skin and inner mucosal linings.
- Glandular: Secretory tissue in exocrine/endocrine glands.
Functions: Protection, absorption, filtration, excretion, secretion, sensory reception.
Characteristics:
- Polarity: Apical (free surface) and Basal (attached layer).
- Continuity: Continuous sheets with tight junctions.
- Support: Anchored by a basement membrane.
- Avascular & Innervated: Lacks blood vessels; supplied by nerves.
- High Regeneration: Rapid cell replacement via mitosis.

Naming System:
- Layers: Simple (1 layer) vs. Stratified ( $2+$ layers).
- Shape: Squamous (flat), Cuboidal (cube), Columnar (tall). Stratified tissue is named by its apical layer shape.

Simple Squamous:
- Function: Rapid diffusion and filtration.
- Location: Kidney glomeruli, lung alveoli, blood vessels, serous membranes.

Simple Cuboidal:
- Function: Secretion and absorption.
- Location: Kidney tubules, small gland ducts, ovary surface.

Simple Columnar:
- Function: Absorption, mucus/enzyme secretion.
- Location: Digestive tract lining (nonciliated); small bronchi and uterine tubes (ciliated).

Pseudostratified Columnar:
- Function: Mucus secretion and ciliary movement.
- Location: Trachea and upper respiratory tract (ciliated); sperm ducts (nonciliated).

Stratified Squamous:
- Function: Protection against abrasion.
- Location: Esophagus, mouth, vagina (nonkeratinized); skin epidermis (keratinized).

Transitional:
- Function: Stretches to allow distension.
- Location: Ureters, urinary bladder, urethra.

Rare Stratified Epithelia:
- Stratified Cuboidal: Large gland ducts (sweat, mammary).
- Stratified Columnar: Pharynx, male urethra.

Endocrine Glands: Ductless; secrete hormones directly into blood/interstitial fluid.
Exocrine Glands: Duct-containing; secrete onto body surfaces or into cavities (e.g., salivary, sweat, liver).

Key Features:
- The nonliving extracellular matrix (ECM) permits load-bearing and stress resistance.
- All derive from embryonic mesenchyme.
- Vascularity ranges from avascular (cartilage) to highly vascularized (bone).
ECM Composition:
- Ground Substance: Interstitial fluid, adhesion proteins, proteoglycans.
- Fibers: Collagen (tensile strength), Elastic (stretch/recoil), Reticular (support network).
Cell Types:
- Blast Cells (Active): Fibroblasts, chondroblasts, osteoblasts.
- Cyte Cells (Mature): Fibrocytes, chondrocytes, osteocytes, blood cells.
- Defense/Storage: Adipocytes, leukocytes, mast cells, macrophages.

Loose Connective Tissues:
- Areolar: Cushions organs; contains all 3 fiber types; key in inflammation.
- Adipose: Packed with adipocytes; white fat (insulation/storage) and brown fat (heat generation).
- Reticular: Reticular stroma supporting blood/lymphoid cells in spleen and lymph nodes.
Dense Connective Tissues:
- Dense Regular: Parallel collagen fibers; withstands unidirectional tension; poorly vascularized (slow repair; found in tendons/ligaments).
- Dense Irregular: Interwoven collagen; resists multidirectional stress (dermis, joint capsules).
- Dense Elastic: High elastic fiber content; allows recoil in arteries and vertebral ligaments.

Avascular, lacks nerves, up to $80\%$ water.
Hyaline: Provides firm support and cushioning (joints, ribs, nose, trachea).
Elastic: High flexibility (external ear, epiglottis).
Fibrocartilage: Resists heavy compression (intervertebral discs, pubic symphysis).

Calcified matrix with collagen; highly vascularized. Functions in support, protection, mineral storage, and hematopoiesis.

Red/white blood cells and platelets in plasma. Transports gases ( $O_2$ , $CO_2$ ), nutrients, and wastes.

Skeletal: Voluntary, striated, multinucleate, cylindrical cells attached to bones.
Cardiac: Involuntary, striated, uninucleate, branching cells with intercalated discs in heart wall.
Smooth: Involuntary, nonstriated, spindle-shaped cells in hollow organ walls.

Composed of Neurons (signal transmission) and Neuroglia (supportive cells). Located in brain, spinal cord, and peripheral nerves.

Cutaneous: Dry skin membrane (keratinized stratified squamous on dermis).
Mucous: Moist membranes lining cavities open to exterior (digestive, respiratory).
Serous: Moist membranes lining closed ventral cavities (pleurae, pericardium, peritoneum).

Inflammation: Chemicals cause vasodilation and vessel permeability; blood clot forms scab.
Organization: Granulation tissue and capillaries form; fibroblasts synthesize collagen; macrophages clear debris.
Regeneration & Fibrosis: Fibrous scar contracts and matures while overlying epithelium regenerates.

Regenerative Capacity:
- Good: Epithelia, bone, areolar CT, dense irregular CT, blood-forming tissue.
- Moderate: Smooth muscle, dense regular CT.
- Weak: Skeletal muscle, cartilage.
- None: Cardiac muscle, CNS nervous tissue.
Germ Layers:
- Ectoderm: Nervous tissue and epidermis.
- Mesoderm: Muscle, connective tissue, endothelium.
- Endoderm: Mucosae of digestive/respiratory systems.
- Note: Epithelium develops from all three germ layers.
Aging Effects: Epithelia thin, repair efficiency decreases, and muscle/bone tissue atrophies.