Tissues and Histology

Introduction to Tissues

Tissues are groups of cells similar in structure that perform a common function. The study of tissues is called Histology.

• Four basic tissue types: Epithelial, Connective, Muscle, Nervous (focus here: Epithelial & Connective)

Epithelial Tissue

Overview and Functions

Epithelial tissue forms sheets of cells that cover body surfaces and line cavities. It serves as a protective barrier and is involved in absorption, filtration, excretion, secretion, and sensory reception.

• Covering and Lining Epithelium: Found on external and internal surfaces (e.g., skin).

• Glandular Epithelium: Forms secretory tissue in glands (e.g., salivary glands).

Characteristics of Epithelial Tissue (ET)

• Polarity: Has an apical (top) surface and basal (bottom) surface. The apical surface may have specializations like microvilli or cilia.

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

• Supported by Connective Tissue: Sits on a basement membrane composed of basal lamina and reticular lamina.

• Avascular but Innervated: No blood vessels, but has nerve supply.

• Regeneration: High capacity for renewal.

Classification of Epithelia

• Number of Layers:

  • Simple Epithelium: Single cell layer (for absorption, secretion, filtration).

  • Stratified Epithelium: Two or more layers (for protection).

• Cell Shape:

  • Squamous: Flattened, scale-like.

  • Cuboidal: Box-like, cube.

  • Columnar: Tall, column-like.

In stratified epithelia, cells are named according to the shape at the apical layer.

Types of Epithelial Tissue

• Simple Squamous Epithelium:

  • Function: Rapid diffusion (e.g., lungs, kidneys).

  • Types:

    • Endothelium: Lining of lymphatic vessels, blood vessels, heart.

    • Mesothelium: Serous membranes in ventral body cavity.

• Simple Cuboidal Epithelium:

  • Function: Secretion and absorption.

  • Location: Kidney tubules, ducts of small glands.

• Simple Columnar Epithelium:

  • Function: Absorption, secretion of mucus, enzymes.

  • Features: May have microvilli, cilia, goblet cells.

• Stratified Epithelial Tissues:

  • More durable, for protection.

  • Stratified Squamous Epithelium: Most widespread; surface cells are squamous, deeper layers cuboidal/columnar. Found in skin (keratinized) and moist linings (nonkeratinized).

  • Transitional Epithelium: Found in ureter, bladder; cells change shape when stretched.

  • Stratified Cuboidal Epithelium: Rare, usually only two layers.

• Pseudostratified Columnar Epithelium:

  • Appears stratified but is a single layer; nuclei at different levels.

  • Function: Secretion and movement of mucus (e.g., trachea, upper respiratory tract).

Glandular Epithelium

Consists of one or more cells that make and secrete an aqueous fluid.

• Classification by Product Release:

  • Endocrine: Internally secreting (hormones into blood/interstitial fluid).

  • Exocrine: Externally secreting (onto body surfaces or into ducts).

• Classification by Cell Number:

  • Unicellular: Single cell (e.g., goblet cells).

  • Multicellular: Many cells (e.g., salivary glands).

Endocrine Glands

• Ductless; secrete hormones via exocytosis into interstitial fluid.

• Hormones enter the circulatory system and act on target organs.

Exocrine Glands

• More common than endocrine glands.

• Secrete products into ducts (e.g., mucus, sweat, oil, saliva).

• Unicellular Exocrine Glands: Goblet cells produce mucin (forms mucus).

• Multicellular Exocrine Glands: Made of duct and secretory unit; e.g., mammary glands.

Modes of Secretion

• Merocrine: Products secreted by exocytosis as produced (e.g., sweat, pancreas).

• Holocrine: Products accumulate, then cell ruptures (e.g., sebaceous glands).

• Apocrine: Products accumulate, apex ruptures (e.g., mammary glands).

Connective Tissue (CT)

Overview and Functions

Connective tissue supports, protects, and binds other tissues. It is the most abundant and widely distributed tissue type.

• Functions: Support, protection, insulation, storage, transport (e.g., blood).

Characteristics of Connective Tissue

• Common Origin: All CT arises from mesenchyme (embryonic tissue).

• Vascularity: Varies by type (cartilage is avascular, bone is highly vascularized).

• Extracellular Matrix (ECM): Cells are suspended in ECM (protein-sugar mesh), allowing CT to bear weight and withstand tension.

Components of Connective Tissue

• Ground Substance: Unstructured gel-like material filling space between cells; made of interstitial fluid, proteins, and water (affects viscosity).

• Fibres:

  • Collagen: Strongest, most abundant; high tensile strength.

  • Elastic: Long, thin, for stretch and recoil.

  • Reticular: Short, fine, highly branched; forms networks for support.

• Cells:

  • "Blast" cells: Immature, actively mitotic (e.g., osteoblasts, chondroblasts, fibroblasts).

  • "Cyte" cells: Mature, less active, maintain matrix health.

  • Other cells: Fat cells, white blood cells, mast cells, macrophages (defense).

Types of Connective Tissue Proper

• Areolar: Loose, space-filled; most widely distributed. Holds fluids, supports other tissues.

• Adipose:

  • White fat: Greater nutrient storage, shock absorption, insulation.

  • Brown fat: Uses lipid fuels to heat blood.

• Reticular: Mesh-like stroma; supports blood cells in lymph nodes, spleen, bone marrow.

• Dense Regular: High tensile strength; closely packed collagen fibres (e.g., tendons).

• Dense Irregular: Thicker, irregularly arranged collagen; resists tension from many directions (e.g., dermis).

• Dense Elastic: Elastic fibres; found in walls of large arteries, ligaments.

Cartilage

• Matrix secreted by chondroblasts (during growth) and maintained by chondrocytes (in lacunae).

• 80% water, packed with collagen and sugar protein.

• Tough yet flexible; avascular (receives nutrients from perichondrium).

• Types:

  • Hyaline: Most abundant; glassy appearance (e.g., long bones, nose).

  • Elastic: More elastic fibres; found in ear, epiglottis.

  • Fibrocartilage: Properties between hyaline and dense regular CT; found in intervertebral discs, knee.

Bone (Osseous Tissue)

• Supports, protects, stores minerals, and enables hematopoiesis.

• Matrix: Collagen fibres and calcium salts.

• Cells: Osteoblasts (build bone), osteoclasts (break down bone), osteocytes (maintain bone).

• Types: Compact (dense outer layer), spongy (trabecular inner mesh).

Blood

• Fluid connective tissue; transports nutrients, gases, wastes.

• Matrix: Plasma (liquid ECM).

• Cells: Red blood cells, white blood cells, platelets.

Key Terms Table

Additional Info

• Equations: No direct equations, but relevant for physiology (e.g., diffusion rate, osmotic pressure).

• Example: Simple squamous epithelium in alveoli of lungs allows rapid gas exchange due to thinness.

• Comparison Table: See above for key terms and tissue types.