Molecular Biology: Epithelium

1. Basic Tissue Types and Histology (Objective: List the four basic types of tissues)

Histology

  • Definition: Microscopic study of cells, tissues, and organs, focusing on the relationship between structure and function. Techniques include colorized staining and utilization of light and electron microscopes.

  • Basic components of a cell: Nucleus and Cytoplasm.

Basic Tissue Types

  • Tissue: A group of closely associated cells performing related functions and sharing structural similarities.

  • Extracellular material: Nonliving substance between cells.

  • The four basic types of tissue:

    1. Epithelial tissue (covering)

    2. Connective tissue (support)

    3. Muscular tissue (movement)

    4. Nervous tissue (control)

2. General Features and Morphology of Epithelial Tissue (Objectives: Describe general features & Discuss function/morphology relationship)

  • Definition: Epithelium is a sheet of cells covering body surfaces or lining cavities; forms most of the body’s glands.

  • Roles: Forms interfaces and boundaries between different environments.

  • Basement Membrane: Supports all epithelial tissues, varying in thickness.

Special Characteristics

  1. Cellularity: Composed almost entirely of closely packed cells.

  2. Specialized Contacts: Cell junctions maintain the integrity and structural framework.

  3. Polarity:

    • Apical Surface: The free upper surface.

    • Basal Surface: Contributes to the basal lamina of the basement membrane.

  4. Avascular but Innervated: Lacks blood vessels; nourished by diffusion and innervated by nerve endings.

  5. Supportive Framework: Underlined by connective tissue.

  6. Regeneration Capability: High turnover and capacity to regenerate.

Functions of Epithelium

  • Protection

  • Absorption

  • Sensory reception

  • Ion transport

  • Secretion

  • Filtration

  • Formation of slippery surfaces for movement.

3. Epithelial Surface Modifications (Objective: Describe different apical modifications)

Apical Surface Features

  • Cilia: Motile extensions aiding movement, found in respiratory and female reproductive tracts.

  • Flagellum: Long, isolated motile structure, only seen in human sperm.

  • Microvilli: Fingerlike extensions increasing surface area for absorption, notably located in the small intestine and kidneys.

Other Surface Features

  • Lateral Surface Characteristics: Features adhesion proteins and cellular junctions that interlock.

  • Basal Surface Characteristics: Composed of basal lamina (protein sheet with reticular fibers) forming the basement membrane.

4. Classification and Specific Types of Epithelia (Objectives: Classify epithelia & List locations, structures, and functions)

Classification

  • By Shape:

    • Squamous: Wider than tall (flattened appearance).

    • Cuboidal: Equal height and width.

    • Columnar: Taller than wide.

  • By Layers:

    • Simple: Single cell layer.

    • Stratified: Multiple layers named according to the shape of the apical layer cells.

Specific Tissue Types (Structure, Function, Location)

  • Simple Squamous Epithelium

    • Structure: Single layer of flat cells with a centrally located flattened nucleus.

    • Function: Ideal for filtration, diffusion, and secretion.

    • Locations: Alveoli of lungs, Glomerular capsule of kidneys, Endothelium of cardiovascular and lymphatic systems, Mesothelium of serous membranes.

  • Simple Cuboidal Epithelium

    • Structure: Single layer of cube-shaped cells with a centrally located nucleus.

    • Function: Primarily for secretion and absorption.

    • Locations: Pigmented layer of retina, Surface of ovary, Follicular cells of thyroid, Ducts of glands (small ducts).

  • Simple Columnar Epithelium

    • Structure: Single layer of columnar cells, with oval nuclei near base; may be ciliated or non-ciliated.

    • Function: Involved in secretion and absorption.

    • Locations: Gallbladder, Digestive system's epithelium, Uterine epithelium (ciliated version in uterine tubes).

  • Pseudostratified Columnar Epithelium

    • Structure: Appears stratified with nuclei at various levels, every cell rests on the basement membrane; may be ciliated or non-ciliated.

    • Function: Protection and secretion.

    • Locations: Ciliated: Upper respiratory tract; Non-ciliated: Ducts of many glands, epididymis, part of male urethra.

  • Stratified Squamous Epithelium

    • Structure: Surface cells are squamous; deeper cells may be cuboidal; capable of regeneration.

    • Function: Protective.

    • Locations: Non-keratinized: Gastrointestinal tract (esophagus, anus), vagina; Keratinized: Skin.

  • Transitional Epithelium

    • Structure: Stratified; allows for stretching, varied appearance in relaxed (4-5 layers thick) vs. stretched states (2-3 layers thick; surface cells flattened).

    • Function: Lines the urinary tract, enables organs to stretch and accommodate variable fluid volume.

5. Cell Junctions (Objective: Describe the structure and functions of cell junctions)

  • Tight Junctions: Create impermeable barriers between cells.

  • Adherens Junctions: Use transmembrane linker proteins to resist separation during contraction.

  • Desmosomes: Anchoring junctions using intermediate filaments that connect to opposite sides.

  • Hemidesmosomes: Integrins attach to intermediate filaments inside and laminin outside, anchoring to the basement membrane.

  • Gap Junctions: Made of connexins, allow small molecules to pass between cells for communication.

6. Glands (Objectives: Compare endocrine/exocrine glands & Describe functional classification of exocrine glands)

Characteristics

  • Formed from epithelial cells; consist of unicellular or multicellular structures.

  • Functions: Production and secretion of substances.

Comparison of Endocrine and Exocrine Glands

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

  • Endocrine Glands: Ductless, releasing hormones directly into the bloodstream; hormones target specific organs (e.g., pituitary gland, thyroid).

Functional Classification of Exocrine Glands

  • Merocrine (Eccrine): Release secretion via exocytosis (primarily proteins).

  • Apocrine: Release lipid products with free, unbroken membrane-bound vesicles (found in mammary and some sweat glands).

  • Holocrine: Whole secretory cells are discharged, leading to cell disintegration (e.g., sebaceous glands).

7. Epithelial Membranes (Objective: Describe types of epithelial membranes)

  • Definition: Thin layer or sheet covering body surfaces, organs, and cavities.

  • Components: Epithelial tissue + connective tissue.

Types of Epithelial Membranes:

  • Cutaneous Membrane: Covers the body's surface (skin), consists of epidermis and dermis.

  • Mucous Membrane: Epithelial layer on loose connective tissue (lamina propria); contains glands adapted for secretion/absorption.

  • Serous Membrane: Two layers of simple squamous epithelium on