Epithelial and Connective Tissues: Structure, Function, and Classification

Epithelial Tissue Introduction

Comparison with Connective Tissue

  • Epithelial Tissue: Cells are very closely packed together, sometimes in a single layer, sometimes in multiple layers or sheets. There is very little extracellular matrix.

  • Connective Tissue: Contains few scattered cells with a lot of extracellular matrix (stuff in between cells).

Cell Polarity in Epithelial Tissue

  • Apical: Refers to the part of the cell or layer of cells that is towards the surface, such as the surface of the skin or lining an organ (open to an opening).

  • Lateral: Refers to the sides of the cells, where they connect to adjacent cells.

  • Basal: Refers to the bottom part of the cell or layer of cells, typically connected to the basement membrane and underlying fibers.

Functions of Epithelial Tissue

  • Protection: Forms multiple layers, protecting underlying structures (e.g., skin).

  • Selective Barrier: Determines what enters and exits or passes through into connective tissue; varies depending on the body location.

  • Secretory Surface: Releases substances onto a surface.

    • Examples: Sweat onto our skin, digestive enzymes into the stomach lining, covering the liver.

    • Secretion vs. Excretion:

      • Secretion: Releasing something that the body uses (e.g., hormones, digestive enzymes).

      • Excretion: Getting rid of waste products that are not needed.

  • Sensation: Provides an area for nerve endings to send signals to the brain control center.

    • Examples: Pain receptors, temperature receptors in skin, pressure receptors in artery linings.

General Classes of Epithelial Tissue

  1. Covering and Lining Epithelium: The most prevalent type.

    • Function: Covers and lines surfaces.

    • Examples: Covers skin, lines stomach, entire digestive tract, respiratory tract, and all blood vessels.

  2. Glandular Epithelium: Forms glands that secrete or release substances.

Classification of Covering and Lining Epithelium

  • Classified based on cell shape and number of cell layers.

  • Cell Shapes:

    • Squamous: Flat cells, allowing substances to pass through readily due to their thinness.

    • Cuboidal: Cube-shaped cells, involved in absorption and secretion.

    • Columnar: Elongated, rectangular, or column-shaped cells, often involved in protection. Can have cilia or not.

    • Transitional: Unique category, cells alternate between squamous and cuboidal shapes depending on organ demands (e.g., urinary tract).

  • Cell Layers:

    • Simple: One layer of cells, typically functioning in passage, diffusion, and absorption of materials.

    • Stratified: More than one layer of cells (multiple layers), typically for protection.

    • Pseudostratified: Appears to have multiple layers due to nuclei being at different heights, but is technically only one layer of cells (all attached to the basement membrane). Cells are often columnar shaped and can be ciliated or non-ciliated.

Specific Types of Covering and Lining Epithelium

  • Simple Squamous Epithelium:

    • Structure: One layer of flattened cells.

    • Function: Allows substances to pass through readily (e.g., diffusion).

    • Locations: Alveoli of the lungs, capillaries (for oxygen passage into blood).

  • Simple Cuboidal Epithelium:

    • Structure: One layer of cube-shaped cells.

    • Function: Absorption and secretion.

    • Locations: Surface of the ovary, capsule of the eye lens, kidney tubules, ducts of certain glands.

  • Simple Columnar Epithelium:

    • Structure: One layer of elongated, rectangular cells.

    • Function: Absorption (especially with microvilli to increase surface area), secretion.

    • Ciliated Type: Cilia move substances along the surface and clean out debris.

      • Locations: Respiratory tract, uterine tubes.

    • Non-Ciliated Type: May have microvilli.

  • Pseudostratified Columnar Epithelium:

    • Structure: One layer of columnar-shaped cells, but appears stratified because nuclei are at different levels and not all cells reach the free surface (though all are attached to the basement membrane).

    • Function: Secretion, movement of mucus (if ciliated), provides some protection due to overlapping cells.

    • Ciliated Type: Seen in parts of the respiratory tract, moving substances and cleaning debris.

  • Stratified Squamous Epithelium:

    • Structure: Multiple layers of flattened (squamous) cells.

    • Function: Protection.

    • Keratinized Type: Cells are filled with keratin (a protective protein) and are dead. Provides an extra layer of protection.

      • Location: Epidermis of the skin.

    • Non-Keratinized Type: Cells are living and lack keratin.

      • Location: Lining of the oral cavity (example previously given in a lab context).

  • Stratified Cuboidal Epithelium:

    • Structure: Multiple layers of cube-shaped cells; typically only the surface layer appears cuboidal, with deeper layers being more squamous or irregularly shaped.

    • Function: Protection, secretion.

    • Location (Rare): Adult sweat glands, parts of the esophagus.

  • Stratified Columnar Epithelium:

    • Structure: Multiple layers of columnar-shaped cells; typically only the apical (top) layer is truly columnar, with deeper layers being irregularly shaped or squamous.

    • Function: Protection.

    • Location (Rare): Part of the urethra, some excretory ducts.

  • Transitional Epithelium:

    • Structure: Cells transition between flattened (squamous) and cuboidal shapes.

    • Function: Allows for distension (stretching) and recoil (resuming original shape and size).

    • Locations: Primarily the bladder, also ureters and urethra (parts of the urinary system).

Clinical Applications (Epithelial Diseases/Disorders)

  • Eczema: Inflammation of the top layer of skin (stratified squamous epithelium), often triggered by various factors.

  • Psoriasis: Autoimmune condition affecting stratified squamous epithelium.

  • Skin Cancers:

    • Basal Cell Carcinoma: Most common type.

    • Squamous Cell Carcinoma: More serious/aggressive.

    • Melanoma: Most serious/aggressive type.

  • Cystic Fibrosis: Primarily a respiratory dysfunction, often affecting pseudostratified cells.

Glandular Epithelium

  • Location: Typically found deep to or underneath the covering and lining epithelium.

  • Function: Release (secrete) substances that the body uses.

  • Gland Definition: Can be a single cell (e.g., goblet cells) or many cells, sometimes considered organs. Glands typically have a combination of epithelial and other cell types.

  • Types of Glands:

    • Endocrine Glands:

      • Function: Release hormones directly into the bloodstream for absorption, without ducts.

      • Mechanism: Hormones travel via blood to target cells with specific receptors.

      • Role: Directly regulates homeostasis.

      • Example: Pancreas (also has exocrine functions).

    • Exocrine Glands:

      • Function: Release substances (non-hormones) onto a covering and lining surface via ducts (channels).

      • Examples: Sweat glands (release sweat onto skin surface), digestive enzymes (travel via ducts to the digestive tract lining).

Connective Tissue Introduction

Diversity and General Characteristics

  • Diversity: A highly diverse