Epithelial Overview

Chapter 4: Tissue Level of Organization

General Overview of Tissues

  • Definition of tissue: Cells (and intercellular material) that work together to perform a function.

    • What term is used for the "study of tissues?"

  • Organs are formed by multiple tissues that come together, performing the same functions.

  • There are only four main types of tissues in the body:

    1. Epithelial

    2. Connective

    3. Muscle

    4. Nervous

    • Importance: It is vital not just to memorize these types but to understand each tissue's specific function.

Cell Junctions

  • Importance of cell junctions: Introduced in the previous section and critical in various tissue types:

    1. Tight Junction (Fluid-sealed): Prevents fluid from passing between cells.

    2. Desmosome (Spot welding): Holds cells together.

    3. Gap Junction (Communication): Facilitates the movement of chemicals between cells.

Epithelial Tissues

Functions of Epithelial Tissues

  1. Protection: Covers surfaces and lines cavities (e.g., skin, body cavity linings).

  2. Absorption and Secretion:

    • Example: Found in kidney tubules and the digestive tract; glands are the secretory part (e.g., sweat glands).

  3. Permeability: Controls what substances are allowed to pass through the tissue.

General Characteristics of Epithelial Tissues

  1. Tightly Packed Cells: Minimal intercellular space present.

  2. Avascular: Epithelial tissues do not contain blood vessels.

    • Nutrient supply: By diffusion from underlying connective tissue.

  3. Free Surface: Typically covers/lines organs.

  4. Basement Membrane:

    • Structure: Comprised of collagen and glycoproteins, attaches epithelium to underlying connective tissue.

    • Functions:
      a. Attachment: Anchors epithelium to underlying connective tissue.
      b. Support: Provides structural support for epithelial cells.
      c. Filtration: Regulates what substances can pass between epithelium and connective tissue.

  5. Regeneration: Epithelial cells can undergo rapid mitosis; intestinal lining cells turn over every 2-3 days.

  6. Nerve Supply: Epithelial tissue is innervated with nerve endings.

  7. Additional Characteristics:

    • Keratinized Epithelia (e.g., skin) are tough and water-resistant.

    • Non-Keratinized Epithelia lack keratin and are moist (e.g., mouth).

    • Cilia: Hair-like structures on epithelial cells that help move substances across the surface.

    • Goblet Cells: Unicellular glands present in epithelia that secrete mucus.

Classification of Epithelial Tissues

  • Classified by layers and shapes:

  1. Shape:

    • Squamous: Flat cells.

    • Cuboidal: Cube-shaped cells.

    • Columnar: Tall cells.

    • Example: Draw simple cuboidal tissue.

  2. Layers:

    • Simple: One layer of cells.

    • Stratified: Two or more layers of cells.

    • Note: In stratified tissues, not all cells are the same shape.

    • Naming: Stratified tissues are named based on the shape of the cells at the free (apical) surface.

Specific Epithelial Tissues

  • For each tissue type, consider:

    1. Appearance.

    2. Location.

    3. Functional significance.

A. Simple Squamous Tissue

  1. Structure: One layer of flat cells.

    • Basement membrane location: Directly under the simple squamous layer.

    • Free surface: Top surface exposed to air or fluid.

  2. Location: Lines blood vessels, body cavities, air sacs, and kidney tubules.

  3. Function: Provides a thin layer for diffusion across cells.

    • Types include:

      • Endothelium: Lines blood vessels.

      • Mesothelium: Lines body cavities (like the peritoneum).

B. Simple Cuboidal Tissue

  1. Structure: One layer of cuboidal-shaped cells.

    • Basement membrane location: Below the cells.

    • Free surface: Topmost layer of cuboidal cells exposed.

  2. Location: Found in ducts, glands, kidney tubules, and in thyroid glands.

  3. Function: Involvement in secretion and absorption.

C. Simple Columnar Tissue

  1. Structure: One layer of tall column-shaped cells.

    • Basement membrane location: Directly beneath cells.

    • Free surface: Exposed surface is the apical side.

  2. Location: Lining the gastrointestinal tract, fallopian tubes, and larger ducts.

  3. Function: Facilitates secretion and absorption; may contain goblet cells for mucus secretion.

    • Additional features: Microvilli may be present on the surface to increase absorption efficiency.

D. Stratified Squamous Tissue

  1. Structure: Several layers of cells; the uppermost cells are flattened.

    • Mitosis: Occurs near the basement membrane, denser cell structure here.

  2. Location: Found in skin, lining of the mouth, esophagus, vagina, and rectum.

  3. Function: Provides protection from abrasion; multiple cell layers form a barrier to the connective tissue below, making this tissue any place where protection from wear and tear is needed.

    • Types:

      • Keratinized: As seen in the skin, provides extra strength and water resistance.

      • Non-Keratinized: Remains moist (found in the mouth).

E. Stratified Cuboidal Tissue

  1. Sketch required: Understand and draw structure.

  2. Location: Found in sweat gland ducts and mammary glands.

  3. Function: Primarily provides protection.

F. Transitional Epithelium

  1. Structure: Can change shape between stratified squamous (when stretched) and cuboidal (when relaxed).

    • Requires sketch showing both states.

  2. Location: Lining urinary bladder and ureters.

  3. Function: Allows for stretching and recoiling as the organ fills.

G. Pseudostratified Columnar Tissue

  1. Appearance: Appears stratified but is single-layered with cells varying in height; all cells contact the basement membrane.

    • Sketch required to show variation in height and that all cells touch the basement membrane while not all reach the free surface.

  2. Location: Lines the respiratory tract and part of the male reproductive tract.

  3. Function: Often has ciliated cells and goblet cells to produce mucin, aiding in mucus movement towards the mouth in the respiratory tract.

H. Glandular Tissue in Epithelial Tissue

  1. Types of glands:

    • Exocrine Glands: Secrete into ducts to the epithelial surface. Examples include sweat and salivary glands.

    • Endocrine Glands: Secrete hormones directly into the blood, no ducts present.

  2. Differences between exocrine and endocrine glands include location and secretion mechanisms.

  3. Types of Multicellular Exocrine Glands:

    • a. Holocrine Secretion: Entire cell fills with secretory product and then bursts (example: sebaceous glands).

    • b. Merocrine Secretion: Secretory product released via exocytosis (example: sweat glands).

    • c. Apocrine Secretion: Secretion involves part of the cell pinching off (example: mammary glands).

Unicellular Gland: Goblet Cell

  • Description: Unicellular gland producing mucin.

  • Importance: Found in respiratory and digestive tracts, helping to trap debris and facilitate the movement of materials.

Summary of Epithelial Functions and Locations

  • Simple Squamous: Best for diffusion, found in lungs and blood vessels.

  • Simple Cuboidal: Secretion and absorption, found in glandular tissues and kidney ducts.

  • Simple Columnar: Absorption and secretion, lines gastrointestinal tract.

  • Stratified Squamous: Provides protection, found in skin and mucosal linings.

  • Stratified Cuboidal: Limited locations, mainly protective roles.

  • Transitional: Stretchy, found in urinary bladder.

  • Pseudostratified Columnar: Appears stratified, found in respiratory tract.