ppt epithelium

Epithelial Tissue

Characteristics of Epithelial Tissue

• Sheets of cells that line surfaces.

• Act as barriers and mediators between two environments.

• Function in sealing and selective transport.

Locations of Epithelial Tissue

• Lines body surfaces, body cavities, and hollow organs.

• Examples include:

  • Pleural cavity

  • Pericardial cavity

  • Abdominopelvic cavity

Functions of Epithelial Tissue

• Absorption: Substances that enter or exit the body must pass through epithelia.

• Secretion: Epithelial cells are involved in the secretion of various substances, such as mucus from Goblet cells and digestive enzymes from those lining the intestines.

Characteristics of Epithelia

1. Cells are tightly bound together, leaving very little extracellular space.

   • Cell membranes interdigitate like a jigsaw puzzle.

   • Junctional complexes are specialized areas that hold cells tightly together and create a seal between two environments.

2. Cells lie on a basement membrane.

   • The basement membrane is a sheet of extracellular material composed of:

     • Collagen filaments (a type of protein)

     • Glycosaminoglycans

   • Thickness is approximately 100 nm.

   • It is secreted by the epithelial cells and serves as:

     • A structural substrate for the epithelium.

     • A barrier to macromolecules.

3. Epithelia are avascular.

   • They contain no blood vessels.

   • Nutrients reach epithelial cells by diffusion from blood vessels in the underlying connective tissue.

4. Cells have polarity (apical and basal).

   • Apical membrane: faces the lumen or outside of the organ.

   • Basal membrane: adheres to the basement membrane.

   • Cells exhibit differing structures on apical and basal surfaces, such as microvilli on apical surfaces in absorptive tissues like the proximal renal tubule.

Classification of Epithelia

By Number of Cell Layers

1. Simple: One layer of cells.

2. Pseudostratified: Appears to have multiple layers due to cell height variations, but is a single layer.

3. Stratified: Multiple layers of cells.

By Shape of Cells in Apical Layer

1. Squamous: Thin, flat cells.

2. Cuboidal: Cube-shaped cells.

3. Columnar: Taller than they are wide, column-like cells.

4. Transitional: Can change shape based on stretching.

Specific Types of Epithelia

• Simple Squamous

• Simple Cuboidal

• Stratified Squamous Nonkeratinized

• Stratified Squamous Keratinized

• Simple Columnar

• Pseudostratified Columnar

• Transitional (relaxed)

• Transitional (distended)

Glandular Epithelium

• Specialized for secretion and develops from covering epithelium through proliferation and downgrowth into the underlying connective tissue.

• Types of Glands:

  • Exocrine glands: Have ducts and secrete substances onto surfaces (e.g., sweat glands, salivary glands).

  • Endocrine glands: No ducts, deliver secretions (e.g., hormones) into the bloodstream.

• Exocrine gland formation includes:

  • Simple glands:

  • Simple tubular

  • Simple coiled tubular

  • Simple branched tubular

  • Simple branched acinar

  • Compound glands:

  • Compound tubuloacinar

  • Compound tubular

  • Compound acinar

Apical Specializations

1. Microvilli: Permanent extensions of the apical plasma membrane that increase surface area about 25 times. Have a microfilament core.

2. Cilia: Motile extensions of the apical plasma membrane that move substances along epithelial surfaces. Composed of microtubules, they tend to beat in synchrony.

3. Stereocilia: Very long microvilli that increase surface area, with a microfilament core.

4. Keratinization: The process where surface cells are filled with intermediate filaments, keratin, making them water and enzyme proof. This provides friction resistance, especially in stratified squamous epithelium.

5. Goblet cells: Unicellular glands that secrete mucus, containing a nucleus and secretory vesicles with mucin.

Functions of Epithelial Tissue Related to Tissue Type

• One Layer (Simple Epithelial Tissues): Generally involved in processes such as diffusion and filtration, as well as secretion and absorption.

• More Than One Layer (Stratified Epithelial Tissues): Generally involved in protection and, in the case of transitional epithelium, stretching. Most of these tissue types are rare in humans.