CM02: Epithelium

What are actin (microfilaments) and their functions? Are they polar or nonpolar?

• A type of cell cytoskeleton that are found in most cells and specialized structures.

• They function as core of microvilli, locomotion of cell, extension of cell processes (like lamellipodia).

• Polar structure

What are microtubules and their functions? Are they polar or nonpolar?

• Found within most cells and specialized structures (like core of cilia)

• Movement of cilia and flagella, organize chromosomes during mitosis, and intracellular vesicular transport.

• Polar structure

What are intermediate filaments (keratins) and their functions? Are they polar or nonpolar?

• Ropelike fibers to withstand mechanical stress

• Ex: Keratins (cytokeratins), vimentin, desmin, neurofilaments, lamins

• Non-polar structures

What are the main characteristics of epithelium? (6)

• exhibit polarity with apical, basal, and lateral domains

• close together with no extracellular matrix between them

• covers external body surfaces, lines internal body cavities, and forms glands

• rests on a specialized connective tissue structure (basement membrane)

• avascular and innervated

• non-specific line of defense against infection

What is the difference between simple and stratified epithelium?

• Stratified: provides protection against friction, abrasion, infection, etc.

• Simple: functionally more important as a regulatory barrier

What is keratinization?

cytoplasmic events in keratinocytes as they move through the different layers of the epidermis to differentiate from mitotic cells to cells filled with keratin filaments

What is orthokeratinized?

Nuclei is lost in the keratinized layer

What is parakeratinized?

Pyknotic nuclei becomes dense and begins the process of karyorrhexis (fragmentation). This is not a complete layer and stays in cornified layer.

What is non-keratinization?

When there is no clear division of strata and nuclei are apparent in the upper layer.

What is the apical domain?

Free surface of the epithelial sheet, it faces an internal space (called a lumen/external environment)

What is the lateral domain?

Surfaces of cell-cell contact.

What is the basal domain?

Surface of the cell that contacts the basement membrane.

What is the microvilli?

They are projections of the apical epithelial cell surface covered with plasma membrane

What are microvilli made of?

Core of actin microfilaments cross-linked with actin-binding proteins

What is the function of microvilli?

Increase the surface area of the apical region of cells that face an internal lumen or the external environment

What are cilia?

They are projections of apical cell surface covered by plasma membrane

What do each cilium contain?

They contain an axoneme made of the 9+2 array of microtubules.

What are stereocilia?

They are extensions of the apical surface covered by plasma membrane

What do sterocilia contain?

They contain a core of actin microfilaments.

What are the three functions of the lateral domains of epithelial cells?

• Provide stability

• Separates apical domain from basolateral domain

• Control movement of solutes, ions, and water across an epithelial sheet

What is zonula?

Adhesion structure that surrounds the entire perimeter of the cell.

What is macula?

A spot-like adhesion structure that is restricted to one small region of the lateral domain of two cells.

What is fascia?

An adhesion junction that covers more cell surface area than a macula.

What type of junction is a tight junction?

Occluding junction

Where are tight junctions found?

Found at the top of lateral domain near the apical domain

What are some functions of tight junctions?

• prevent diffusion of membrane lipids and proteins

• regulates the paracellular pathway

What type of junction is zonula adherens?

adherens junction

Where are zonula adherens found?

Beneath the tight junction

What is the function of the zonula adherens?

Adhesion of two epithelial cells

What type of junction are macula adherens?

Desmosome (type of anchoring junction)

Where are macula adherens found?

Found beneath the zonula adherens as part of the junctional complex.

What is the function of the macula adherens?

Adhesion of two epithelial cells and holds stratified squamous epithelium as an intact sheet.

What type of junction is gap junction?

Communication junction

Where are gap junctions found?

Between two epithelial cells

What is the function of gap junction?

• connexons form channels in the plasma membranes between two cells

• a gap junction is formed by many connexons

What is a hemidesmosome?

It is a cell-matrix anchoring junctions

Where are hemidesmosomes found?

Between one epithelial cell and basement membrane.

What is the function of hemidesmosome?

It attaches epithelial cells to the basement membrane.

What is a basement membrane? What is it made of?

It is the extracellular matrix between epithelial cells and underlying connective tissue. Collagen type IV and other proteins.

What are the functions of the basement membrane?

• Supports connective tissue structure for epithelial cells and a transition layer between epithelium and underlying connective tissue

• Serves as filtration of plasma in the kidney

• Guides migration of cells during embryogenesis

• Scaffold for tissue regeneration

• Contributes to epithelial cell polarity