Unit 2 - Epithelial Tissue (copy)

Unit 2 - TISSUES, ORGANS, DEVICES AND SYSTEMS IN THE HUMAN BEING

Epithelial Tissue

• Epithelial tissue is composed of cells laid together in sheets with the cells tightly connected to one another.

•  Epithelial layers are avascular, but innervated. Epithelial cells have two surfaces that differ in both structure and function. 

• Glands, such as exocrine and endocrine, are composed of epithelial tissue and classified based on how their secretions are released.

Functions of Epithelium

• Epithelial tissue forms boundaries between different environments, nearly all substances must pass through the epithelium. Role as interface tissue, epithelium accomplishes functions, including: 

• Protection for the underlying tissues from radiation, desiccation, toxins, + physical trauma.

•  Absorption of substances in the digestive tract lining with distinct modifications. 

• Regulation & excretion of chemicals between the underlying tissues and the body cavity.

• Secretion of hormones into the blood vascular system.  Secretion of sweat, mucus, enzymes, and other products that are delivered by ducts come from the glandular epithelium. 

• Detection of sensation.

Characteristics of Epithelial Layers

• Composed of cells laid out in sheets with strong cell-to-cell attachments.

•  These protein connections hold the cells together to form a tightly connected layer that is avascular but innervated in nature.

• Epithelial cells are nourished by substances diffusing from blood vessels in the underlying connective tissue.

•  One side of the epithelial cell is oriented towards the surface of the tissue, body cavity, or external environment and the other surface is joined to a basement membrane. 

• The basement layer is non-cellular in nature and helps to cement the epithelial tissue to the underlying structures.

• Typically characterised by the polarised distribution of organelles and membrane-bound proteins between their basal and apical surfaces.

•  Particular structures found in some epithelial cells are an adaptation to specific functions.

•  Certain organelles are segregated to the basal sides, whereas other organelles and extensions, such as cilia, when present, are on the apical surface.

• Cilia are microscopic extensions of the apical cell membrane that are supported by microtubules.

•  Beat in unison and move fluids as well as trapped particles. 

• Ciliated epithelium lines the ventricles of the brain where it helps circulate the cerebrospinal fluid. 

• The ciliated epithelium of your airway forms a mucociliary escalator that sweeps particles of dust and pathogens trapped in the secreted mucus toward the throat. It is called an escalator because it continuously pushes mucus with trapped particles upward. 

• In contrast, nasal cilia sweep the mucous blanket down towards your throat. In both cases, the transported materials are usually swallowed, and end up in the acidic environment of your stomach.:

Types of Epithelial Tissue

• Identified by both the number of layers and the shape of the cells in the upper layers. 

• 8 basic types of epithelium: 6 identified based on both the number of cells and their shape; 2 of them are named by the type of cell (squamous) found in them. 

Cell to Cell Junctions:

Cells of epithelia are closely connected and are not separated by intracellular material. Three basic types of connections allow varying degrees of interaction between the cells: tight junctions, anchoring junctions, and gap junctions

Cellular Junctions and Their Role in Epithelial Tissue Stabilization

Tight Junctions:

• Separate cells into apical and basal compartments.

• Anchoring junctions:

• Stabilise epithelial tissues.

•  Common on lateral and basal surfaces of cells.

•  Provide strong, flexible connections

• Types include desmosomes, hemidesmosomes, and adherens.

Desmosomes:

• Structural proteins on the inner surface of cell membranes.

• The adhesion molecule, cadherin is embedded in these patches

• Projects through the cell membrane to link with the cadherin molecules of adjacent cells.

Hemidesmosomes:

• Link cells to extracellular matrix e.g basal lamina

• Include integrins, not cadherins.

Adherens junctions:

• Use either cadherins or integrins depending on cell or matrix link.

• Characterised by contractile protein actin on cytoplasmic surface.

• The actin can connect isolated patches or form a belt-like structure inside the cell. Junctions influence the shape and folding of the epithelial tissue

Gap Junctions:

• Form intercellular passageway between adjacent cell membranes.

• Facilitate movement of small molecules and ions.

•  Allow electrical and metabolic coupling of adjacent cells.