HB 3 (Epithelial Cell)

• Epithelia are the closely applied sheet of cells with minimal intercellular substance that covers all free surface of the body, large internal body cavities, and the internal surface of blood & lymph vessels.
• Epithelia are derived from three germ layers: Ectoderm, Mesoderm, and Endoderm.
• Epithelia can be divided into two main groups according to their structure and function: Covering Epithelia and Glandular Epithelia.
• Covering Epithelia have cells organized in layers and cover the external surface or line the cavities of the body.
• Covering Epithelia are classified according to the number of cell layers and the morphologic features (shape) of the cells in the surface layer.
• Simple Epithelia have one layer of cells and are further classified into Stratified Epithelia and Pseudostratified Epithelia.
• Microvilli are cytoplasmic processes extending from the cell surface that move substances over the surface.
• Cilia appear as hair-like structures from the apical surface and are anchored into the apical cytoplasm by basal bodies.
• Stereocilia are long microvilli, nonmotile processes, found in the male reproductive tract, including the epididymis and ductus deferens, and in the ear, specifically in receptor hair cells.
• Cilia are motile cytoplasmic structures that propel particles along the cell surface.
• Junctions between cells can be classified as adhering junctions, impermeable junctions, and communicating junctions.
• Stratified Epithelia have two or more layers of cells and can be further classified into Simple Stratified Epithelia and Pseudostratified Epithelia.
• Hemidesmosomes are the contact zone between epithelial cells and basal lamina, taking the form of half of a desmosome, binding the epithelial cell to the subjacent basal lamina.
• Pseudostratified Epithelia have a single layer of cells but of different heights and gives false appearances of stratification.
• The outlines of individual cells in epithelia are not always visible, making it difficult to distinguish between different types.
• The shape, location, and spacing of the nuclei can be used to identify the type of epithelia.
• Simple Epithelia have only one layer of cells and can be squamous, cuboidal, or columnari.
• Simple Squamous Epithelia have a single layer of thin flat cells, are more width than height, and their nuclei are flat, oval, or egg-shaped.
• Simple cuboidal epithelium is a single layer of cells with height and width almost equal, rounded and centrally located nuclei, and is associated with secretion.
• Stratified squamous epithelium can also be non-keratinized, which is found in the lining of the mouth cavity, nasal openings, vagina, urethral opening, esophagus, anus, and cornea.
• Stratified squamous epithelium consists of many layers of thin “flat” cells, is protective and acts as a barrier, with basal cells at the deepest cells contact with the basement membrane, and is mitotically active and replaces the cells of the epithelium which are lost by "wear and tear".
• Stratified epithelium consists of more than one layer of cells and is classified according to the cell shape of its superficial layer: squamous, cuboidal, columnar, or transitional.
• Simple squamous epithelium is a single layer of cells with height and width almost equal, rounded and centrally located nuclei, and is associated with secretion.
• Stratified squamous epithelium can be keratinized, which makes the skin waterproof and flattened, cornified cells cover the surface as the stratum corneum, these are dead cells impregnated with a dense amount of fibrous keratin.
• Pseudostratified epithelium is a type of stratified epithelium where the superficial layer is pseudostratified, meaning it appears to be stratified but is not.
• Simple columnar epithelium is a single layer of cells with height more than width, elongated and basally located nuclei, and is associated with absorption or secretion.
• Central nuclei in Simple Squamous Epithelia appear as protrusions.
• Simple Squamous Epithelia allow for easy movement across a single thin cell (exchange).
• Basal lamina functions include structural attachment, compartmentalization, providing a barrier that regulates exchanges of macromolecules, polarity induction, tissue scaffolding, and serving as a guide or scaffold during regeneration.
• Adhesion is due in part to the binding action of a family of transmembrane glycoproteins called cadherins.
• Intercellular adhesion is especially marked in epithelial tissues that are subjected to traction and pressure.
• Zonula occludens function to form a seal that prevents the flow of materials between epithelial cells in either direction, forming an impermeable barrier.
• Simple Squamous Epithelia form tissues that wrap to form capillaries, alveoli, etc.
• Zonula occludens are the most apical intercellular junctions and form a band completely encircling the cell.
• Intercellular junctions serve as sites of adhesion and seals to prevent the flow of materials, provide a mechanism of communication between adjacent cells, and provide tissue scaffolding.
• Adhesions maintain the structural integrity of the epithelium, provide cell to cell attachment, and cell to basement membrane attachment.
• Intercellular junctions are specialized structures that link individual cells into a functional unit.
• Gap junctions can occur almost anywhere along the lateral membranes and permit exchange between cells of molecules with molecular mass < 1500 Dalton, facilitating cell-to-cell communication.
• Lateral membranes exhibit several specializations that form intercellular junctions.
• Desmosome/Macula adherens are complex disk-shaped structures that are matched with an identical structure at the surface of adjacent cell, providing a firm adhesion among cells.