Cell Junctions and Epithelial Tissue

• Gap junctions mediate cell to cell communication by allowing the passage of ions and small molecules
• Gap junctions connect the cytoplasm in adjacent cells through the use of connexins, which are integral proteins
• Different combinations of connexions create channels that differ in permeability and regulation
• Tight junctions hold cells closely together and prevent molecules from leaking across the epithelium
• Tight junctions are primarily claudin and occludin transmembrane proteins
• Claudins are a selectively permeable seal
• If claudin function is impaired, cancer, IBD, and impaired barrier function can happen
• A pathogenic breach of tight junctions create an overactive inflammatory response and leads to impaired barrier function
• Epidermal growth factors contribute to tumorigenesis when tight junctions are impaired
• Anchoring junctions are complexes that adhere cells to other cells or to the extracellular matrix
• Anchoring junctions include adherens junctions and desmosomes found laterally, and hemidesomosomes and focal adhesions found in the basal surface
• Adherens junctions contain cadherins as transmembrane proteins and link to actin
• Desmosomes contain cadherin as transmembrane proteins and link to intermediate filaments
• Hemidesmosomes contain integrins as transmembrane proteins and link to intermediate filaments
• Focal adhesions contain integrins as transmembrane proteins and link to actin
• Adherens junctions are located on the apical side of the cell and are below tight junctions
• Cadherin structure depends on Ca2+ ions and are named for the tissue they're found in
• Loss of E-cadherin function is associated with the development of cancer
• Desmosomes play a big role in tissues subjected to mechanical stress
• Hemidesmosomes and focal adhesion connect the cell to the extracellular matrix, and both have integrins
• Cell junctions function in attachment for mechanical support, separation for protection and traffic regulation, communication between cells, and cell polarity for orientation of the cell
• Epithelial tissue covers all of the surfaces of the body exposed to the outside world and lining the outside of organs
• Simple squamous epithelium lines blood vessels and body cavities and regulates the passage of substances
• Simple cuboidal epithelial tissue is found in glandular tissue and kidney tubules
• Simple columnar epithelium is specialized for absorption and has apical cilia and microvilli, and line the stomach and intestines
• Stratified squamous epithelium has protective functions against microorganisms and against water loss
• The outer layer of skin is made of stratified squamous epithelium
• Stratified cuboidal epithelium is found in the excretory ducts of salivary and sweat glands
• Stratified columnar epithelium is found in the conjunctiva, where it is protective and mucus secreting
• Pseudostratified columnar epithelium lines the upper respiratory tract and has a lot of cilia
• Transitional epithelium is made up of several layers of cells that become flattened when stretched
• the characteristics of epithelial tissue are: specialized contacts, polarity, avascular, innervated, and regenerative
• The functions of epithelial tissue are: protection, secretion, absorption, excretion, filtration, diffusion, and sensory reception