Epithelia Flashcards
What is epithelia?
- Cells that line the inner and outer surfaces of the body by forming cellular layers (epithelium) or masses.
- To line the surface of external (skin and GI tract) and internal cavities and lumen of bodies (endothelium, genitito-urinary tract and respiratory tract)
- Epithelia form the top layer, than we have connective tissue underneath, and then muscle at the bottom.
- Epithelia important in human disease: 85% of human cancers originate from cells of epithelial origin.
- Cystic Fibrosis is a disease of epithelial tissue.
What are the functions of epithelia?
- Protection from the external environment - Prevents dissection (drying out) as the old, dry cells are replaced with new epithelial cells. - Exchange- alveoli - Filtration (lining of kidney tubules) - Sensation (taste buds, olfactory epithelium) - Secretion (lining of glands)
How do epithelia form a functional unit?
Epithelial layers are attached to the extracellular matrix of the basal lamina.
Epithelial cells contain specialised structures that link the cells to one another and the basal lamina:
◦Tight junctions
◦Anchoring junctions
◦Channel forming junctions
What are tight junctions?
Tight junctions AKA occluding junctions
They are membrane proteins that seal adjacent cells together
The ensure molecules cannot leak freely between cells.
Also prevent lateral migration of membrane proteins, so they don’t move to the bottom of the cell. For example a sodium/pottasim pump moving to the wrong side of the microvilli in ileum would ruin the concentration gradient for absorbtion.
This creates distinct cell membrane domains generating cell polarity
What are anchoring juctions?
Anchor cells to the basal lamina or to other cells.
Provide mechanical stability and allows functioning as a cohesive unit.
Anchoring junctions categorised by the type of cytoskeletal filament they interact with:
- Those that interact with actin filaments can be cell-cell (adherens junction) or cell-basal lamina (focal adhesions).
- Those that interact with intermediate filaments can also be cell-cell (desmosomes) or cell-basal lamina (hemidesmosomes).
What is the basement membrane?
- The basement membrane anchors epithelial cells to connective tissue below.
- It acts as a mechanical barrier, and is important in angiogenesis.
- It is made up of the basal lamina and reticular lamina (type 11 collagen)
What is a channel forming/ gap junction?
A channel that forms laterally in between cells to allow diifusion/ communication between cells. Like bridges.
Types of epithelial tissue: simple squamous
Function: Absorption, filtration, minimal barrier to diffusion
Location: Capillaries, alveoli, abdominal and pleural cavities
Types of epithelial tissue: simple cuboidal
Function: Secretion , transportation.
Location: Glands and ducts, kidney tubules, covering of ovary
Types of epithelial tissue: Simple columnar
Function: Absorption, protection, secretion
Location: Digestive tract
Stratified squamous (Keratinised or Non-Keratinised)
Function: Protection
Location: Skin, mouth, upper throat, oesophagus
Pseudostratified columnar
Function: Absorption and protection
Location: Upper Respiratory tract, trachea
A pseudostratified epithelium is a type of epithelium that, though comprising only a single layer of cells, has its cell nuclei positioned in a manner suggestive of stratifiedepithelia.
Transitional Relaxed Epithelium
Function: Stretchable layer
Location: Bladder
Epithelial Surface Specialisations: Microvilli
Epithelial surface specialisation of GI tract.
Formed by network of cytoskeletal proteins.
Massively increases surface area, massively increases absorption
Epithelial Surface Specialisations: Keratinised surface of skin
Skin = keratinised squamous epithelium
Keratin provides inert protective layer
Phospholipid around upper layer of cells = water proofing
Epithelial Surface Specialisations: Upper Respiratory Tract
•Cilliated psuedostratified columnar
