epithelia Flashcards
what is epithelia
-generally the boundary between controlled internal environment and uncontrolled external environment (ependymal cells which line parts of the nervous system and is the only exemption to epithelia cells)
-cover surfaces
-line cavities
-form glands (bags on epithelial tissue folded in on itself)
-develop from all germ layers such as endoderm (GI lining), mesoderm (lining of CV system), and ectoderm (epidermis)
-present in every organ
-very widespread
common properties of epithelia
-dymanic barrier
-able to import or export substances (sometimes against steep concentration gradients)
-properties that facilitate this: connected via tight junctions decrease free diffusion, and distinct apical and basolateral domains with different membrane properties and functions (polarised)
-entirely cellular: avascular, lack extracellular fibres and menial extracellular space
what are basement membranes
-separates cells from underlying connective tissue
-ECM proteins secreted by epithelial cells (collagen, laminin, proteoglycans)
-provides structural support and organisation (basal lamina and reticular lamina where reticular fibres anchor basal lamina to the underlying connective tissue
what are junction complexes
-used for cell adhesion and communication
-tight junctions obstruct paracellular movement between cells which holds them together, and stops paracellular movement between cells
-protein strands called Claudins determine tightness
-high barrier function e.g. renal thick ascending limb w
-the amount of Claudins effects the tightness of the cell
-leaky e.g. renal proximal tubule
Claudins
-24 different Claudins
-permeability can vary because for example, you dont want a leaky bladder so less permeable
-combo of different Claudins determine permeability
what are adhering junctions
-they form a belt around the cell under the tight junction
-they are linked actin filaments and cadherins
-disruption causes a decrease organisation in epithelial tumours (metastasis)
-disruption may be that the cells are migrating away from one another
how can epithelial-mesenchymal interactions regulate epithelial fate
-epithelial cells can disassemble and migrate away
-switched expression of adhesion molecules (cadherins), induces mesenchymal cells e.g. fibroblasts to form an epithelium
-close contact with the underlying mesenchyme which control epithelial cell-fate
-epithelial assembly is a reversible process (epithelial-mesenchymal transition EMT)
what are gap junctions
-lateral communication between cells via gap junctions
-permit small molecules diffusion between cytoplasm of neighbouring cells because they have gaps- permit small molecules to move between cytoplasms of different cells
-therefore cells electrically coupled
what are desmosomes
-they form strong adhesion points between cells
-extracellular domains (cadherin family)
-anchor proteins (plaques) link cadherin cytosolic domains to intermediate filaments (stress-bearing cytoskeleton)
-myosin filaments interactions for contraction
what occurs in cell replacement
-hostile the external environment causes great cell death e.g. stomach is v hostile due to acidic conditions meaning a quicker cell death
-continual cell replacement from stem cells is tissues homeostasis
-there is substantial variation as in the intestine set-renewal is 5 days, inter follicular epidermis is 4 weeks, and the lung epithelium is up to 6 months
what are the two main types of epithelia
-simple (single layer of cell e.g. lung)
-stratified (many layers of cells e.g. skin)
-however some don’t fit into categories such as psuedo-stratified in the supper respiratory tract and transitional in the urothelium
-psuedo-stratified is one layer of cell but the nuclei aren’t in line with each other
-transitional is many layers of cells but they change sizes as you go down the layers
what does simple squamous epithelia look like
-thin “scales”
-facilitate rapid passage of molecules
-secretion/ absorbtion of molecules via active transport
what is simple cuboid epithelia used for
-secretion/absorbtion of molecules via active transport
-many epithelial cells e.g. lining kidney tubules
-have single, non-motile cilium (central cilium) on apical membranes
-deformation senses fluid flow
what are the different types of columnar epithelia
-simple columnar epithelia:
-with/without cilia or microvilli
-absorption/secretion via active transport
-majority of GI tract
-ciliated surfaces
-line fallopian tubes- moves eggs
-sections of respiratory system- remove particulate matter
-psuedostratified columnar epithelia:
-single cell layer- multi-layer appearance (nuclei at different levels)
-all in touch with basolateral membrane
-ciliated or unciliated
ciliated cells often interspersed with goblet cells
stratified squamous epithelia
-most common type of stratified epithelium in the body
-apical cells appear squamous, basal cells seem cuboidal or columnar
-areas of high absorption
