principles of tissue organisation- epithelia and mesenchyme

features of the epithelia

• they are bound to their adjacent epithlial cells by juctions that provide epithelial cells by junctions that provide the epithelial cells with mechanical strength and rendering it impermeable

• this sheet rests on a basal lamina (basement membrane)

• cells are polarised such that the apical side faces the lumen

• rest of cell is described as basolateral

• basal side of the cell lies on the basal lamina and the lateral edges touch adjacent cells

• initially they are not differenitated to a particular function but during development specific epithelial cells differentiate

features of mesenchyme

• have a changeable shape and are highly migratory

• they have no polarity and travel all over the matrix with all sides able to contact the matrix

• they can be loosely attached to each other but often have no attachments

• they dont sit on basal lamina

examples of EMT and MET

• in gastrulation when epiblast cells change from being epithelial and become mesenchymal so they can migrate away

• once they encounnter the hypoblast cell layer they turn from mesenchymal to epithelial to form the endoderm

what does the neural tube develop from

• the ectoderm at the midline moving ventrally towards the endoderm so that for a while it is v shape

• it then forms a tube by the top edges separaing away from the ectoderm

how are epithelial cells connected

• through tight junctions, adherens junctions, desmosomes and gap junctions

• are in stereotypes positions within the cell and are recognisable by the distance between adjacent cell membranes and their interactions with the cytoskeletons

adherens junction

• what do they mediate

• what do they consist of

• initiate cell to cell contacts

• mediate the maturation and maintenance of the contact

• consist of transmembrane protein E-cadherin, and intracellular components p120 catenin, b catenin and a catenin

tight junctions

• what do they regulate

• what do they consist of

• regulate the paracellular pathway for the movement of ions and solutes in between cells

• consist of transmembrane proteins occludin and claudin and cytoplasmic scaffolding proteins ZO1,2,3

desmosomes

• resist mechanical stress because they adopt a strongly adhesive state where they are hyper adhesive

• failure can result in diseases of the skin and heart

• can change between low and high affinity- switching is signalled by protein kinase c

• can also act as signalling centres

  • regulate the availability of signalling molecules

  • cell prolifration, differentiaion and morphogenesis

gap junctions

• what do they allow

• what are they encoded

allow the exchange of ions, second messengers and small metabolites between adjacent cells

in chordate animals gap junction channels are endocded by a famil of genes called connexins