Embryology Biology 441 Spring 2011 Albert Harris
Seal off spaces from each other;
Have distinct apical surfaces and baso-lateral surfaces
(incidentally, "endothelium" is a special kind of epithelium that lines blood vessels)
During embryonic development, epithelia do the following:
* Fold (" Invaginate ") For example, the archenteron of sea urchins.
Sometimes a rolling fold called " Involution "
* Convert into mesenchyme (" Ingression ")
An example is formation of the primary mesenchyme
Mammal, bird and reptile gastrulation is by ingression!
And then many of those convert back to being epithelial!
* Fuse with other epithelia.
** Epithelia sometimes form from mesenchymal cells
(for example: mammal and bird digestive tracts, the epithelial lining is made of cells that had ingressed)
The neural tube of teleost fish forms as a solid rod of (non-epithelial cells) which then "hollows out" to become a tube made of epithelial cells.
Some developmental biologists call this "secondary neurulation"
The extreme posterior sixth of bird neural tubes also form by hollowing out of what had been a solid mass of (mesenchymal?) cells.
AND, if you dissociate neural tube cells from a mammal or a bird or a frog or salamander embryo, the cells will spontaneously rearrange into a hollow ball, or tubes.
categories of cell reactions to adhesion, chemicals, shapes etc.
Turn randomly when attractant concentration decreases
(Turn when concentration at front is lower than at rear)
Slow down, or stop when concentration is larger than some threshold.
(Slow if concentration high; Stop entirely if front and back both maximum.)
Maybe you can invent some other sets of rules that will cause cells to move toward increasing concentrations of some molecule.
Also, note that chemotaxis never exerts an actual pulling force on the cells.
(Nor does the smell of good food exert a pull on people's noses.)
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