Embryology   Biology 441   Spring 2011   Albert Harris

 

Epithelia (Epithelium, singular)
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.
Another example, the neural tube.

Sometimes a rolling fold called "Involution"
(This is how frogs and salamanders gastrulate.
=internalize their future mesoderm and endoderm)

* Convert into mesenchyme ("Ingression")

An example is formation of the primary mesenchyme

Mammal, bird and reptile gastrulation is by ingression!
Our future mesoderm and endoderm start as parts of an epithelium;
But some of these epithelial cells convert into mesenchyme,

And then many of those convert back to being epithelial!
(An example is formation of the digestive tract of mammals)

* Fuse with other epithelia.
(A good example is the neural folds fusing in neurulation;
another example is fusion of the stomodeum with the tip of the archeteron.

** 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"
as if naming a phenomenon explains it.

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.

Contact Guidance

Contact Inhibition

Haptotaxis

Galvanotaxis

Chemotaxis,

    This word is used for several fundamentally different phenomena.

      Turn toward direction of larger concentration of attractant phenomenon

      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.

People will call the result "Chemotaxis", regardless of how the result is achieved.

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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