Lecture Notes for April 1


Monday, April 1, 2013 Lecture topics (which are different than now listed on the syllabus)

1) Apoptosis         2) Chemical pattern generation          3) Limb bud regeneration (salamanders)


ONE) Programmed Cell Death  Apoptosis


TWO) 3 1/2 chemical reactions that can generate spatial patterns:
"Reaction-Diffusion systems"

(none of which actually is known for sure to operate in embryos)

I) Turing's mechanism (as was described last week)

A causes increase in A and B
B causes DECREASE in A and B
B diffuses faster than A

We have a Pascal computer simulation of the Turing mechanism
(which shows the chemical concentrations changing at each location

II) "Liesegang rings ( = Liesegang rings)"
Silver nitrate diffusing into a gel containing potassium nitrate.
(or many other combinations of ions that produce an insoluble salt)
(the same patterns are also produced by oxidation-reduction reactions)
(almost ANY two chemicals that react to produce an insoluble product will work)
(Gelatin, agar silica gel, or ANY gel will produce the effect?

Rings are progressively farther apart (but maybe we could change that).

The mechanism of ring formation has been debated since the 1890s
(yes, eighteen ninety)
Easy to reproduce in the lab.

We also have a Pascal computer simulation of Liesegang Ring formation
(which also shows the chemical concentrations changing at each location)

II ½) Mechanochemical systems (George Oster, Jim Murray et al.)

Analogous to Turing, except that A and B are forces or cell population densities, or any other physical variables that produce long range effects, instead of diffusion.

Many different variations can be invented. You just need two variables, one of which increases both; the other of which decreases both; with the later producing longer range inhibition and the former producing shorter range stimulation. Faster diffusion is just one of many alternative longer range effects.

A and B can be population densities of two cell types.
Or one or both can be any other physical variables, like tension or osmotic pressure.
Even more complex patterns result if the A or B variables are vectors or tensors.

Patricia Warner and David Stopak got such a set of reactions to produce biological-looking patterns.
Cell density acted as "A", collagen tension acted as "B".

We only scratched the surface of possibilities!

III) Clock and Wave-front hypotheses Convert time oscillations into spatial wave-lengths.

Invented to explain feather locations in bird embryos.
New molecular genetic evidence indicates this mechanism causes somite formation instead.


Salamander Limb Regeneration:

Do cartilage cells only regenerate cartilage cells? (but some cartilage started as dermal fibroblasts)
Do muscle cells only regenerate muscle cells?
Do epidermal cells only regenerate only epidermal cells?

The answer to all these questions is YES.
This was proven by radioactive labeling of DNA and also by grafting cells containing Green Fluorescent Protein (GFP).
Which means regeneration is by "Morphallaxis", a word invented and defined by T. H. Morgan.

(As opposed to his word "Epimorphosis", which Morgan invented to mean regeneration by means of cells switching from one differentiated cell type to another)

An excellent recent review paper by D, L. Stocum and Jo Ann Cameron was published in The journal Developmental Dynamics, vol. 240 pages 943-968 (2011)

Freely available on line at the following URL: [PubMed PMID 21290477]


back to syllabus