Genes, Morphogenesis, Evolution: Life and ALife Aspects

Regulative Morphogenesis

Polar Coordinate Model

Classical embryological analysis has lead to the concept of a morphogenetic field. It can be defined operationally as the domain within which regulation can occur in response to surgical manipulation (French, Bryant and Bryant, 1976). In several organisms it has been shown that up to certain stage the whole embryo can regulate and it therefore constitutes a single primary field. But later surgical interventions have more localized effects, restricted to developmentally autonomous parts of the embryo. These units French, Bryant and Bryant (1976) called secondary fields.

French, Bryant and Bryant (1976) carried out a set of key experiments and correlated their findings on very diverse epimorphic regenerating systems (cockroach legs, Drosophila imaginal discs and newt limbs) with their elegant polar coordinate model. The model accounts formally and in a simple and unified way for the kind of developmental regulation seen in the secondary fields of both invertebrates and vertebrates.

This is as contrasted with "morphollactic" regeneration which take place in the absence of cell division but rather by modeling of existing tissue, and in the present terms, assigning a new value of morphogen. A common example of morphollactic regeneration occurs in the hydra. The situation of the epimorphic regeneration of "imaginal discs" of insects (from which many of adult appendages such as legs, antennae, wings, sex and mouth parts arise) , which take place after surgical intervention of various kinds, is summarized below.

Polar coordinate model. Each cell is assumed to have information with respect to its position on a radius (A through E) and its position around the circle (0 through 12). According French, Bryant and Bryant (1976).

The following two rules for the behavior of cells in epimorphic fields are sufficient for regulations.

1. Shortest intercalation rule. "When normally nonadjacent positional values in either the circular or the radial sequence are confronted in a graft combination or as a result of wound healing, growth occurs at the junction until cells with all the intermediate positional values have been intercalated; then growth ceases" (French, Bryant and Bryant ,1976).
2. Complete circle rule for distal transformation. "The entire circular sequence at a particular radial level may undergo distal transformation to produce cells with all of the more central (distal) positional values. …this distal transformation occurs only when cells with a complete circular sequence of positional values are either exposed at an amputation site or generated by intercalation".

Application of the model to the results obtained by culturing imaginal disk fragments from Drosophila. According French, Bryant and Bryant (1976).

1. Results of culturing 90 or 270 degrees sectors of the wing disk.
2. Results obtaining following bisection of the wing disc.
3. Results of culturing different fragments in the leg disk.
4. Results obtained by culturing central and peripheral fragments of the bilaterally symmetrical genital disk.

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