Control of Pattern Formation
Limbs such as all other organs have a pattern. What factor (or factors), environmental affects etc. are responsible for specific positioning of its different parts? For example, what determines that there will be one femur in the thigh but two parallel bones in the shank? What manages that the thumb will form on the anterior and the little finger on the posterior side? How is it that the toes of duck are webbed but those of chick are not even though both build up from a paddle shaped autopodial segments of the embryonic limb? Numerous such questions have been and are being asked by biologists also laymen interested in the phenomenon of development.
We still do not have a reply to the basic question "how pattern formation is controlled?" that can be applicable to just one or all organs of any one or all organisms. In this subsection we will discuss some information acquired by experiments related to the problem of control of pattern creation in the vertebrate limbs. You know that the limb has three polarities or axes, A-P, D-V and P-D which are programmed into the limb mesoderm cells early in the embryo even though not simultaneously. Are these programmes rigidly fixed in the cells from the start and their morphological expression in the limb realized by the mesodermal cells themselves? or do the cells stay labile for sometime in these respects and some other influences, say from the ectoderm, as well operate in realization of the programmed polarities? Experiments to find answers to these questions have been performed on limb buds of chick embryos in the recent past.