Precision of Development in Chick Limb Morphogenesis

IT is important for an understanding of pattern formation in development to know how precisely the patterns are specified. One type of mechanism may involve the specification of positional information in the form of some graded cellular parameter. This is interpreted by cells in terms of molecular differentiation according to their genome and past history. How precisely must positional information be specified or interpreted1? To make the situation more concrete, one can consider a line of 50 cells, the size of a typical positional field, and ask how accurately the position of any one could or need be specified. If one half were to be different from the other how accurately would the boundary be located? It is very difficult to obtain such data on the precision of development at the cellular level. One of the few situations where this type of information is available is in sea urchin development in which about 50 of the 1,000 cells of the blastula become primary mesenchyme. This process is rather imprecise with a coefficient of variation of about 10%2 and corresponds with an overall impression of the lack of precision in sea urchin development1,3. Maynard-Smith4 has provided an original discussion of continuous and quantized variation in adult structures but does not directly relate them to developmental processes. While geneticists have been conscious of what has been termed developmental noise5 or special environmental variance6, their studies, too, are not easily relatable to developmental processes at the cellular level. One approach is of particular interest, however, and that is the comparison of bilaterally symmetrical structures7, for differences between these could provide an estimate of the precision of development. Most studies of this type have been carried out on Drosophila and made use of numbers of bilateral structures such as bristles and ovarioles, though it has been briefly reported8 that the coefficient of variation of the ratio of wing lengths is 0.40%.