TISSUE AND CELLULAR PATTERNING OF THE MUSCULATURE IN CHICK WINGS

Development of the musculature involves generation of a precise number of individual muscles arranged in appropriate locations, each with th e correct cellular patterning. To find out the rules that govern muscl e number and arrangement, the forearm musculature of chick wing buds w as analysed following grafts of the polarizing region or application o f retinoic acid. Muscle patterns appear symmetrical with 'posterior' m uscles now forming in the anterior part of the wing. When the number o f muscles that develop is reduced, pattern symmetry is maintained, wit h loss of anterior muscles in the mid-line, especially dorsally. Stric t anteroposterior ordering of muscles in duplicated patterns does not always occur. The number of muscles that develops bears some relations hip to the number of forearm elements. Each muscle has a characteristi c pattern of fast and slow fibres. In duplicated wings, each pair of s ymmetrically arranged muscles has the same fibre type pattern. Not onl y are proportions of fast and slow fibres similar, but local variation s in fibre type arrangement within the muscle are also reproduced. Thi s suggests that the cellular pattern within the new 'posterior' muscle s at the anterior of the limb has been re-specified. In manipulated li mb buds, which will develop a duplicated muscle pattern, there are no detectable changes in distribution and number of potentially myogenic cells, and fibre type patterning within early muscle masses also appea rs normal. In contrast, the splitting process that divides up muscle m asses is altered. The appropriate fibre type arrangement only emerges after splitting is complete. This suggests that tissue patterning and cellular patterning occur at different times during muscle development .