Discontinuities of pattern and rules for regeneration in limbs of crayfish

The most caudal limb in crayfish, the uropod, has two rami, the exopodite and the endopodite. Results of earlier experiments (J.E. Mittenthal et al. (1985) W. Roux's Arch. Dev. Biol. 194, 121-130) indicated that ramus morphogenetic fields in the two rami are equivalent and tandem. Thus the proximal (inner) junction of the rami, where intersegmental membrane separates them from each other and from the coxa, is analogous to a boundary between segments of the body or of a leg. In this region a discontinuity in the positional information carried by the ramus fields might occur. To characterize the morphogenetic fields in the region near this junction we have exchanged the medial and lateral margins of the two rami, performing the four possible grafting operations of this kind. While an experimentally generated discontinuity between the lateral margin of the exopodite and the medial margin of the endopodite (outer-to-outer junction) triggers intercalation of supernumerary rami, a discontinuity of pattern between the medial margin of the exopodite and the lateral margin of the endopodite (inner-to-inner junction) is stable despite the absence of intervening intersegmental membrane. Where intercalation does occur, it can proceed in either direction along the margin of a supernumerary ramus. These results suggest that there is no discontinuity of positional value at the boundary between the rami. The results of all of our experiments on the uropod indicate that a conjunction of separate proximodistal, dorsoventral, and mediolateral component fields may give positional information for generating the uropod. Intercalation restores the continuity of pattern in the proximodistal field. In the mediolateral field a discontinuity of pattern may result from a preferred polarity of intercalation: Outer cells may be competent to generate inner cells, but not vice versa. According to this hypothesis the two rami have tandem ramus fields but mirror-symmetric polarity of competence. Alternatively, intercalation may eliminate a mediolateral discontinuity only if the mismatch of mediolateral positional values at the discontinuity exceeds a threshold. The threshold criterion may be a weighted sum of limb field and ramus field positional values.