Formation of extra-digits induced by surgical removal of the apical ectodermal ridge of the chick embryo leg bud in the stages previous to the onset of interdigital cell death.

In the present work we have studied the mechanism of formation and the possible morphogenetic significance of the process of ectopic chondrogenesis induced by surgical removal of AER of the interdigital spaces of the chick leg bud at stage 28-30 (Hurle and Gañan 1986). Our results show that ridge removal causes condensation and rounding of the underlaying mesenchymal cells followed by chondrogensis. The long-term study of the fate of these ectopic cartilages shows that in a high percentage of the cases the cartilages undergo morphogenesis taking by day 10 of incubation the appearance of the two distal phalanges of an extra-digit. These extra-digits lack tendons and are joined by thin interdigital membranes to the neighboring digits.