Pathogenesis of preaxial polydactyly of the hand in human embryos.

Hand plates with preaxial polydactyly from 13 human embryos of the Carnegie stages 17-23 were examined macro- and microscopically. Morphological features in early pathogenesis of preaxial polydactyly are (1) an abnormal extension and a delayed involution of the apical ectodermal ridge on the preaxial border of the hand plate in stages 17 and 18, (2) a precocious development of an interdigital notch between the duplicated thumbs in stages 17 and 18, and (3) bifurcation of the distal part of the first digital ray in stage 19. A disorder of the interaction between limb ectoderm and mesoderm is considered to be the pathogenetic event.