Pattern formation and regulation of gene expressions in chick recombinant limbs

Recombinant limbs were performed by ensembling dissociated-reaggregated win g bud mesoderm inside an ectodermal hull. The zone of polarizing activity w as excluded from the mesoderm used to perform the recombinant limbs (non-po larized recombinants), and grafted when desired (polarized recombinants). R eorganization of patterning progressively occurred in the newly formed prog ress zone under the influence of the apical ectodermal ridge (AER), explain ing the proximo-distal gradient of morphogenesis observed in developed reco mbinant limbs. The AER, without the influence of the polarizing region (ZPA ), was sufficient to direct outgrowth and appropriate proximo-distal patter ning, as observed in the expression of the Hoxa-11 and Hoxa-13 genes. The d evelopment of the recombinant limbs coursed with symmetric AER and downregu lation of Bmp expression in the mesoderm supporting a negative effect of Bm p signaling upon the apical ridge. The recombinant ectoderm maintained prev iously established compartments of gene expressions and organized a correct dorso-ventral patterning in the recombinant progress zone. Finally, the ZP A effect was only detected on Bmp expression and pattern formation along th e antero-posterior axis. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.