EFFECTS OF FGFS ON THE MORPHOGENIC POTENCY AND AER-MAINTENANCE ACTIVITY OF CULTURED PROGRESS ZONE CELLS OF CHICK LIMB BUD

The apical mesodermal region of chick limb buds (progress zone, PZ) wh ich is essential for limb pattern formation contains uncommitted cells that change their positional values instructed by the apical ectoderm al ridge (AER). Reciprically, the PZ cells maintain the AER activity. FGF-P and FGF-4 can substitute for the AER to maintain normal outgrowt h and gene expression in the limb bud. We examined the effects of FGF on the maintenance of PZ cells characteristics in culture by making re combinant limbs with anterior PZ cells that were pre-cultured in the p resence of FGF-P or FGF-4 and analyzed their morphogenic potency and r esponsiveness to positional cues arising from the zone of polarizing a ctivity (ZPA). The limb buds expressed distal Hox genes and could form a segmented digit. Recombinant limb buds consisting of anterior PZ ce lls cultured without FGF failed to express Hox genes and formed instea d a small cartilage nodule. These results indicate that addition of FG F-2 or FGF-4 to cultured PZ cells maintains their competence for Hox g ene expression and digit formation, but not their responsiveness to po sitional cues from the ZPA. We also found that when anterior PZ cells which had been pre-cultured with FGF-2 or FGF-LF were implanted undern eath the AER, they could maintain Fgf-8 expression in the AER, whereas this expression was not detected in the AER on the grafted anterior P Z cells that had been pre-cultured without FGF, indicating that FGF ma intains AER-maintenance activity of PZ cells in culture.