Effects of retinoic acid on regenerating normal and double half limbs of axolotls : Histological studies.

Retinoids induce proximodistal (PD) pattern duplication in zeugopodial (lower arm or leg) regenerates of normal limbs and PD pattern duplication plus anteroposterior (AP) pattern completion in double anterior half zeugopodial regenerates. In contrast, retinoids inhibit the regeneration of double posterior half zeugopodia (Kim and Stocum, 1986). Here we describe the developmental histology of regenerating normal, double anterior half and double posterior half zeugopodia in axolotls after intraperitoneal injection of retinoic acid (RA) at the stage of initial blastema cell accumulation. In all three classes of RA-treated limbs, the accumulation of blastema cells disappeared within 3 days after injection, and dedifferentiation continued to a much more proximal extent than in controls. Subsequently, however, the developmental histology of the three limb classes was different. RA-treated double posterior limbs exhibited the histological features typical of non-regenerating limbs: the premature appearance of a thick basement membrane under the wound epidermis, formation of a thick connective tissue mat between the basement membrane and the cut ends of the stump cartilages, and failure of blastema formation. In contrast, RA-treated normal zeugopodia reformed single blastemas which grew out in a posterior or posterodorsal direction. RA-treated double anterior zeugopodia formed twin blastemas that were spatially separated to varying degrees and which grew distally. The blastemas of both these RA-treated limb types consisted of a proximal, low-density cell population that formed the girdle of the regenerate and a distal, high-density cell population that formed the free limb. In the free limb portion of the blastema, the density of the mesenchymal cell population was higher than in controls. Blastemas of RA-treated normal and double anterior zeugopodia appeared similar in size and proportions to controls at the medium bud stage, but subsequently took on the characteristics of stylopodial blastemas. These observations suggest that the extra pattern induced by RA in regenerating urodele limbs may be correlated with an increase in the number of defifferentiated cells per unit of blastema volume.