Nigel Holder

Pattern Regulation in the Regenerating Limbs of Urodele Amphibians

Pattern formation is the process by which cells in regenerating and developing systesm are spatially organized to form the complex structures of the adult organism. This aspect of morphogenesis has intrigued developmental biologists for over a century. The urodele limb has been used extensively in the study of morphogenesis during regeneration and several models have been proposed to account for a variety of experimental results. The purpose of this paper is to review the experiments that have been published since the turn of the century that deal with patter formation in the regenerating limbs of urodele amphibians and to discuss these results in the context of recent theoretical models.

Regeneration of symmetrical forelimbs in the axolotl, Ambystoma mexicanum☆

Abstract Surgically constructed symmetrical double-anterior and double-posterior upper forelimbs of the axolotl were amputated immediately after surgery. Double-anterior limbs either failed to regenerate or formed single digits or spikes. Double-posterior limbs formed symmetrical double-posterior regenerates in 60% of the cases, thus extending the previous finding that the amount of distal transformation in surgically constructed double-half limbs is inversely proportional to the time between grafting and amputation ( Tank and Holder, 1978 ). When these symmetrical regenerates were amputated through the forearm region, all but one formed a symmetrical secondary regenerate. The majority of the secondary regenerates had a larger number of digits than did their corresponding primary regenerates. Reamputation of the secondary regenerates resulted in symmetrical tertiary regenerates, and the majority of these also had a larger number of digits than did their corresponding primary regenerates. The results are compared to those of Slack and Savage (1978a, b) on embryonically derived double-posterior limbs and they are discussed in terms of a formal model for distal transformation (Bryant and Baca, 1978).

Positional information and pattern formation in plant morphogenesis and a mechanism for the involvement of plant hormones.

Abstract I discuss the possibility of examining pattern formation and morphogenesis in plants in terms of the concept of positional information. Experiments performed on shoot, floral and root apices are interpreted in terms of the theory presented. A model for floral morphogenesis and the interaction of phyllotaxis and shoot morphogenesis is also presented. Finally, some genetic abnormalities of floral morphogenesis are discussed in terms of the main theme of the study.

The effect of healing time on the proximodistal organization of double-half forelimb regenerates in the axolotl, Ambystoma mexicanum☆

Abstract The effect of healing on the proximodistal organization of regenerates from double-half forelimbs was studied. Double-anterior and double-posterior upper forelimbs were prepared surgically and amputated at 5, 10, 15, 20, 30, and 60 days after grafting. All experimental groups regenerated hypomorphic skeletal patterns. Double-half forelimbs amputated at Days 5 and 10 regenerated more distally complete skeletal patterns than did limbs amputated at Days 30 and 60. The mean numbers of skeletal elements regenerated were seen to decrease as a function of time after grafting, with the maximal suppression of skeletal patterns observed to occur when limbs were amputated 30 days following grafting. There was no appreciable difference between limbs amputated at Days 30 and 60. These results suggest that healing time has a profound effect on the proximodistal organization of limbs regenerated from double-half forelimb stumps.

Morphogenetic interactions occurring between blastemas and stumps after exchanging blastemas between normal and double-half forelimbs in the axolotl, Ambystoma mexicanum

Abstract Regeneration blastemas were exchanged between surgically constructed forelimbs comprised of symmetrical tissues (double-anterior and double-posterior) and normal, unoperated forelimbs. Normal blastemas grafted at the stage of medium bud (MB) onto double-half forelimb stumps regenerated normal skeletal patterns in nearly all cases. Double-half blastemas transplanted at the stage of MB onto normal forelimb stumps did not regenerate complete limb patterns. These results indicate that a double-half blastema cannot be “rescued” by transplantation to a normal stump and that a double-half limb stump does not interfere with the ability of a normal blastema to distally transform. The regeneration blastema possesses sufficient positional information at the stage of MB to permit it to develop autonomously. Supernumerary forelimbs resulted from several types of graft-stump combinations. The location and handedness of these supernumerary limbs are predicted by the rules of a recently presented model for pattern regulation in epimorphic fields [French, V., Bryant, P. J., and Bryant, S. V. (1976). Science 193, 969–981].