Retinoic acid proximalizes level-specific properties responsible for intercalary regeneration in axolotl limbs

The objective of this study was to determine whether retinoic acid (RA) coordinately proximalizes positional memory and the cellular recognition system that detects pattern discontinuity in regenerating amphibian limbs. The strategy was to test the capacity of RA-treated blastemas to evoke intercalary regeneration when grafted to an amputation level proximal to their level of origin. Control wrist and ankle, or elbow and knee blastemas treated with the retinoid solvent, dimethylsulphoxide, evoked intercalary regeneration as effectively as untreated blastemas, when grafted to the midstylopodial amputation surface of host limbs. RA-treated wrist and ankle or elbow and knee blastemas were proximalized and formed complete limbs that were at an angle to, or continuous with, the midstylopodium of the host limb. No intercalary regeneration, from either graft or host, was observed in these cases. The results indicate that the cellular mechanism that recognizes disparities between non-neighbouring cells and initiates intercalary regeneration is coordinately proximalized with positional memory. Thus the recognition mechanism and positional memory are directly related. Intercalary regeneration and corrective displacement (affinophoresis), both of which restore a pattern of normal cell neighbours by different means in regenerating axolotl limbs, appear to use the same mechanism to recognize pattern discontinuity.

Disruption of segmentation in a short germ insect embryo

A heat shock (of 15min at 48° C) given to early embryos of the locust, Schistocerca gregaria, results in localized abnormalities in the segment pattern subsequently formed. Most defects involve two consecutive segments of the thorax or abdomen, and these are analysed in detail. The abdominal defects fall into three main classes each of which involves the absence of a particular region of the segment pair and, in one class, duplication of the region which remains. The thoracic defects similarly involve absence of parts of the segments and the formation of a single limb base from which one, two, or three limbs develop. Heat shock may result in the absence of parts of segments in two distinct ways. It may interfere with the process of segmentation or it may delete parts of already formed segment primordia. These possibilities are discussed although, at present, neither can be excluded. The duplication observed in some abdominal disruptions and the formation of triple limbs indicates that the absence of parts of embryonic segments is followed by pattern regulation similar to that occurring in regeneration studies on larval segments and appendages of other insects. Two out of the three classes of abnormality can be explained in terms of intercalary regeneration restoring pattern continuity, but it is possible that discontinuities persist in the remaining class.

Cell division during intercalary regeneration in the cockroach leg

In a series of grafting operations on cockroach legs, epidermal cells from different positions or from the same position on the circumference of the femur were placed together. Where cells from different positions were confronted, new cuticular structures corresponding to the positions which would normally have lain between them were formed during the following moults. At the control junctions, where cells from the same positions were placed together, no new structures were formed. Grafted legs were examined histologically at various times after the operation. The events following grafting fell into four phases: wound healing — when epidermal cells migrated over the wound to re-establish epidermal continuity and cells adjacent to the wound divided to compensate for cell emigration; intercalation — when cell divisions took place at the host-graft borders where there was a positional discrepancy; proliferation — when the general growth of the epidermis occurred by widespread cell division; cuticle secretion — when apolysis occurred, cell division ceased, and cuticle secretion began. The results show that intercalary regeneration is associated with local cell division at the graft-host borders, and that these divisions are not confined to the normal proliferative phase of the moult cycle, but begin much earlier in the cycle, as soon as wound healing is complete. These results support epimorphic models (such as the Polar Coordinate Model) of pattern regulation, where change of positional value is tied to cell division, but they do not discount the possibility of a limited initial morphallactic phase.