Development 102. 687-698 (1988) 687 Printed in Great Britain © The Company of Biologists Limited 1988 Retinoic acid coordinately proximalizes regenerate pattern and blastema differential affinity in axolotl limbs KAREN CRAWFORD* and DAVID L. STOCUM University of Illinois, Department of Cell and Structural Biology, 506 Morrill Hall, 505 S. Goodwin Ave., Urbana, IL 61801, USA •Present address: Department of Biology, Swarthmore College, Swarthmore, PA 19081, USA Summary An assay that detects position-related differences in proximalized the pattern of donor forelimb regener- affinity of axolotl regeneration blastema cells in vivo ates to the level of the girdle and abolished their was used to test whether retinoic acid, which proxi- displacement behaviour on untreated host hindlimbs. malizes regenerate pattern, simultaneously proxi- Conversely, untreated forelimb donor blastemas dis- malizes blastema cell affinity. The assay involved placed distally to their corresponding levels on host autografting or homografting late bud forelimb blas- ankle regenerates, that had been proximalized to the temas derived from the wrist, elbow or midupper arm level of the girdle by retinoic acid. These results levels to the dorsal surface of the blastema-stump indicate that positional memory in regenerating limbs junction of an ipsilateral, medium-bud-stage hindlimb is directly related to blastema cell affinity, and that regenerating from the midthigh level. The grafted very similar or identical sets of level-specific affinity blastemas consistently displaced to their correspond- properties are shared by forelimb and hindlimb cells. ing levels on the proximodistal axis of the host regener- ate, indicating the existence of level-specific differ- Key words: differential affinity, positional memory, ences in blastema cell affinity. Retinoic acid retinoids, axolotl, limb, regeneration, pattern formation. Introduction restoration, via intercalary regeneration, of the pat- tern normally occupying the space between the two Mesenchymal cells of the amphibian limb regener- levels (Iten & Bryant, 1975; Pescitelli & Stocum, ation blastema inherit from their parent limb cells a 1980; Muneoka & Bryant, 1984). memory of their level of origin. This positional Despite the importance of positional memory in memory (Carlson, 1975) or value (Wolpert, 1971) reestablishing a normal pattern of cell neighbours specifies the proximal boundary of the regenerate by during regeneration, little is known of its cellular or preventing blastema cells forming structures proximal molecular basis. One approach to this problem is to to their level of origin (Stocum, 1984). Since the limb identify cellular properties that exhibit position- regenerates exactly what is amputated from any level, related differences along a limb axis and which therefore might be directly related to positional its proximodistal (PD) axis can be viewed as a series memory. Demonstration of a direct relationship re- of such potential regenerate boundaries. Similar sets quires showing that experimental modification of of level-specific memories appear to exist along the regenerate pattern is accompanied by modification of anteroposterior (AP) and dorsoventral (DV) axes of the cellular property. the limb (Carlson, 1975). The boundary function of Cell affinity is one property that might be related to positional memory in regeneration is seen in exper- positional memory, as shown by an assay of blastema iments that juxtapose cells from different axial levels cell adhesiveness in vitro (Nardi & Stocum, 1983). In (i.e. cells with different positional memories). The this assay, binary combinations of wrist, elbow or result is the dedifferentiation and/or mitosis of cells upper arm blastema mesenchyme were cultured in on either side of the interface, followed by the vitro, with one member of each pair being marked by 688 K. Crawford and D. L. Stocum [3H]thymidine. The proximal blastema of each pair Materials and methods always engulfed the distal one in hierarchial fashion. This result is consistent with Steinberg's (1963, 1978) Animals and maintenance differential adhesion hypothesis, and suggests the Larval axolotls (Ambystoma mexicanum) were provided by existence of position-related differences in cell ad- the Indiana University Axolotl Colony or were obtained hesiveness along the PD axis of the limb. from spawnings of our laboratory stock. They were reared individually in 50% Holtfreter solution in waxed paper Vitamin A and its derivatives (retinoids) are agents cups, at room temperature (21 °C), and fed freshly hatched that modify regenerate pattern. The effect of reti- brine shrimp or frozen brine shrimp every other day. At the noids is to proximalize regenerate pattern in the PD time of amputation, the animals weighed 2-5 g and were axis, resulting in serial repetition in the regenerate of 6-9 cm in length. structures proximal to the amputation plane (Niazi & Surgical operations Saxena, 1978; Maden, 1982; Thorns & Stocum, 1984). (1) General procedures In the AP axis, retinoids posteriorize the pattern, Animals were anaesthetized in Benzocaine (Sigma) dis- resulting in mirror-image duplications (in regenerat- solved in 100% Holtfreter solution (0-007% w/v). Oper- ing anuran limbs and embryonic chick limbs; Niazi & ations were performed with sharp watchmaker's forceps Saxena, 1978; Maden, 1983; Tickle etal. 1982), or the and iridectomy scissors. Amputations were done in air after completion of the missing posterior pattern following placing anaesthetized animals on Benzocaine-soaked filter amputation of an anterior half limb (in urodeles; Kim paper. Grafting was done in deep Petri dishes lined with filter paper, with the animal submerged in anaesthetic & Stocum, 1986a). All these effects have been inter- solution. After grafting, the animals were maintained in the preted as a reprogramming of the positional memory anaesthetic solution for 5-6 h at room temperature to of blastema cells to a new level. Measurements of facilitate healing, then placed in 100 % Holtfreter solution. endogenous retinoic acid in embryonic chick limb They were left undisturbed in the dark for 2 days and buds have yielded results suggesting that this retinoid special precautions in feeding and changing water were observed for an additional 5 days postgrafting to avoid might be a morphogen that specifies anteroposterior dislodging grafts. All grafts healed in 24 h and blood axial pattern in the limb bud (Thaller & Eichele, circulation in the grafts was observed by 1-2 days post- 1987). operation. Retinoids modify the phenotypic differentiation of a variety of cultured normal and tumour cells. These (2) Differential affinity assay in vivo modifications are accompanied by quantitative Fig. 1 diagrams the assay used to detect level-specific changes in cell-cell and cell-substrate adhesiveness differences in blastema cell affinity in vivo. Forelimbs were that are correlated with qualitative and/or quantitat- amputated bilaterally through the wrist, elbow or distal midupper arm levels, and hindlimbs of the same animal ive changes in cell surface glycoconjugate compo- were amputated bilaterally through the midthigh. The sition (Lotan, 1980; Shapiro & Mott, 1981; Roberts & forelimbs were allowed to regenerate to the late bud stage, Sporn, 1984, for reviews). Taken in conjunction with at which time the hindlimb regenerates were at medium bud the effects of retinoids on regenerate axial pattern, [staging according to Stocum (1979)]. The forelimb blas- these findings suggest that positional memory and temas were autografted to the blastema-stump junction of the ipsilateral hindlimb after removing a piece of blastema blastema cell differential affinity might be directly epidermis and stump skin from the dorsal surface of the related. junction. The AP axis of the grafted blastema was aligned A direct test of this hypothesis requires an assay with that of the host limb. Late bud blastemas rather than that can reveal changes in both blastema cell affinity earlier stages were chosen as donors because they were easier to manipulate and they exhibit a high frequency of and in regenerate pattern in response to retinoid. The autonomous development when grafted to the dorsal fin major disadvantage of an assay for blastema cell (Stocum, 1968). The prediction was that level-specific adhesiveness in vitro (Nardi & Stocum, 1983) is that differences in blastema cell affinity would be detected as the blastema mesenchyme does not undergo morpho- displacement of the grafts to their corresponding levels on genesis in vitro, so correlations between changes in the PD axis of the host regenerate. adhesiveness and pattern in response to retinoid cannot be made. Therefore, we developed an assay (3) Test for coordinate proximalization of which detects level-specific differences in blastema regenerate pattern and blastema cell affinity cell affinity in vivo, and in which blastema morpho- For each experiment, a fresh stock solution of a\]-trans retinoic acid (RA, Sigma) was made by dissolving 50 mg of genesis takes place. Here we describe this assay and RA in 1 ml of dimethyl sulphoxide (DMSO) under subdued use it to show that retinoic acid coordinately proxi- light (to minimize photo-isomerization). Two homografting malizes blastema cell affinity and regenerate pattern. experiments were done, in which either donor or host Cellular basis of positional memory 689 Donor forelimbs regenerating from wrist, their displacement behaviour was correlated with their final elbow