Test of a model for the effects of retinoic acid on urodele limb regeneration

Amphibian regeneration and stem cells

Larval and adult urodeles and anuran tadpoles readily regenerate their limbs via a process of histolysis and dedifferentiation of mature cells local to the amputation surface that accumulate under the wound epithelium as a blastema of stem cells. These stem cells require growth and trophic factors from the apical epidermal cap (AEC) and the nerves that re-innervate the blastema for their survival and proliferation. Members of the fibroblast growth factor (FGF) family synthesized by both AEC and nerves, and glial growth factor, substance P, and transferrin of nerves are suspected survival and proliferation factors. Stem cells derived from fibroblasts and muscle cells can transdifferentiate into other cell types during regeneration. The regeneration blastema is a self-organizing system based on positional information inherited from parent limb cells. Retinoids, which act through nuclear receptors, have been used in conjunction with assays for cell adhesivity to show that positional identity of blastema cells is encoded in the cell surface. These molecules are involved in the cell-cell signaling network that re-establishes the original structural pattern of the limb. Other systems of interest that regenerate by histolysis and dedifferentiation of pigmented epithelial cells are the neural retina and lens. Members of the FGF family are also important to the regeneration of these structures. The mechanism of amphibian regeneration by dedifferentiation is of importance to the development of a regenerative medicine, since understanding this mechanism may offer insights into how we might chemically induce the regeneration of mammalian tissues.

Regeneration of the urodele limb: a review

Urodele amphibians have been widely used for studies of limb regeneration. In this article, we review studies on blastema cell proliferation and propose a model of blastemal self-organization and patterning. The model is based on local cell interactions that intercalate positional identities within circumferential and proximodistal boundaries that outline the regenerate. The positional identities created by the intercalation process appear to be reflected in the molecular composition of the cell surface. Transcription factors and signaling molecules involved in patterning are discussed within the context of the boundary/intercalation model.

Effects of concanavalin A on limb regeneration in the axolotlAmbystoma mexicanum

The purpose of this investigation was to determine the effects, if any, of concanavalin A (Con A) on the process of pattern formation during limb regeneration in the axolotl Ambystoma mexicanum. Con A was administered to regenerating axolotl forelimbs in a Hydron disk that released the Con A slowly over a period of time. The results indicate that Con A is teratogenic to the regenerating axolotl limb, causing reductions in the number, size, and quality of skeletal elements present, with increasing reductions and malformations as the dose is increased. Low doses resulted in a reduction in the number of carpals or phalanges. Higher doses resulted in more drastic malformations, including the absence of digits. The implications of this teratogenic effect of Con A on limb regeneration are discussed in the context of the possible mechanisms of pattern formation.

Stage- and region-dependent responses of chick wing-bud mesenchymal cells to retinoic acid in serum-free microcultures

Retinoic acid (RA) has been shown to affect skeletal patterning in vivo in both developing and regenerating limbs. Regional differences in RA concentrations alone cannot account for the region-specific cell behaviors involved in limb-skeletal morphogenesis. The present study explores a role for regional differences in signal interpretation in RA's effects along the anteroposterior and proximodistal axes of stage 21-22 and 23-24 chick wing-buds. Mesenchymal cells isolated from specific limb regions were grown in chemically defined medium and exposed to 5 or 50 ng/ml of RA for 4 days in high-density microtiter cultures. Previous studies showed that RA's effects on chondrogenesis and growth in such cultures differed depending on the position along the limb's proximodistal axis from which the cells were isolated. The present study is the first to show that such differences in RA-responsiveness also exist along the limb's anteroposterior axis, especially in the distal subridge mesenchyme. The region-dependent relationships between RA's effects on growth and chondrogenesis suggest that RA affects these two behaviors through different mechanisms. The regional differences in the responsiveness of these cells to exogenous RA are discussed with respect to their correspondence to the in vivo patterns of expression of RA-binding proteins, RA-receptors, and other patterning-related genes.