The role of cartilage and fibronectin during respecification of pattern induced in the regenerating amphibian limb by retinoic acid

Mechanisms of urodele limb regeneration

This review explores the historical and current state of our knowledge about urodele limb regeneration. Topics discussed are (1) blastema formation by the proteolytic histolysis of limb tissues to release resident stem cells and mononucleate cells that undergo dedifferentiation, cell cycle entry and accumulation under the apical epidermal cap. (2) The origin, phenotypic memory, and positional memory of blastema cells. (3) The role played by macrophages in the early events of regeneration. (4) The role of neural and AEC factors and interaction between blastema cells in mitosis and distalization. (5) Models of pattern formation based on the results of axial reversal experiments, experiments on the regeneration of half and double half limbs, and experiments using retinoic acid to alter positional identity of blastema cells. (6) Possible mechanisms of distalization during normal and intercalary regeneration. (7) Is pattern formation is a self-organizing property of the blastema or dictated by chemical signals from adjacent tissues? (8) What is the future for regenerating a human limb?

Limb regeneration in higher vertebrates: developing a roadmap

We review what is known about amphibian limb regeneration from the prospective of developing strategies for the induction of regeneration in adult mammals. Prominent in urodele amphibian limb regeneration is the formation of a blastema of undifferentiated cells that goes on to reform the limb. The blastema shares many properties with the developing limb bud; thus, the outgrowth phase of regeneration can be thought of as cells going through development again, i.e., redevelopment. Getting to a redevelopment phase in mammals would be a major breakthrough given our extensive understanding of limb development. The formation of the blastema itself represents a transition phase in which limb cells respond to injury by dedifferentiating to become embryonic limb progenitor cells that can undergo redevelopment. During this phase, rapid wound closure is followed by the dedifferentiation of limb cells to form the blastema. Thus, the regeneration process can be divided into a wound-healing/dedifferentiation phase and a redevelopment phase, and we propose that the interface between the wound-healing response and gaining access to developmentally regulated programs (dedifferentiation) lies at the heart of the regeneration problem in mammals. In urodele amphibians, dedifferentiation can occur in all of the tissues of the limb; however, numerous studies lead us to focus on the epidermis, the dermis, and muscle as key regulators of regeneration. Among higher vertebrates, the digit tip in mammals, including humans, is regeneration-competent and offers a unique mammalian model for regeneration. Recent genetic studies in mice identify the Msx1 gene as playing a critical role in the injury response leading to digit tip regeneration. The results from regeneration studies ranging from amphibians to mammals can be integrated to develop a roadmap for mammalian regeneration that has as its focus understanding the phenomenon of dedifferentiation.

Examination of fibronectin distribution and its sources in the regenerating newt limb by immunocytochemistry and in situ hybridization

Using monoclonal antibodies (mAbs) reactive to newt limb regenerates, we hope to gain insight into the identity and function of regeneration significant molecules. mAb MT4 (matrix 4) identifies an extracellular matrix (ECM) protein that is strongly up-regulated first in the distal stump and then in the blastema during regeneration. Within the first 24 hr after amputation the MT4 antigen is localized to an acellular space beneath the wound epithelium, and first appears in the basal cells of the wound epithelium between days 5 and 7. At mid-bud blastema stages, the MT4 antigen is homogeneously distributed as thin fibers in the blastema ECM, and is later largely restricted to the distal tip of the blastema and the areas of cartilage condensation. After extraction and immunoblotting, the MT4 antigen was observed as three reduced species of M(r) 225, 250, and 260. Taken together, the immunoblot and immunocytochemistry results suggested that mAb MT4 recognized newt fibronectin (FN). Sequence from a cDNA (NvFN.10) obtained by screening a newt blastema cDNA expression library with mAb MT4 conclusively identified the MT4 antigen as FN. To further investigate the expression of FN in regeneration, cDNA NvFN.10 was used to construct a riboprobe and in situ hybridization was done. In the unamputated limb only a few scattered cells expressed the FN gene. Within the first 3 days after amputation strong hybridization signal was observed in the basal cells of the wound epithelium. Most of the stump cells that dedifferentiated and accumulated beneath the wound epithelium at 7 days expressed the FN gene, while the basal cells of the wound epithelium maintained their expression. At mid- and late-bud blastema stages the vast majority of the blastema cells were strongly expressing the FN gene, but the wound epithelial cells now showed only weak FN transcription. Thus initially FN comes from the plasma. Then FN is synthesized by both the wound epithelium and mesenchyme. Finally, at blastema stages FN is produced primarily by the mesenchyme. The expression pattern of FN throughout regeneration suggests that this glycoprotein has roles in wound epithelial and mesenchymal cell migration and mesenchymal cell proliferation and differentiation.

Production and analyses of transgenic mice with ectopic expression of cellular retinoic acid-binding protein

Transgenic mice with ectopic expression of bovine cellular retinoic acid-binding protein (CRABP) under the control of human metallothionein IIA promoter have been generated. From a total of 10 independent transgenic live borns, six lines have expressed the bovine CRABP messages in a variety of tissues. Among the six lines, three appear to be normal and healthy in the founders as well as their offspring, one has generated transgenic offspring with retarded growth and two have produced only female transgenic mice that are all sterile.

Molecular cloning and transcriptional mapping of the mouse cellular retinoic acid-binding protein gene

The gene encoding the mouse cellular retinoic acid-binding protein (CRABP) has been isolated from a mouse genomic library and its structure has been determined. This gene spans approximately 10.5 kb and consists of four exons encoding 24, 59, 38, and 16 amino acid residues, respectively. This gene structure is very similar to the structures of other related genes belonging to the same protein family such as the human cellular retinol-binding protein, the rat cellular retinol-binding protein II, the rat fatty acid-binding protein, and the mouse adipocyte P2 protein. The site for transcription initiation has been mapped to the 93rd nucleotide upstream from the translation initiation codon ATG using both primer extension and RNase protection assays. From the DNA sequence, the promoter of the CRABP gene resembles those found in the "housekeeping" genes in that it is very G/C rich, lacks a TATA box, and contains multiple copies of the sequence GGGCGG. The deduced amino acid sequence of the translated region is identical to the amino acid sequence of the known bovine CRABP, and the DNA sequence of the transcribed region from the mouse gene shows approximately 78% homology to that of the bovine cDNA.