What enables some organisms, but not others, to regenerate lost body parts? The answer to this question has profound implications for the field of regenerative medicine, which seeks to understand and harness regenerative mechanisms for repairing or replacing damaged human tissues. The problem of regeneration has challenged scientists ever since Abraham Trembley's experiments on freshwater Hydra launched the era of experimental biology more than 260 years ago; nonetheless, the mechanisms involved in regenerative processes remain poorly understood.
Our model for studying the molecular mechanisms underlying metazoan regeneration is the planarian flatworm, a subject of classic regeneration experiments. The choice of planarians as a system to study the problem of regeneration was based on their remarkable developmental plasticity, the rapidity of their regenerative response, the ease with which they can be cultured in the laboratory, and the stem cell population that gives rise to their regenerative abilities. The development of functional genomic tools for studying the planarian Schmidtea mediterranea has revitalized studies of these fascinating organisms and permits detailed analyses of the mechanisms underlying regeneration.
After a planarian has been transected, the wounded area is rapidly covered by a thin layer of epidermal cells. Stem cells called neoblasts are then signaled to proliferate beneath the wound epithelium, giving rise to an unpigmented structure referred to as the regeneration blastema. As regeneration proceeds, neoblasts continue to accumulate within the blastema, causing it to grow exponentially. Within 1 week of the transection, differentiation of the missing structures occurs. In uninjured planaria, neoblasts are distributed throughout the parenchyma (mesenchyme) and, as the only mitotic somatic cells in the animal, serve as the source of replacement cells during tissue renewal.
Research in my laboratory utilizes the tools of molecular cell biology and functional genomics to address several major biological problems for which planarians serve as excellent models.
Differentiation of the Regenerative Stem Cells: Roles in Regeneration and Tissue Maintenance
How are stem cells specified to adopt specific fates? How is their differentiation choreographed to correctly replace the missing structures? How are newly differentiated cells integrated into functional tissues and organs, during regenerative and homeostatic processes? We are addressing these and related questions by combining high-throughput in situ hybridization and monoclonal antibody screens to identify cell type-specific markers, with microarray analyses and functional studies using double-stranded RNA-mediated genetic interference (RNAi).
Regulation of Germ Cell Development and Differentiation
In addition to their impressive regeneration of somatic tissues, planarians are also able to regenerate their germ cell lineage. Furthermore, in contrast to several other well-studied model organisms (e.g., Caenorhabditis elegans, Drosophila, Xenopus, and zebrafish), planarians do not appear to segregate their germ cell lineage during embryogenesis. Rather, the germ cells appear to be derived postembryonically from neoblasts, with inductive signals directing the neoblasts to give rise to germ cells at the appropriate time and in the appropriate place in the animal. This inductive specification of germ cell fate is observed in many basal metazoans and mammals. The nature of the inductive signals that specify germ cell fate in basal metazoans and whether these signals have been conserved evolutionarily remain open questions.
The species that we have been studying, Schmidtea mediterranea, provides a unique opportunity to study this issue. S. mediterranea exists in both sexually and asexually reproducing strains; the sexual organisms are hermaphroditic and produce egg capsules when mated with another planarian, whereas the asexual organisms reproduce strictly by fission. From a molecular genetic standpoint, the asexuals are interesting because they harbor a Robertsonian translocation that is correlated with the switch from sexual to asexual reproduction. We (and others) have shown that asexual planarians still possess germ cells, but our work suggests that these presumptive germ cells are incapable of differentiating properly. By analyzing global changes in gene expression between animals that possess germ cells at various stages of development, as well as animals that lack germ cells entirely, and then characterizing such genes functionally, we seek to define the genetic programs underlying germ cell development.
Neural Regeneration and the Role of the Nervous System
The plasticity of the nervous system is a fascinating aspect of planarian biology. In addition to the rapid regeneration of the cephalic ganglia (brain), ventral nerve cords, and all of the sensory structures following amputation, the nervous system is sufficiently plastic to grow and shrink with the planarian, depending upon food availability. Little is known about the process of nervous system regeneration in planarians, or to what extent cell turnover in the nervous system allows the plasticity seen during growth and degrowth. Our work seeks to understand how new neurons are formed during regeneration, how the nervous system is repatterned and rewired following amputation, how the nervous system is maintained in the negligibly senescent planarian, and the role played by neural signals in regulating regenerative processes.
