Why does regeneration occur? And why, when it manifests itself, does
it do so in some but not all metazoan species? Hence, what are the permissive
or inhibitory factors operating behind this phenomenon? When it comes to
regeneration, many questions, such as these, remain unanswered. In fact,
the problem of animal regeneration has withstood the probing of scientific
inquiry for over 250 years and still awaits a satisfactory mechanistic explanation.
In this essay, I will review the distribution and the modes of regeneration
that are found in the different metazoan phyla. Also, I will re-examine
ideas on its evolutionary origins, and discuss its possible relationship
to both asexual reproduction and embryogenesis. This endeavor has two objectives.
First, to bring forward an interpretation of regeneration which integrates
evolutionary and developmental considerations into its discussion. And second,
to suggest a comparative experimental approach to this problem that may
bring us closer to understanding the molecular basis of this long-standing