Cell Differentiation and Morphogenesis

Once cellular resources are available, the last phase of regeneration initiates: rebuilding the lost structure. During this phase, morphogenetic mechanisms are deployed to pattern the new structure, driving cells to multiply, sort out, and differentiate into the regenerated tissues. Once patterning is complete, the regenerate becomes evident, usually as a smaller, more rudimentary version of the original structure. This rudiment keeps growing and developing, eventually reaching a size and morphology closely resembling the replaced structure.

In many cases, regeneration reconstructs less than the original amount of tissues lost: for example, squamate lizards often regenerate tails that are smaller and less flexible than the original (Alibardi 2014); many annelids show a maximum number of anterior segments rebuilt during head regeneration (Berrill 1952). This can result in a discontinuity of tissue identity between the regenerate and the stump. For example, after anterior regeneration in an annelid that lost ten anterior segments to amputation but can regenerate only four, segment 4 will form adjacent to segment 11, and the worm will be missing all structures specific to segments 5 to 10. Such discontinuities usually trigger a transformation process of stump tissues to adjust to their new positional identities. In our example, stump segments transform so that segment 11 adopts the morphology of segments 6, and so on. Other body-wide adjustments besides positional identity are possible: for example, in many worms, both growth of the regenerate and thinning plus elongation of the stump combine to restore the original body proportions (Coe 1929). End results may vary; while in some species regenerated structures eventually become indistinguishable from the original, in others the regenerate might differ morphologically or even functionally. In some cases, the morphogenetic process results in the regeneration of a different structure than the one that was lost (e.g., a posterior end regenerating after amputation of an anterior end). Such cases of heteromorphic regeneration tend to occur at a low frequency and are thought to be due to developmental errors caused by faulty signaling.

Since in most cases injuries remove terminal structures from appendages or the main body axis, the regenerate develops along a proximodistal axis. Interestingly, in most cases, morphogenesis proceeds by first rebuilding the distalmost, terminal tissues, and then intercalating the remaining structures. When regeneration is incomplete, tissues not restored are usually those that were proximal to the wound site; correspondingly, species that show poor structure regeneration are often still capable of restoring the terminal tips of many structures. This pattern is particularly evident in axial regeneration of segmented animals, where both anterior and posterior terminal regions share a non-segmental origin that differentiates them from the intervening segmental tissues.

In this final phase of regeneration, developmental trajectories converge the most with those found during embryogenesis. While detailed mechanisms are still far from understood, many studies have uncovered a large degree of similarity between regeneration and embryonic development in the genetic toolkit deployed during tissue patterning and cell differentiation. For this reason, regeneration is often used as a proxy of embryogenesis to study development of specific tissues and organs.

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