Regeneration in vertebrates

One way or another, all species possess the ability to regenerate damaged tissues. The degree of regeneration, however, varies considerably among tissues within a body and among species, with urodeles being the most spectacular. Such differences in regenerative capacity are indicative of specific mechanisms that control the different types of regeneration. In this review the different types of regeneration in vertebrates and their basic characteristics are presented. The major cellular events, such as dedifferentiation and transdifferentiation, which allow complex organ and body part regeneration, are discussed and common molecular mechanisms are pinpointed.     

Left-right asymmetry in vertebrates

A mechanism for the generation of the morphological left-right asymmetry in higher organisms is proposed, based on the idea that chirality at the molecular level is the primordial source for macroscopic asymmetry. This mechanism accounts for a variety of experimental results on artificial production of situs inversus and fits well with mutations in mice causing visceral transposition.     

Resource polymorphisms in vertebrates

Discrete resource polymorphisms occur in various vertebrate species and probably occur more frequently than is generally appreciated. They are manifested in a number of ways, including morphological, behavioral and life history characters. Research on a number of unrelated taxa suggests that resource polymorphisms may be underestimated as a diversifying force and potentially play important roles in population divergence and initial steps in speciation. In an ecological context, they are important in resource partitioning and reducing intraspecific competition. Recent research suggests that the mechanisms maintaining these polymorphisms may be similar in diverse taxa, that phenotypic plasticity is important, and that some are under simple genetic control.     

Ontogeny of performance in vertebrates

When competing for food or other resources, or when confronted with predators, young animals may be at a disadvantage relative to adults because of their smaller size. Additionally, the ongoing differentiation and growth of tissues and the development of sensory-motor integration during early ontogeny may constrain performance. Because ectothermic vertebrates show different growth regimes and energetic requirements when compared to endothermic vertebrates, differences in the ontogenetic trajectories of performance traits in these two groups might be expected. However, both groups of vertebrates show similar patterns of changes in performance with ontogeny. Evidence for compensation, resulting in relatively high levels of performance in juveniles relative to adults, appears common for traits related to locomotor and defensive behaviors. However, there is little evidence for compensation in traits associated with feeding and foraging. We suggest that this difference may be due to different selective regimes operating on locomotor versus feeding traits. As a result, relatively high levels of locomotor performance in juveniles and relatively high levels of feeding performance in adults are observed across a wide range of vertebrate groups.     

Retinal stem cells in vertebrates

In fish and amphibia, retinal stem cells located in the periphery of the retina, the ciliary marginal zone (CMZ), produce new neurons in the retina throughout life. In these species, the retina grows to keep pace with the enlarging body. When birds or mammals reach adult proportions, however, their retinas stop growing so there appears to be no need for such a proliferative area with stem cells. It is a surprise, therefore, that recent data suggest that a region similar to the CMZ of fish and amphibia exists in the postnatal chick and the adult mouse.