Mechanisms of Interspecies Senescence Senescence

The nature of human experience has impelled us throughout time to ponder mortality and immortality. Today, biologists are actually beginning to provide answers to what were formally purely philosophical and religious questions. What follows is a discussion of the mechanisms underlying biological mortality and immortality, otherwise known as senescence.

In terms of biological immortality, the cnidarian Hydra stands out. Some hydra species have been shown to survive indefinitely under laboratory conditions, by relying on asexual reproduction (Bosch, 2009, p. 484). Bosch suggests that asexual budding confers an evolutionary advantage to Hydra, because it provides a mechanism for generating enough offspring to survive competition and predation pressures from other species. Rapid asexual budding, in turn, requires cells to proliferate continuously. In other words, the stem cell populations in Hydra, which give rise to the various cell types required to make a complete organism, appear to be immortal. This is consistent with recent research findings that suggest accelerated aging (progeria) in humans is due in part to defects in the stem cell populations.

The progeny of Hydra stem cells differ from their counterparts in more complex metazoans, as well (Bosch, 2009, p. 481). For example, ectodermal epithelial cells have been shown to retain the capacity for transdifferentiation into a variety of other cellular phenotypes. For example, these cells can change their shape, function, and interactions with other cells within the organism. Differentiation...

In contrast, phenotypic commitment is a part of normal development for bilateral metazoans. It should be noted though, that the three stem cell populations that give rise to all the cellular phenotypes in Hydra are not capable of transdifferentiation and are therefore committed to a specific cellular phenotypes.
The ability of the Hydra stem cell progeny to differentiate seems to depend on epigenetic reprogramming (Bosch, 2009, p. 481), which suggests that the maintenance of a pluripotent, undifferentiated stem cell state, and thus biological immortality, also depends on maintenance of a defined epigenetic state. The epigenetic connection between biological immortality and mortality appears to extend to all metazoans. In the nematode Caenorhabditis elegans, the maintenance of a stem cell population depends in part on maintaining a specific epigenetic state (Rando and Chang, 2012, p. 47-51). In Drosophila, the epigenetic reprogramming of disc imaginal cells, which give rise to adult structures like legs and wings, can be induced by tissue fragmentation and regeneration. In addition, the cloning of mammals has depended on the ability to reprogram the epigenetic state of germ and somatic cells. This was accomplished as early as 1952 by transferring the nuclear material of a somatic cell to an enucleated oocyte.

The aging process in all metazoans therefore seems to be selectively controlled in terms of cellular phenotype. The stem cells in Hydra are essentially immortal, and the same could…