Stem cell dynamics in Cnidaria: are there unifying principles?

Abstract

The study of stem cells in cnidarians has a history spanning hundreds of years, but it has primarily focused on the hydrozoan genus Hydra. While Hydra has a number of self-renewing cell types that act much like stem cells--in particular the interstitial cell line--finding cellular homologues outside of the Hydrozoa has been complicated by the morphological simplicity of stem cells and inconclusive gene expression data. In non-hydrozoan cnidarians, an enigmatic cell type known as the amoebocyte might play a similar role to interstitial cells, but there is little evidence that I-cells and amoebocytes are homologous. Instead, self-renewal and transdifferentiation of epithelial cells was probably more important to ancestral cnidarian development than any undifferentiated cell lineage, and only later in evolution did one or more cell types come under the regulation of a "stem" cell line. Ultimately, this hypothesis and competing ones will need to be tested by expanding genetic and developmental studies on a variety of cnidarian model systems.

Reconsidering regeneration in metazoans: an evo-devo approach

“…Such responses are tightly bound to some sort of "immune response, " i.e., the infiltration in the wound area of immune cells. This varies between organisms, and ranges from the transient intervention of amoeboid cells in cnidarians to the complex adaptive immune response of vertebrates (Kawakami and Nakanishi, 2001;Eming et al, 2009;Nakanishi et al, 2011;Palmer et al, 2011;Gold and Jacobs, 2013;Dubuc et al, 2014;Wenger et al, 2014). The relationship between wound healing, particularly the immune response phase, and the regenerative event is not clear, and is probably not a conserved feature (see next section).…”

“…However, multipotent somatic cells, called interstitial cells (i-cells), have only been described in hydrozoans (Leclère et al, 2012;Gold and Jacobs, 2013) where they can coexist with other restricted stem cells capable of differentiation into more specialized cell types (Müller et al, 2004;Hobmayer et al, 2012;Gold and Jacobs, 2013). Homologues of i-cells seem to be absent in other classes of cnidarians.…”

“…Homologues of i-cells seem to be absent in other classes of cnidarians. Taken together with their phylogenetic position it is likely that i-cells are a derived feature of hydrozoans, rather than ancestral to the cnidarians (Technau and Steele, 2011;Gold and Jacobs, 2013). Instead, other mechanisms seem to provide the regenerative cells in the other classes.…”

Aging and longevity in the simplest animals and the quest for immortality

“…3G). The cells of the main body column divide continuously and these replace all of the non-dividing, differentiated cells in the hydra within 20 days; thus the hydra’s body is renewed continuously - at least 60 times over the 4 years of the Martínez study (Martínez, 1998; Watanabe et al, 2009; Tanaka and Reddien, 2011; Gold and Jacobs, 2013). Hydras typically maintain a steady adult size; most excess cells are formed into buds that develop into daughter hydras that separate from their mother (Fig.…”

Maristem—Stem Cells of Marine/Aquatic Invertebrates: From Basic Research to Innovative Applications

“…Studies revealed that, in contrast to the prevalence of diverse oligopotent and unipotent stem cells of vertebrates, marine invertebrates appear to display the communal spread of multipotency and pluripotency [10], with adult stem cells that give rise to cell lineages characteristic of more than a single germ layer, sometimes with somatic and germ line potential. In addition, unlike vertebrates, in many aquatic/marine invertebrates, stem cells are disseminated and widespread inside the animal body, i.e., not associated with a regulatory microenvironment (niche) [12,13]. It is also worthy of note that transdifferentiation (today, a topic of great interest when trying to understand how to "reprogramme" a cell) is prevalent in both anatomically simple and "morphologically complex" invertebrates [14,15].…”