Pluripotent Gametogenic Stem Cells of Asexually Reproducing Invertebrates

Abstract

In this review, morphological and some functional properties of stem cells in different representatives of animals with asexual reproduction (sponges, hydroids, planaria, colonial rhizocephalan crustaceans and colonial ascidia) are considered in comparison with metazoan germline cells and in vitro mammalian embryonic stem cells. Stem cells are an essential and defining feature developed during evolution of all multicellular organisms (Lohman, 2008; Batygina, 2010; Funayama et al., 2010). The study of mammalian embryonic stem cells has become a hot, intensely developing field in biology, biotechnology, and biomedicine. However, comparative studies of stem cells in various multicellular organisms are required to understand molecular mechanisms of maintaining pluri/totipotency, “stemness”, which still remain far from clear, as well as mechanisms that regulate gametogenesis, reproduction, development and regeneration. According to the generally accepted view, stem cells are cells of embryos or adult organisms capable of self-renewing by mitotic reproduction and differentiation into specialized cell types (Weissman et al., 2001; Cogle et al 2003; Muller, 2006; Lohman 2008; Rinkevich et al., 2009; Skold et al., 2009; Funayama et al., 2010). Two main types of stem cells should be considered: the cells of the germline and the somatic stem cell lineages (Hogan, 2001; Rinkevich, 2009; Srouji & Extavour, 2011). Germ cells are the only cell type capable of generating a new whole organism in animals; everlasting germline cycle continues from one generation to the next, thus the germline escapes the mortality that all somatic cells of an organism ultimately confront (the common view from Weismann, 1892, to Cinalli et al., 2008). The true innovation in the evolution was the generation of gametogenic germ line, the loss of gametogenic potential from the majority of cells of the organism and protection of the germ line throughout development (Extavour, 2008; Strouji & Extavour, 2011). During development, germ cells are set aside from all somatic cells of the embryo (Cinalli et al., 2008). E. Davidson and colleagues have developed the “set-aside” hypothesis (Davidson et al., 1995; Cameron et al., 1998; Jenner, 2000; Collins & Valentine, 2001). Cameron et al. (1998) called “set-aside cells” the patches of larval cells that give rise to the adult body plan in animals with indirect development while most of the larval cells have a dead-end fate. These cells are developmentally set aside from the embryo-larva differentiation process,