The stem cell system in demosponges: suggested involvement of two types of cells: archeocytes (active stem cells) and ch
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REVIEW
The stem cell system in demosponges: suggested involvement of two types of cells: archeocytes (active stem cells) and choanocytes (food-entrapping flagellated cells) Noriko Funayama
Received: 19 April 2012 / Accepted: 16 September 2012 / Published online: 9 October 2012 # Springer-Verlag Berlin Heidelberg 2012
Abstract Major questions about stem cell systems include what type(s) of stem cells are involved (unipotent/totipotent/ pluripotent/multipotent stem cells) and how the self-renewal and differentiation of stem cells are regulated. Sponges, the sister group of all other animals and probably the earliest branching multicellular lineage of extant animals, are thought to possess totipotent stem cells. This review introduces what is known about the stem cells in sponges based on histological studies and also on recent molecular biological studies that have started to reveal the molecular and cellular mechanisms of the stem cell system in sponges (mainly in demosponges). The currently proposed model of the stem cell system in demosponges is described, and the possible applicability of this model to other classes of sponges is discussed. Finally, a possible scenario of the evolution of stem cells, including how migrating stem cells arose in the urmetazoan (the last common ancestor of metazoans) and the evolutionary origin of germ line cells in the urbilaterian (the last common ancestor of bilaterians), are discussed. Keywords Archeocytes . Choanocytes . Stem cells . Germ cells . Origin
Introduction Stem cell systems only occur in multicellular organisms. How stem cells first arose and how “the stem cell system” Communicated by Volker Hartenstein N. Funayama (*) Laboratory of Molecular Developmental Biology, Department of Biophysics, Graduate School of Science, Kyoto University, Kitashirakawa-Oiwake, Sakyo-ku, Kyoto 606-8502, Japan e-mail: [email protected]
(the stem cells and the regulatory mechanisms that control their differentiation and self-renewal) developed during evolution are two of the fundamental questions of developmental biology. Recently, many studies have focused on the mechanisms of the maintenance and differentiation of embryonic stem (ES) cells and induced pluripotent stem (iPS) cells, especially for the purpose of regenerative medicine. However, early branching organisms in the multicellular animal lineage already have totipotent/pluripotent/multipotent somatic stem cells and mechanisms to maintain them or induce their differentiation to requisite cell types for homeostasis and to produce the next generation through sexual and/or asexual reproduction. Sponges (Porifera) are thought to be the sister group of all other animals (Philippe et al. 2009; Pick et al. 2010) and the earliest branching multicellular lineage of extant animals, although this is still controversial (for a review, see Philippe and Roure 2011). Thus, they can yield insights into the acquisition of the primordial stem cells in the last common ancestor of metazoans (the urmetazoan), potentially clarifying t
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