Multicellularity, stem cells, and the neoblasts of the
planarian Schmidtea mediterranea
Alejandro Sa´nchez Alvarado*, Hara Kang
University of Utah School of Medicine, Department of Neurobiology and Anatomy, Salt Lake City, UT 84112, USA
Received 4 March 2005, revised version received 4 March 2005
Available online 25 April 2005
All multicellular organisms depend on stem cells for their survival and perpetuation. Their central role in reproductive, embryonic, and post-
embryonic processes, combined with their wide phylogenetic distribution in both the plant and animal kingdoms intimates that the emergence
of stem cells may have been a prerequisite in the evolution of multicellular organisms. We present an evolutionary perspective on stem cells and
extend this view to ascertain the value of current comparative studies on various invertebrate and vertebrate somatic and germ line stem cells.
We suggest that somatic stem cells may be ancestral, with germ line stem cells being derived later in the evolution of multicellular organisms.
We also propose that current studies of stem cell biology are likely to benefit from studying the somatic stem cells of simple metazoans. Here,
we present the merits of neoblasts, a largely unexplored, yet experimentally accessible population of stem cells found in the planarian
Schmidtea mediterranea. We introduce what we know about the neoblasts, and posit some of the questions that will need to be addressed in
order to better resolve the relationship between planarian somatic stem cells and those found in other organisms, including humans.
D 2005 Elsevier Inc. All rights reserved.
Keywords: Planarians; Stem cells; Neoblasts; Schmidtea mediterranea