The Centriole Paradox in Planarian Biology: Why Acentriolar Stem Cells Divide and Centriolar Somatic Cells Do Not

Planarians (free-living flatworms) present a fundamental paradox in cell biology: their proliferative stem cells (neoblasts) completely lack centrioles, while their post-mitotic, differentiated cells possess them. This review synthesizes evidence to resolve this inverse correlation. We demonstrate that neoblasts employ a robust, evolutionarily conserved acentrosomal pathway for mitotic spindle assembly. This mechanism relies on chromatin-mediated nucleation via a RanGTP gradient and motor protein-driven self-organization. This adaptation confers significant advantages, including enforced asymmetric division, metabolic economy, and a drastically reduced risk of centrosome amplification-driven genomic instability, which may underpin planarians' extensive regenerative capabilities and resistance to tumors. Conversely, the quiescence of centriole-bearing somatic cells is not caused by the organelles themselves but is a consequence of an irreversible terminal differentiation program. These cells epigenetically silence core cell cycle machinery and repurpose their centrioles as basal bodies for ciliogenesis. Thus, the presence of centrioles is a marker, not a driver, of the differentiated state. This system represents a profound uncoupling of the mitotic apparatus from the centriole, offering novel insights into stem cell biology, alternative modes of cell division, and providing conceptual frameworks for regenerative medicine and cancer research.