An MCM2-related gene is expressed in proliferating cells of intact and regenerating planarians

The minichromosome maintenance (MCM2-7) gene family encodes conserved prote ins, which are essential for DNA replication licensing in eukaryotes, They are abundant in proliferating cells, and specific MCM transcripts undergo c ell cycle-dependent oscillations. Here we report the characterization of a planarian MCM2 homologue, DjMCM2, which represents the first molecular mark er for detecting proliferating cells in planarians, DjMCM2-expressing cells are broadly distributed in the mesenchymal space of the body, with the exc eption of the cephalic region, and are preferentially accumulated in the pe ripheral area of the dorso-lateral mesenchyme, along the anteroposterior ax is. During regeneration, no DjMCM2 transcripts are observed within the blas tema, according to the current view that this structure is not a proliferat ion site in planarians, Spatio-temporal changes in DjMCM2 RNA expression pa ttern in the stump parallel blastema growth, coordinately with the orientat ion of the cut. X-ray irradiation results in the disappearance of DjMCM2 ex pression, thus confirming that these transcripts are detected specifically in proliferating cells, visualized as neoblasts by in situ hybridization in dissociated cells. In addition to neoblasts, rare large DjMCM2-expressing cells are observed in macerates of tissues excised just below the wound, su ggesting that cell types other than neoblasts may be sporadically recruited for proliferation in planarians, (C) 2000 Wiley-Liss, Inc.