Marrow stromal cells, mitosis, and neuronal differentiation: Stem cell and precursor functions

To define relationships among marrow stromal cells (MSCs), multipotential progenitors, committed precursors, and derived neurons, we examined differentiation, mitosis, and apoptosis in vitro. Neural induction medium morphologically converted over 70% of MSCs to typical neurons, which expressed tau, neuronal nuclear antigen, neuron-specific enolase, and TUC-4 within 24 hours. A subset decreased fibronectin expression, consistent with mesenchymal to neuroectodermal conversion. More than 35% of differentiating neurons incorporated bromodeoxyuridine (BrdU) and divided, increasing cell number by 60%, while another subpopulation differentiated without incorporating BrdU or dividing. Inhibition of mitosis and DNA synthesis did not prevent neural differentiation, with 70% of blocked cells expressing tau and displaying neuronal morphologies. By deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling assay, less than 1% of cells underwent apoptosis at 36 and 72 hours, suggesting differentiation without cell-selective mechanisms. Apparently, MSCs may directly differentiate into neurons without passing through a mitotic stage, suggesting that distinctions among stem cells, progenitors, and precursors are more flexible than formerly recognized.