The development of mouse spinal cord in tissue culture. II. Development of neuronal precursor cells.

Neural tubes of mouse embryos at Theiler Stages 14, 15, and 16 were grown in cultures for 21 d with 0.5 MicroCi/ml tritiated thymidine or cold growth medium. It was found that 50 to 60% of the neurons formed in the outgrowth zone were labeled, indicating that they formed from precursor cells that proliferated in the cultures. The unlabeled neurons must have formed from cells that were already postmitotic when the cultures were started. By comparing the total number of neurons per neuromere formed in vivo and in vitro, it seems that the postmitotic precursor cells survive better in cultures and only a small percentage of proliferative precursor cells in cultures enter the postmitotic stage and form neurons.