The development of chick spinal cord in tissue culture. III. Neuronal precursor cells in culture.

On culturing fragments of neural tube of Hamilton and Hamburger (H & H) Stage 10 chick embryos, large multipolar neurons developed. The aim of this investigation was to determine whether these neurons in culture developed from dividing neuronal precursor cells, from postmitotic precursor cells, or both. Of the neurons formed during the 20 d of culturing in the presence of [3H]thymidine, 26% were unlabeled, indicating that they originated from cells that were already postmitotic at the time of explantation. By labeling cells of the neural tube in vivo and determining the total number of cells in the neural tube, we estimated that the neural tube of chick embryos of H & H Stage 10 contained approximately 1000 (3.3%) postmitotic cells. By estimating the total number of neurons that formed in 20-d cultures and the percentage of labeled and unlabeled neurons, we concluded that the postmitotic neuronal precursor cells survived well in cultures and proceeded on their predetermined path of differentiation. By considering the number of neurons found in the spinal cord in vivo and the number of labeled neurons found in cultures, we concluded that only a relatively small fraction of the dividing neuronal precursor cells entered the postmitotic stages of differentiation and formed neurons in cultures. The majority of cells that did this, entered the postmitotic stage of differentiation during the first 5 d in culture.