The development of dorsal root ganglia and ventral horns in the opossum. A quantitative study.

Numbers of cells, of mitotic figures, and of degeneration sites have been counted in dorsal root ganglia and ventral horns during the development of the opossum, from embryonic to pouch-young stages. Tn the embryo, from forelimb bud stages onwards, the cervico-brachialganglia growrapidly, at first with very high mitotic rates (about 5 %) and little cell degeneration. One day before birth, cell death is widespread through the whole series though cell numbers at all levels continue to increase. In the new-born, the growth of brachial ganglia is checked by very high local rates of cell death. In the cord, cells of the lateral motor columns, or ventral horns arise by outward migration of cells from the basal half of the mantle layer. In the later embryo, these cells are recognizable at cervico-brachial levels during their migration, and are then accompanied by degenerating cells, the axial distribution of which foreshadows that of the lateral column of the new-born. In the pouch-young, there is a slow loss of neurons from dorsal root ganglia and from ventral horns. This begins later at lumbo-sacral than at cervico-brachial levels. Comparison of these results with data from other species emphasizes the variety of patterns in neural development among tetrapods.