The establishment of polarity by hippocampal neurons: the relationship between the stage of a cell's development in situ and its subsequent development in culture.

Neurons removed from the embryonic hippocampus and placed into culture develop structurally and functionally distinct axonal and dendritic processes. The central issue addressed in this study concerns the extent to which the sequence of events which results in the differentiation of neurites by hippocampal neurons in culture is influenced by the cell's state of development in situ. [3H]thymidine was administered to pregnant rats either on Embryonic Day 15 (E15) or on E18.5 to label hippocampal neurons at known stages of their development. All fetuses were sacrificed on E19. Some of the fetal brains were sectioned and examined by autoradiography to determine the location of labeled cells in the hippocampus. The remaining brains were used to prepare hippocampal cell cultures. Neurons labeled at E18.5 remained confined to the ventricular zone at E19. Those labeled at E15 had completed their migration to the cortical plate. Other data suggest that the former cells had not yet initiated process outgrowth, while the latter cells had begun to elaborate both axons and dendrites. When introduced into culture, both populations of cells developed axons and dendrites and both compartmentalized MAP2 to the dendritic domain. Moreover, despite marked differences in their developmental state at the time of introduction into culture, both underwent the same sequence of developmental events leading to axonal and dendritic development. In a few cases cells that incorporated [3H]thymidine in situ at E18.5 apparently underwent mitosis in culture. These neurons also developed axons and dendrites appropriately. These results indicate that hippocampal neurons become polarized in culture, even if they have never developed axons or dendrites in situ, and do so as efficiently as cells that have become polarized before being placed into culture. Moreover, they indicate that the same sequence of events leading to the establishment of polarity occurs for hippocampal neurons with different developmental histories prior to culturing.