Early in development, the behavior of neuroepithelial cells is controlled by several factors acting in a developmentally regulated manner. Recently it has been shown that diffusible factors contained within embryonic cerebrospinal fluid (CSF) promote neuroepithelial cell survival, proliferation, and neurogenesis in mesencephalic explants lacking any known organizing center. In this paper, we show that mesencephalic and mesencephalic+isthmic organizer explants cultured only with basal medium do not express the typically expressed mesencephalic or isthmic organizer genes analyzed (otx2 and fgf8, respectively) and that mesencephalic explants cultured with embryonic CSF-supplemented medium do effect such expression, although they exhibit an altered pattern of gene expression, including ectopic shh expression domains. Other trophic sources that are able to maintain normal neuroepithelial cell behavior, i.e., fibroblast growth factor-2, fail to activate this ectopic shh expression. Conversely, the expression pattern of the analyzed genes in mesencephalic+isthmic organizer explants cultured with embryonic cerebrospinal fluid-supplemented medium mimics the pattern for control embryos developed in ovo. We demonstrate that embryonic CSF collaborates with the isthmic organizer in regulation of the expression pattern of some characteristic neuroectodermal genes during early stages of central nervous system (CNS) development, and we suggest that this collaboration is not restricted to the maintenance of neuroepithelial cell survival. Data reported in this paper corroborate the hypothesis that factors contained within embryonic CSF contribute to the patterning of the CNS during early embryonic development.
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