Early in development, the behavior of neuroepithelial cells is controlled by several factors, which act in a developmentally regulated manner. Diffusible factors are secreted locally by the neuroepithelium itself, although other nearby structures may also be involved. Evidence suggests a physiological role for the cerebrospinal fluid in the development of the brain. Here, using organotypic cultures of chick embryo neuroepithelial explants from the mesencephalon, we show that the neuroepithelium in vitro is not able to self-induce cell survival, replication, and neurogenesis. We also show that the embryonic cerebrospinal fluid (E-CSF) promotes neuroepithelial stem cell survival and induces proliferation and neurogenesis in mesencephalic explants. These data strongly suggest that E-CSF is involved in the regulation of neuroepithelial cells behavior, supporting the hypothesis that this fluid plays a key role during the early development of the central nervous system.

During early stages of brain development, neuroepithelial stem cells undergo intense proliferation as neurogenesis begins. Fibroblast growth factor 2 (FGF2) has been involved in the regulation of these processes, and although it has been suggested that they work in an autocrine-paracrine mode, there is no general agreement on this because the behavior of neuroepithelial cells is not self-sufficient in explants cultured in vitro. In this work, we show that during early stages of development in chick embryos there is another source of FGF2, besides that of the neuroepithelium, which affects the brain primordium, since the cerebrospinal fluid (E-CSF) contains several isoforms of this factor. We also demonstrate, both in vitro and in vivo, that the FGF2 from the E-CSF has an effect on the regulation of neuroepithelial cell behavior, including cell proliferation and neurogenesis. In order to clarify putative sources of FGF2 in embryonic tissues, we detected by in situ hybridization high levels of mRNA expression in notochord, mesonephros and hepatic primordia, and low levels in brain neuroectoderm, corroborated by semiquantitative PCR analysis. Furthermore, we show that the notochord segregates several FGF2 isoforms which modify the behavior of the neuroepithelial cells in vitro. In addition, we show that the FGF2 ligand is present in the embryonic serum; and, by means of labeled FGF2, we prove that this factor passes via the neuroepithelium from the embryonic serum to the E-CSF in vivo. Considering all these results, we propose that, in chick embryos, the behavior of brain neuroepithelial stem cells at the earliest stages of development is influenced by the action of the FGF2 contained within the E-CSF which could have an extraneural origin, thus suggesting a new and complementary way of regulating brain development.

Foetal cerebro-spinal fluid (CSF) has a very high protein concentration when compared to adult CSF, and in many species five major protein fractions have been described. However, the protein concentration and composition in CSF during early developmental stages remains largely unknown. Our results show that in the earliest stages (18 to 30 H.H.) of chick development there is a progressive increase in CSF protein concentration until foetal values are attained. In addition, by performing electrophoretic separation and high-sensitivity silver staining, we were able to identify a total of 21 different protein fractions in the chick embryo CSF. In accordance with the developmental pattern of their concentration, these can be classified as follows: A: high-concentration fractions which corresponded with the ones described in foetal CSF by other authors; B: low-concentration fractions which remained stable throughout the period studied; C: low-concentration fractions which show changes during this period. The evolution and molecular weight of the latter group suggest the possibility of an important biological role. Our data demonstrate that all the CSF protein fractions are present in embryonic serum; this could mean that the specific transport mechanisms in neuroepithelial cells described in the foetal period evolve in very early stages of development. In conclusion, this paper offers an accurate study of the protein composition of chick embryonic CSF, which will help the understanding of the influences on neuroepithelial stem cells during development and, as a result, the appropriate conditions for the in vitro study of embryonic/foetal nervous tissue cells.