Brain filament proteins in primary cultures derived from chick embryos early in development

Brain filament expression and neurofilament post‐translational modifications (phosphorylations) were studied in primary cultures derived from whole 3–4 day chick embryos. After 2–3 days in culture, neurofilament‐positive cells formed neuronal aggregates connected by bundles of neurites in a distinctive pattern similar to that observed in cultures derived from embryonal rat brain and neonatal rat cerebellum. Aggregates and neuritic bundles were stained with several monoclonal antibodies reacting with phosphorylated neurofilament epitopes. With two monoclonal antibodies reacting with phosphorylated forms of the high molecular weight neurofilament subunit, staining was only observed after 8 and 10 days in vitro. There was a major difference between rat and chicken with respect to astrocyte differentiation in culture. In chicken, the flat cells surrounding the neuronal aggregates remained constantly GFAP‐negative throughout the whole experimental period (10 days). GFAP‐positive cells were first observed within the neuronal aggregates on day 8 in vitro.