Hypothalamic c-Fos expression changes during late chicken embryonic development and increases following experimental stimulation

The timing of functional hypothalamic responsiveness during embryogenesis remains incompletely defined. We examined hypothalamic activation in chicken embryos from embryonic day (ED) 12–17 following experimental stimulation consisting of a localized foot injection of paraformaldehyde (PFA) together with the associated handling procedures required for its administration. Hypothalamic activation was quantified as the proportion of c-Fos-positive cells within the paraventricular nucleus (PVN). Generalized linear modelling revealed significant effects of both embryonic age and stimulation condition on c-Fos expression, with no evidence of an Age × Stimulation interaction. Across development, stimulated embryos exhibited higher proportions of c-Fos-positive cells than age-matched controls. Independent of stimulation, the only significant difference between adjacent developmental stages occurred between ED14 and ED15, suggesting a candidate transition in hypothalamic maturation. Exploratory analysis indicated that these findings are consistent with an early (ED12–ED14) and a late (ED15–ED17) developmental phase and suggest that ED14–ED15 may represent a candidate phase boundary in hypothalamic maturation.