Selective effects of light exposure on distribution of motility in the chick embryo at E18.

It is well established that orderly patterns of motor neuron activity, muscle recruitment, and limb movement are generated in chicks during motility by embryonic day (E)9, the midpoint in embryonic development. However, our recent work suggests that some attributes of motility, such as the rhythm of repetitive limb movements and distribution of activity, become less orderly after E9. In this study, we extend these observations by performing continuous force recordings over a 24-h period in ovo at E18 with augmented sampling of synchronized video and electromyogram (EMG) recordings. We report the distribution of three repetitive behaviors, rapid limb movement, respiratory-like movement, and beak clapping, identified in force recordings, and the general distribution of motility. We also test a model recently proposed to account for age-related changes in motility parameters. In the model, we proposed that circadian networks contribute to the age-related changes in distribution of motility. As a first test of this hypothesis, we examine whether light exposure contributes to the variable distribution of motility by comparing motility parameters at E18 for embryos incubated and tested under either a 12-h light/dark cycle or continuous light. Results suggest that exposure to light increases the total amount of activity and hastens the onset of extended respiratory-like movement sequences but does not impact expression of repetitive limb movement or beak clapping at E18. The possible influence of circadian mechanisms on embryonic behavior and insensitivity of repetitive limb movements to light exposure are discussed.