Development of the rhythmic melatonin secretion in the embryonic chicken pineal gland

In order to elucidate details on the development of the circadian clock, the effects of light on the in vitro melatonin (MT) release and the presence of mRNAs of several clock genes in the embryonic chicken pineal gland were investigated. Chicken embryos of various developmental stages were exposed to stimuli of light in vitro in dynamic, four day long bioassay (perifusion). MT secretion and mRNA levels of Cry1, Cry2, Clock and Bmal2 clock genes were determined. Our conclusions: (1) environmental illumination modified MT secretion from explanted embryonic pineal glands as early as on the 13th embryonic day, (2) daily rhythm of MT release develops between embryonic days 16 and 18 under periodic environmental illumination. (3) Chicken Cry1, Cry2, Clock and Bmal2 clock gene mRNAs were also detected in glands of animals of 15th embryonic day. Although both MT secretion and clock genes have been developed by then, the circadian MT rhythm appears first on the 17th embryonic day. Either the mechanisms coupling the clock with the melatonin output or the synchronization of the individual pinealocytes develop around this age. Rhythmic MT release in the embryonic chicken pineal gland evolves only if the egg is exposed to rhythmic environmental stimuli.