Early Development and Epigenetic Programming of Body Functions in Birds

Basic research on early ontogeny of body functions is carried out mainly in domestic bird species. Summarizing the results, it can be concluded that in birds the development of body functions but also of adaptation mechanisms related to changes in incubation environment starts early during the embryonic phase. However, during the final incubation period bird embryos, especially of precocial species, have well-developed physiological mechanisms, which enable them to react on environmental influences similar to the post-hatching period in a higher degree. Most functional systems of the organism develop from open loop systems without feedback control into closed control systems regulated by feedback mechanisms, which is a ‘critical period’ in the development of body functions. Some examples will be given as follows. Changes in incubation temperature, for instance, induce activation of thermoregulatory mechanisms [1]. After internal pipping, on increase or decrease in incubation temperature chicken and duck embryos are able to react with increase or decrease in blood flow of the chorioallantoic membrane, respectively. Respiratory rate is increasing with warming and distress call rate is increasing with cooling the eggs. Further, in Muscovy duck embryos the neuronal thermosensitivity of the thermoregulatory centre in the hypothalamus is similar with that of the post-hatching period [2, 3]. In chicken embryos the hypothalamic-pitatury-thyroidal axis is functional on embryonic day 19 [4], it means that feedback mechanisms are developed. The acoustic-sensory-cardiac axis in Muscovy ducks, for instance, is functional from embryonic day 27 and heart rate responses to acoustic stimulation are detectable [5]. In the domestic fowl circadian melatonin synthesis starts during embryonic life [6, 7]. The amplitude of the rhythm increased considerably during the last two days of incubation, which indicates that melatonin may play an essential role in adapting the embryo to the postnatal environment [8].