Differential embryo development among Tibetan Chicken, DRW and Shouguang Chicken exposed to chronic hypoxia

Asian - Australasian Journal of Animal Sciences, March, 2009 by Mei Li, Chun-Jiang Zhao, Chang-Xin Wu

T, a kind of highland indigenous animal, having successfully adapted itself to chronic hypoxia, can develop normally at high altitude (2.2 to 4.1 thousand meter). However, DRW and S are two lowland chicken breeds where altitude is less than 400 m. Unfortunately, T has been rarely used in chronic hypoxia studies, though hypoxic incubation of avian development has been studied for decades.

In the present study, three subjects (DRW, S and T), two stress factors (hypoxia and normoxia) and four stages (HH 39, 41, 43 and 45) were considered together to design the experiment, which was more comprehensive than the previous studies (single subject and the equal length of incubation) (Dzialowski et al., 2002). The selection of the studied developmental stages was based on the knowledge of hormonal regulated physiology. The elaborate experimental design and the results from the present study may be helpful to study animal physiology of the adaptation in hypoxic environment.

Morphological responses to chronic hypoxia

The results of the present study showed that chronic incubation induced significant changes in embryo and organ mass including brain, heart, liver in the three chicken breeds. Prior to incubation, the egg weight of DRW is the heaviest while the ones of S is the second, and T egg weight is the lightest among the three chicken breeds (p<0.01) (Figure 1). However, in hypoxia, embryo mass in T is always much heavier than those in DRW and S at the four different developmental stages. The results suggested that the embryonic growth of DRW and S was significantly retarded in hypoxia (p<0.01), whereas the embryo mass of T had no significant change between hypoxic and normoxic conditions (p>0.05). At the same time, group of feed withdrawal of broiler chickens weighed less (p<0.05) compared to control group of heat acclimation on male broiler and layer chickens responses to acute heat stress at four weeks of age (Mahmoud and Yaseen, 2005).

Similarly, our present results in part were in good agreement with previous reports and further confirmed that hypoxic exposure during development resulted in decreased embryo growth in chicken (Ruijtenbeek et al., 2000; Dzialowski et al., 2002; Ruijtenbeek et al., 2003; Crossley and Altimiras, 2005). There was also the demonstration that the embryo of fertilized eggs that laid by sea level hens incubated at high altitude showed the restricted fetal growth (Miller et al., 2002). In particular, the retarded growth is correlated with the duration of high altitude residence: the longest resident population exposed to hypoxia, the least decline in birth weight with altitude, whereas the shortest historical residence groups, the greatest decline (Hass et al., 1980; Moore, 1990). For instance, the Black Leghorn chickens had lived at high altitude city of La Paz (3.6 thousand meter) only for at least six generations and sea level hens lived in the city of Santa Cruz (420 m). The fertilized eggs of both high altitude and lowland chickens were carried out the experiment in Bolivia (Giussani et al., 2007). As a result, incubation at high altitude of fertilized eggs laid by sea level hens markedly restricted fetal growth. Similarly, the embryo mass of fertilized eggs laid by high altitude hens that were incubated at high altitude also showed retarded fetal growth, but to a lesser extent compared to that of the lowland chicken embryos. Giussani et al. (2007) doubted in the discussion whether six generations for the chicken inhabited at high altitude is sufficient time for physiological adaptations to high altitude. Giussani (2007) also described that epigenetic mechanisms maybe highlight the hypoxia protection, which can lead to the alteration of gene expression and physiological response in a comparatively shorter period. On the other hand, the body mass of high altitude coot embryos (Fulica americana peruviana) incubated in the Peruvian Andes (3.5 thousand meter) did not differ from those at the sea level embryos at hatching (Cynthia Careyl, 1989), which was consistent with the embryo mass change of T between the two different treatments in the present study. Of note, according to the previous reports (Dragon and Baumann, 2003; LeonVelarde and Monge, 2004), the abilities of the elevated embryo mass in response to the gas exchange in hypoxia could increase the oxygen carrying capacity, decline the erythrocyte ATP level, early stimulate 2,3-bisphosphoglycerate (2,3-BPG) synthesis during embryonic development and promote oxygen utilization. Therefore, the results in the current study indicate that T has a genetically based adaptation to hypoxia, while DRW and S showed the blunted growth in hypoxic incubation, supporting that chronic hypoxia has a significant influence on fetal growth efficiency in different chicken breeds. Furthermore, fetal resource uptake for tissue accretion was induced by hypoxia (Giussani et al., 2007). For instance, fetal resource uptake was greater in fertilized eggs laid by high altitude hens compared to the ones laid by lowland chicken. However, the mechanism of resource utilization trigged by hypoxia remains unknown.