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

INTRODUCTION

Oxygen concentration is essential for embryo development (Jaffee, 1974; Strick et al., 1991; Meuer et al., 1992; Miller et al., 2002; Rouwet et al., 2002; Villamor et al., 2004; Chan and Burggren, 2005). Hypoxia increased embryo mortality (Villamor et al., 2004), induced the retarded growth (Wangensteen et al., 1974; Stock and Metcalfe, 1987; Burton and Palmer, 1992; Rouwet et al., 2002; Villamor et al., 2004), depressed the metabolic rate (YC, 1993), blunted the cardiovascular response to the hypoxic challenge (Crossley and Altimiras, 2005), altered the physiological and morphological trajectories of developing chicken embryos (Dzialowski et al., 2002), also affected the rennin-angiotensin system, the hypothaLamicpituitary-adrenal axis, vascular endothelial function and sympathetic intervation (Jansson and Lambert, 1999; Ruijtenbeek et al., 2000; Ruijtenbeek et al., 2003).

Chicken embryo is a good tool to study developmental physiology. In particular, fetal oxygenation, independent of maternal nutrition during development, plays a vital role in the control of embryonic growth. In the present study, three chicken breeds were involved, which included Tibetan chicken (T), a Chinese indigenous breed living in Tibetan areas with an altitude of 2.2 to 4.1 thousand meter for more than one thousand years; Shouguang Chicken (S), another Chinese indigenous breed from Shandong province with an altitude of 40 to 200 m, and Dwarf Recessive White Chicken (DRW), an imported breed, reared at the Experimental Farm of China Agricultural University in Beijing where altitude is less than 60 m. It was reported that T had a hatchability of 79.51% at high altitude of 2.9 thousand meter while the hatchability of DRW and S at the same altitude was 31.70% and 32.21%, respectively (Zhang, 2005). More and more embryos died at the late embryonic developmental stages during the incubation at high altitude, which is due to that hypoxia produces greater effects later in gestation (Mulder et al, 1998; Dragon and Baumann, 2003) and compensatory mechanism such as activation of the autonomic nervous system and catecholamine release redistribute cardiac output to the heart and brain in response to hypoxia (Mulder et al., 1998; Mulder et al., 2002).

Chick embryonic red blood cell (RBC) played a role in this process of oxygen transportation from the environment to the tissue, which was regulated by hormonal effectors stimulating the cAMP pathway (Dragon and Baumann, 2003). RBC population expanded rapidly until Hamburger and Hamilton stage (HH) 43 (Dragon and Baumann, 2003). The erythroid carbonic anhydrase II (CAII) also improved the CO2 transport properties in hypoxia to respond to the respiratory condition change (Baumann et al., 1986; Dragon and Baumann, 2003), whose transcription peak acted by cAMP appeared at HH 41 during chicken embryonic development. In addition, catecholamine was critical for the adaptive regulation of embryonic blood gas transport properties as well as the control of cardiovascular development from HH 39 to 40 (Dragon, 1999). The peak of norepinephrine and epinephrine released at HH 45 in the chicken embryos. Collectively, the physiology regulated by hormone above in the avian development suggested that a critical window appeared to be present in response to hypoxia. Therefore, the development critical windows of the avian embryos were determined at HH 39, 41, 43 and 45 in the present study.

The previous study mainly focused on the subjects of White Leghorn chicken, turtle, fish as well as Alligator and the incubation stages divided into three discrete periods with equal length to assess physiological morphology (Dzialowski et al., 2002). Yet there remains some confusion about how the physiological and cellular designs of the avian respiratory system have evolved under the selective pressure of multiple requirements for gas exchange (Leon-Velarde and Monge, 2004). Moreover, there was little information of T during the mid-late incubation periods of the physiological development. The objective of the present study is to investigate differential effects induced by chronic hypoxia on tissue growth and development in DRW, S and T at HH 39, 41, 43 and 45 of avian incubation.

MATERIALS AND METHODS

Eggs and incubation

Fertilized eggs of DRW, S and T were obtained from the Experimental Chicken Farm of China Agricultural University. Prior to incubation, eggs in all of the three chicken breeds were weighed and they were incubated under hypoxic condition (13% [O.sub.2]). At the same time, control fertilized eggs were incubated in another incubator exposed to ambient oxygen (approximately 21% [O.sub.2]) for the same duration. And samples were collected from two groups at HH stage 39, 41, 43, 45, respectively (Hamburger and Hamilton, 1951; Corresponding to embryonic day: Day 13, 15, 17 and 19). The simulated hypoxic incubator was developed by Wu's lab (Wu et al., 2005) with a gas mixture containing 13% oxygen and 87% nitrogen. Temperature (37.8[degrees]C) and humidity (60%) in the two incubators were adjusted to be the same. Oxygen concentration was maintained 12.5 to 13% during the exposure period. To consider repeated sampling and mortality, the following numbers of eggs were placed in each incubator at the beginning of incubation. The eggs number of DRW was identical with that of S in hypoxia (n = 150) and normoxia (n = 120). And 120 fertilized eggs of T were put in each incubator under hypoxic or normoxic condition, respectively.