Egg, embryo and tissue weight
Egg weight, embryo weight, embryo: egg weight ratio, and tissue: embryo weight ratio are shown in Table 2. COBB eggs were 14.87g (31%) and 14.26g (30%) heavier than ACRB egg on E14 and E18 (P<0.0001), respectively. COBB embryos were 5.97g (56%) and 6.91g (31%) heavier than ACRB on E14 and E18 (P<0.0001), respectively. Embryo weight relative to egg weight was higher on E18 without breed difference. Relative breast muscle was significantly higher on E14 when compared to E18, and COBB was heavier than ACRB on E14, but no breed difference was observed on E18. Relative liver weight was higher on E18, whereas no breed difference was observed on either day. Relative fat weight was higher on E18 than on E14 (P=0.03), and COBB embryos had higher fat tissue weight than ACRB on E18 (P=0.0002).
Gene expression in the liver and muscle
Comparison of hepatic lipogenesis and lipid metabolism related gene expression between ACRB and COBB is shown in Figure 1. The main effect of embryonic day was observed in ABCA-1, Apo-B, MTTP, PPAR-α and SREBP; the expression of these genes were significantly higher on E18 when compared to E14. Main effect of breed was shown in Apo-B and MTTP; the gene expression was higher in ACRB than in COBB (P<0.05). The main effect of breed was also shown in SREBP-1 expression, which was higher in COBB (P,0.05). The only interaction of embryonic age and breed was observed in FASN expression; ACRB on E18 had significantly lower FASN expression compared to ACRB on E14 and COBB on E18.
Comparison of key myogenic genes between ACRB and COBB is shown in Figure 2. The main effect of embryonic day was observed in MEF2c, MYF6, MYOD, MYOG, PAX3, PAX7 and PTCH expression, which indicates higher expression levels of these genes on E14 compared to E18 (P<0.05). No effect of breed or interaction was detected in myogenesis gene expression.
Fatty acid composition analysis
FA composition (mole %, w/w) of egg yolk prior to incubation is shown in Table 3. Egg yolk at E0 was abundant in oleic acid followed by palmitic acid. linoleic acid, an essential n-6 fatty acids, is the dominate among other polyunsaturated fatty acids (PUFA). Compared to ACRB at this age, COBB had lower level of oleic acids (33.63% vs 42.99%, P<0.001) and total monounsaturated FA (37.01% vs. 46.37%, P<0.001), whereas they had higher essential fatty acids linoleic (22.36% vs 15.56%, P<0.01), γ-linolenic (0.31% vs 0.18, P<0.01) and α-linolenic acid (1.18% vs 0.60%, P<0.001), leading to higher total n-6 (26.62% vs 19.18, P<0.01) and n-3 FA (2.78% vs 2.07%, P<0.01).
FA composition of egg yolk on E14 and E18 is shown in Table 4. While the major FAs are same as E0, the age differences were not identified in all of the FAs analyzed except docosahexaenoic acid (DHA, C22:6n-3, P= 0.027), a major n-3 PUFA showing significant breed and age interaction effect (P=0.025); ACRB on E14 had significantly higher DHA but lower at E18 compared to COBB on the same age. Arachidonic acid level was decreased with age. Compared to ACRB, COBB had higher myristic-, palmitic-, margaric-, γ-linolenic (18:3n-6)-, gondoic acids, total saturated fatty acids (SFA), but had lower palmitoleic, linoleic, α-linolenic (18:3n-3) and total n-6 FA
In the liver (Table 5), unlike egg yolk, both n-6 and n-3 PUFA became the major fatty acids, especially with arachidonic acid and DHA. In terms of age differences, the most pronounced difference was identified in oleic acid, which was increased 2.4-2.5 times higher in E18 in comparison to E14 in both breeds (P<0.0001). At E18, both n-6 and n-3 essential FAs, linoleic- and α-linolenic acids were also significantly (p<0.0001) increased in both breeds, while decreasing saturated, and arachidonic acid and DHA. In terms of breed difference, compared to ACRB, COBB had significantly higher palmitic-, plamitoleic, godonic and eicosadienoic acids but lower in stearic, α-linolenic acid and DHA.
In the muscle (Table 6), palmitoleic-, stearic, oleic, linoleic, α-linolenic (18:3n-3), gondoic, arachidonic, eicosapentaenoic acids, DHA, total SFA, total mono, and total n-3 FA showed breed and age interaction. Similar to the result of the liver, ACRB on E14 had significantly higher DHA (P=0.004) and total n-3 FA (P=0.003) compared to the other groups. α-linolenic (18:3n-3), gondoic, eicosadienoic acids, DHA total n-3 FA and total n-6 FA showed breed difference. Myristic, palmitic, linoleic, gondoic and and total n-6 FA showed age difference.
Histology
Little differences in lipid deposition were observed between the ACRB (Figure 3. A and C) and COBB (Figure 3. B and D) chicken at the same age of embryonic development. However, lipid deposition on E18 was considerably higher than E14 regardless of the breeds.