Breeder eggs (53.66±0.33 g) from sixty 35-week-old Sanhuang broiler breeders (Gallus gallus; a local crossbred broiler breed), purchased from Sanhuang broiler breeding farm (Wen's group, Guangdong, China) were allocated to two groups (80 in each group) and injected into the albumen of the eggs with 0 (Control, Con) or 5.0 μg (Leptin, Lep) of recombinant murine leptin in 100 μL of phosphate-buffered saline (PBS) before incubation. For the dose of leptin, we referred to the study of Lamosova et al. (2003), in which in ovo injection of 1.0 μg leptin into quail eggs significantly affected
posthatch growth and metabolism. Chicken eggs are approximately 5 times heavier than the quail eggs, therefore 5.0 μg of leptin was injected in ovo in the present study. The hatchings were raised under the same condition, and fed ad libitum the same starter diet (Table 2) under the same standard condition from 1 to 21 days of age. The growth performance of the offspring was recorded weekly. The blood was collected from the jugular vein at D21 and the serum was gathered and kept in a −20 °C freezer. All tissue samples were snap frozen in liquid nitrogen and stored at −80 °C prior to quantification of gene expression and enzyme activity. The experiment was undertaken following the guidelines of Animal Ethics Committee in Nanjing Agricultural University, China.