ABSTRACT Incubating temperature and timing or duration is critical to determine the optimum protocol of thermal manipulation (TM), which underlines muscle growth improvement. Therefore, the aim of the present study is to determine the optimum period of embryonic TM that may result in the improvement of pectoral and thigh muscle myogenesis. This is done by investigating the level of mRNA expression of creatine kinase (CK) and lactate dehydrogenase (LDH). An additional goal is measuring the blood levels of CK and LDH as a biomarker of muscle injury due to the experimental thermal challenge on post-hatch day 35. The study was conducted on 1,440 fertile eggs (Ross broilers) that were divided randomly and equally into a control group and four treatment groups (TM1, TM2, TM3, and TM4). The treatment groups were daily subjected to TM at 39 ºC for 18h with 65% relative humidity (RH) during embryonic days (EDs) 7-11, 11-15, 15-18, and 7-18, respectively. Among the thermally manipulated groups that were investigated, TM1 (ED 7-11) resulted in significant improvement of mRNA expression and enzymatic concentration of CK and LDH in muscle during embryogenesis, as compared to the control. Six hours of TC showed the highest significant CK and LDH expression and concentration levels in the control as compared to TM groups. Thus, the results of this study indicate that TM during ED 7-11 improves pectoral and thigh muscles’ response to heat stress without adversely affecting their performance. This finding could be used by commercial breeders to enhance local broiler production.

ABSTRACT Thermal manipulation (TM) during broiler chicken embryogenesis has been shown to promote muscle development and growth. However, the molecular bases of promoting broiler muscle development and growth are not fully understood. The aim of this study was to investigate the molecular bases of muscle growth and development in broiler chickens subjected to TM. This included the investigating of the changes in mRNA expression levels of muscle marker genes, namely MyoD, myogenin, paired box transcription factor (Pax7) and proliferating cell nuclear antigen (PCNA), and muscle growth factors namely insulin-like growth factor 1 (IGF-1), myostatin and growth hormone (GH) during embryogenesis and on posthatch days 10 and 28. Fertile Cobb eggs (n=1500) were divided into four groups. Eggs in the first group (control) were incubated at 37.8°C and 56% RH, whereas, eggs in the second group (TM1), third group (TM2), and fourth group (TM3) were subjected to 39 ºC and 65% RH daily during embryonic days (ED) 12-18 for 9, 12, and 18 hours, respectively. Body weight (BW) during embryogenesis and posthatch days (1, 3, 5, 7, 14, 21, 28 and 35) was recorded. mRNA expression levels of muscle marker genes and muscle growth factor genes during ED 12, 14, 16 and 18 and on posthatch days 10 and 28 were analyzed using real-time RT-PCR. TM upregulated the mRNA expressions of muscle marker and growth factors genes. This upregulation was accompanied by improvement of body weight near and at market age.