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 The aim of this study was to determine the optimum timing of thermal manipulation during early, mid, late and long lasting embryogenesis that may result in improvement of body performance and myofiber development (fiber diameter and thickness of perimysium) of pectoral and thigh muscles by a comparative morphometrical investigation on post-hatch day 35. 1440 fertile Ross broiler eggs were divided randomly and equally into 5 treatment groups: control (no TM), TM1, TM2 TM3 and TM4 were thermally subjected to 39ºC for 18h with 65% relative humidity daily during embryonic days ED 7-11, ED 11-15, ED 15-18 and ED 7-18 respectively. Out of TM conditions that were investigated, TM1 resulted in a significant improvement of hatchability rate (95.14%)and body performance when compared to the control. On post-hatch day 35, there were significant differences in BW, skinned carcass, breast, thigh and gut weight for all groups when compared to each other with a higher significant in TM1 and TM2 when compared to those of the control. FCR in this study was lower in TM groups when compared to the control group. In TM3 and TM4 the chicks show a significantly lower value of body performance when compared with those of the control group, TM1 and TM2.The histological evaluation of Pectoral muscle revealed myofibers of different diameter (38.39-40.27 µm) and perimysial thickening (17-30.5 µm), while myofibers diameter of thigh muscle ranged between (29-32.9 µm) and the perimysial thickening ranged between (14.36-22.15 µm). Thus, these results indicate that TM duringED7-11 improved muscle growth and body performance and this finding may be applied by commercial breeders to produce more enhanced broiler chickens.

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.