Abstract
Background:
Native Saudi chickens have the genetic capacity to develop well and produce eggs in hot environments, allowing them to withstand heat stress. Thermal alteration is one of the most significant suggested tactics that aim to improve muscle growth and growth performance during embryogenesis genetically.

Aim:
This study assessed the impact of temperature modulation on the hatchability, body weight, and myofiber diameter of pectoral and thigh muscles in native Saudi chickens.

Methods:
In this study, 300 viable eggs were equally divided and at random into three groups. The control group was maintained at 37.8°C with 56% relative humidity. The other two groups (TM1 and TM2) were subjected to temperature manipulation and kept at 39°C for 18 hours daily at 65% relative humidity during the embryonic days ED 4–7 and ED 7–10, respectively. Changes were described by analyzing paraffin slices of the thigh and pectoral muscles.

Results:
The hatchability rate was somewhat higher in the TM1 and TM2 groups than in the control group, but there was no appreciable shift in the embryonic body weight. TM2 group had a significantly higher body weight at post-hatch days 10 and 15 than TM1 and the control group. Furthermore, compared to TM1 and the control group, the TM2 group's pectoral and thigh myofiber diameters were considerably greater. Thus, without negatively impacting the hatchability rate or embryonic body weight, morphometrical outcomes of TM2 pectoral and thigh muscles during ED 7–10 increased the sensitivity of pectoral and thigh muscles to heat stress. Gene expression analysis revealed that the heat shock proteins were highly expressed in the thigh muscle of the TM1 than the control group, with Hsp70 showing a higher level of expression than Hsp60, Hsp90, and Hsp108. In contrast, there was no appreciable change in the mRNA expression of Hsp60, Hsp70, and Hsp108 in thigh muscle when TM1 was heated in ED18.

Conclusion:
Thus, the thermal manipulation technique could be exploited by commercial breeders to improve the muscle growth of local chickens in the Kingdom of Saudi Arabia.

Key words: Embryogenesis, Fiber diameter, Pectoral muscle, Thigh muscle, Thermal manipulation