Thermal manipulation during mid-embryogenesis alters myogenic gene expression and muscle morphometry in broilers

This study evaluated the effects of embryonic thermal manipulation (TM) at 39.5°C and 40.5°C for 6 or 12 h daily, from days 7 to 16 of embryonic development (ED), on hatchability, embryo mortality, hatching pattern, chick quality, yolk sac weight, breast muscle gene expression of Heat Shock Protein 70 (HSP70), Myogenic Differentiation 1 (MYOD), Type II Iodothyronine Deiodinase (DIO2), serum hormones and breast muscle histomorphometry at pulling. A total of 4300 hatching eggs from a 39-week-old Ross 308 breeder flock were used. Eggs were randomly assigned to five treatments (860 eggs each) in a completely randomized design: a control treatment (37.5°C and 60% relative humidity (RH) until 18 ED) and four TM protocols applied daily: 39.5°C/6h, 39.5°C/12h, 40.5°C/6h, and 40.5°C/12h, all with 65% RH. The 40.5°C/12h treatment showed the lowest hatchability (31.55%) and Tona chick quality score (75.30) (P < 0.05). Mid-term embryonic mortality was higher in the 40.5°C/12h treatment (18.77%) compared with other treatments (P < 0.05), while late mortality was higher in the 40.5°C/6h (12.30%) and 40.5°C/12h (43.55%) treatments compared with other treatments (P < 0.05). Yolk sac relative weight at hatch was higher in the 40.5°C/12h treatment (15.85%) than in other treatments (P < 0.05). HSP70 gene expression in breast muscle post-hatch was markedly upregulated in 40.5°C/12h (fold change = 380.38) compared with control (1.0), 39.5°C/6h (1.33), 39.5°C/12h (1.13), and 40.5°C/6h (0.7) (P < 0.05). Conversely, MYOD expression was suppressed in all TM treatments (fold change = 0.03, 0.02, 0.03, and 0.03) compared with control (1.00; P < 0.05). At pulling, myocyte area was greater in the 39.5°C/6h (22.23 μm²) and 40.5°C/6h (21.84 μm²) treatments compared with control (21.14 μm²), 39.5°C/12h (19.61 μm²), and 40.5°C/12h (20.26 μm²; P < 0.05). In conclusion, embryonic TM under the 39.5°C/6h program showed potential benefits for early muscle development by downregulating MYOD and increasing myocyte area at pulling, without impairing hatchability or day-old chick quality.