THYROID-HORMONES ON EMBRYO DEVELOPMENT AND APPEARANCE OF MYOSIN HEAVY-CHAIN ISOFORMS IN TURKEYS (MELEAGRIS-GALLOPAVO)

Effects of thyroid hormones on embryo development and the appearance o f myosin heavy chain (MyHC) isoforms were determined by injecting dail y thyroxine (Tk) and methimazole (MMI) to turkey embryos and poults fr om E18 through D8. When injected with T-4 at 0.5 mu g/100 g body weigh t, the morality was increased, but body weights remained comparable to the euthyroid control, When injected with MMI, the hatching was delay ed and body weights were reduced. At doses of 1.0 mg/100 g body weight , no eggs hatched in 32 days of incubation, In Pectoralis muscle, embr yonic, neonatal and adult MyHC isoforms appeared in a sequence, The ne onatal isoform began to replace the embryonic isoform by D2 and the ad ult isoform began to appear at D8, When injected with either T-4 or MM I at the dose of 0.5 mu g or 0.25 mg/100 g body weight, respectively, the appearance of the neonatal isoform at D2 was blocked, However, the appearance of the neonatal isoform to replace the embryonic isoform w as accomplished by D8 despite continued T-4 and MMI treatments, It was concluded that thyroid hormones were involved, but not absolutely req uired for the timely expression of isoforms of myosin heavy chain.