Temporary exposure of ovine embryos to an advanced uterine environment does not affect fetal weight but alters fetal muscle development.

Embryo transfer techniques may result in fetuses that are heavier at birth and that have been described as highly muscled. The aim of this study was to investigate myogenesis in lambs derived from embryo transfer. Embryos were transferred at Day 3 (estrus = Day 0) to a 3 days-advanced uterine environment, maintained there for 3 days, recovered, and then returned to a synchronous (Day 6) uterus; these fetuses comprised the asynchronous group. Control animals were created by synchronous embryo recovery and single transfer at Day 3. Asynchronous transfer did not affect fetal weight or curved crown-rump length between 46 and 135 days of gestation. No differences were detected between groups at Days 110-135 with respect to muscle mass or protein, RNA, and DNA content. However, total muscle fiber number was significantly increased in plantaris muscles from the asynchronous groups at Day 110 and Day 125, suggestive of prolonged hyperplasia. In addition, the levels of Myf 5 protein and the secondary-to-primary fiber ratio were altered in plantaris muscle from the asynchronous group. The growth data are in contrast to previously reported findings. The results show that fetal myogenesis can be altered by very early events in embryogenesis and suggest that any inferences made solely on the basis of fetal or muscle weight may be fallacious.