Melatonin supplementation does not improve ovine pre- or post-hatching development in vitro

Melatonin plays a crucial role in the regulation of reproductive seasonality in sheep. Melatonin supplementation has been widely used both in vivo to increase fertility and prolificacy, and in vitro during oocyte maturation, fertilization, and embryo culture. However, its effects have only been assessed in vitro up to the hatched blastocyst stage in conventional embryo culture systems. This study aimed to evaluate the effects of melatonin supplementation from oocyte in vitro maturation (IVM) through post-hatching embryonic stages in vitro on the development of the first three embryonic lineages. Supplementation with melatonin at 10⁻⁸ or 10⁻⁶ M during IVM, in vitro fertilization (IVF), and in vitro culture (IVC) did not affect cleavage and blastocyst rates at day (D) 8, nor embryo survival and growth at D12. While hypoblast development was not affected, epiblast survival was reduced in D12 embryos treated with 10⁻⁶ M melatonin from the blastocyst stage onward. In conclusion, melatonin supplementation did not show a clear beneficial effect on ovine in vitro embryo production or lineage development during post-hatching embryo culture.