Effects of broodmares' age, parity and lactation on the embryo, the placenta and foal development.

International audienceAlthough puberty can occur as early as 14ʹ15 months of age, depending on breed and use, the reproductive career of mares may continue to advance age. Once mares are used as broodmares, they usually produce foals once a year until they become unfertile. The objective of this work was to study the effects of age, parity and lactation of the mare at insemination on the embryo, placenta at term,lactation and foal. Groups of young or old, nulliparous/primiparous or multiparous mares, barren or nursing at the time of breeding were used. Gene expression was analyzed by RNA sequencing in embryos and placentas. Placental morphology was studied by histology and the growth and metabolism of the foals was monitored to 18 months.Effect of age: Mares >10 years old were less fertile, with significant gene expression changes in the embryo, especially in the inner cell mass, related to energy metabolism and mitochondria. Their placenta was macroscopically similar to that of younger mares but with histological and gene expression changes indicating functional adaptations resulting in the birth of a normal foal. These effects were amplified in nulliparous animals.Effect of parity: In young nulliparous mares, modifications in gene expression were observed in the embryo, but embryo viability was not affected. Placental growth, however, was reduced, resulting in intrauterine growth retardation. At birth, the foals were smaller, but they catch up in their postnatal life.Effect of nursing: Changes in placental gene expression were observed in mares that were lactating at the time of insemination, but normal sized foals were produced. Nevertheless, their growth is reduced at the end of lactation compared to controls, possibly related to a less efficient lactation.Altogether, these data indicate that maternal age in mares still considered as young (10 years old in a mare would be equivalent to 30-35 years of age in a woman) affect embryo and placental gene expression and development, even though the offspring phenotype appears to be normal. Moreover, nulliparity is associated with intra-uterine growth retardation and combined ageing and nulliparity seems to worsen age-related effects. Finally, nursing at the time of conception also affects the offspring post-natal growth. These effects should be considered in the management of individuals in the horse industry and may be indicative of potential confounding factors to be investigated when studying DOHaD in other species