A mathematical model of the interaction between bovine blastocyst developmental stage and progesterone-stimulated uterine factors on differential embryonic development observed on Day 15 of gestation

A complex interaction between the developing bovine embryo and the growth potential of the uterine milieu it inhabits, results in an embryo capable of developing past the maternal recognition stage onto a successful pregnancy. Previously we have observed variation in the lengths of embryos recovered eight days after bulk transfer of Day 7 in vitro produced (IVP) blastocysts into the same uterus. Potential causes behind the differential embryonic growth are examined and modelled using two rounds of bulk (n = 4-6) IVP transfers and recovery of these embryos eight days later. Morphological and gene expression measurements of the embryos were performed and the progesterone concentration of the cows was measured throughout as a reflection of the status of the uterine environment. This data was used to assess the model which describes the interaction between the uterine environment and the growth rate of the developing embryo. Expression of six trophectoderm genes was correlated with the conceptus length (P < 0.001). The only gene tested with inconsistent expression was orphan nuclear receptor 5A2 (NR5A2) expression, an activator of alpha-fetoprotein. The model determined that if the embryo develops to blastocyst stage; the uterine environment, driven by progesterone, is a more important component in the stimulation of embryonic growth rate to ensure adequate IFNT for pregnancy recognition than the blastocyst size. Consideration must also be given to any gene expression discrepancies, such as absence of NR5A2 indicated in this trial or irregular ED formation, which may contribute to the pregnancy failure in the following weeks as placentation is initiated.