Bovine yolk sac: from morphology to metabolomic and proteomic profiles

In several species, placentation involves the presence of two different membranes responsible for maternal-fetal exchanges: the yolk sac and the chorioallantoic placenta. The yolk sac plays important roles in embryonic survival, mainly during the early stages of gestation. In bovine, it is a transitional membrane that is present until day 50-70 of pregnancy. Herein, we evaluated the morphological and molecular aspects of the yolk sac of bovine embryos during 24 to 52 days of gestation. A total of 69 embryos were allocated into three groups according to the crown-rump length and estimated ages. Yolk sac samples were then subjected to morphological and molecular analysis using mass spectrometry and nuclear magnetic resonance techniques. In contrast to alanine, which was observed only in Group I, during all gestational stages, we identified important metabolites such as aspartate, taurine, glycerophosphocholine, creatinine, creatine, hydrouracil, glutamate, glutamine, lactate, lysine, valine, myo-inositol, cadaverine, and choline. In addition, 314 random sequences of proteins were identified in the bovine yolk sac, and 47 of these were considered to be specific. Changes in alpha-fetoprotein and carcinoembryonic antigen concentrations during gestation were also evaluated. In conclusion, the majority of these proteins are related to the development of secondary metabolites that are involved in the activation of other proteins and metabolites, and in signaling pathways that are responsible for maternal-fetal exchanges, activation of programmed cell death mechanisms, and cellular differentiation, and also in proteins that are responsible for the yolk sac involution that is required to establish chorioallantoic placentation.