An increase in lipid was observed in compact morulae produced in vitro compared with those produced in vivo. Interestingly, this increase occurred regardless of the composition of the medium in which the embryos were cultured. It has been suggested that an increased amount of lipid in embryos cultured in vitro results from uptake of lipid from serum in the culture medium. However, on the basis of data presented in this study, increases in lipid density occurred equally in compact morulae cultured in a completely serum-free medium and in serum-supplemented media. Therefore, the increased volume density of lipid in embryos cultured in vitro may result from membrane breakdown in response to a nonphysiological culture environment rather than uptake from the culture media.
Alternatively, lipid may have accumulated as a result of insufficient metabolism by mitochondria present in compact morulae produced in vitro. This latter mechanism would be consistent with the observed reduction in volume density of mature mitochondria for embryos produced in all three in vitro culture media. The sequestering of lipid substrates into the citric acid cycle occurs in the mitochondria. In addition, the numerous folds of cristae within mature mitochondria contain the enzymes directly responsible for the production of ATP. Therefore, a reduction in the volume density of mature mitochondria may contribute to the buildup of nonmetabolized lipid.