Preimplantation mouse embryos of various stages of development, from single-cell zygotes to late blastocysts, were fixed on microscope slides and were stained using the periodic acid-Schiff method. Half of the slides were treated with diastase before staining to determine whether the positive material was glycogen.Glycogen was found in all of the stages studied, regardless of whether development had occurred in viuo or in viho. Single-cell zygotes were not as heavily stained as the two-cell embryos, but this could be due to the decrease in volume which occurs between these two stages. Large amounts of positive material were found in the cleavage stages, from two-cell to morula, but a decrease occurred during the growth of the blastocyst. By the late blastocyst stage, glycogen is no longer found in the trophoblast cells and is present only in small amounts in the inner cell mass. The decrease in PAS-positive diastase-removable material during the blastocyst stages suggests that glycogen may serve as a n energy source during the fifth day of development, when the blastocyst is undergoing expansion and hatching. Culture experiments and enzyme analyses have provided evidence that the cleaving embryo undergoes some basic changes in the pattern of energy metabolism, and it is possible that additional changes might occur in the blastocyst stage.The energy metabolism of preimplantation mouse embryos has been investigated recently through in vitro culturing techniques (Brinster, '65a; Brinster and Thomson, '66). These experiments have led to a number of interesting observations regarding the energy requirements of the early embryo. First, cleavage of the onecell mouse zygote will not occur completely in vitro (Biggers, Gwatkin and Brinster, '62) ; second, two-cell mouse embryos will develop into blastocysts in vitro in a medium containing pyruvate or lactate, but not glucose, as the only energy source (Brinster, '65a) ; third, eight-cell embryos will form blastocysts in vitro with pyruvate, lactate or glucose as the only source of energy (Brinster and Thomson, '66).The possibility that an endogenous energy supply might exist in and be ulitized by the early mouse embryo led to the present investigation of glycogen content at various stages of development. Fridhandler ('61 ) has suggested that a stored energy source might be used by the early rabbit embryo. Earlier histochemical studies of ovarian sections of the rat (Wislocki, Bunting and Dempsey, '47; Hater, '48; Deane, ' 5 2 ) , mouse (Goldman, '12), and cow (Kenney, '64) If it could be shown that the mouse embryo utilizes stored glycogen at some time during early development, some of the questions concerning the in vitro nutrient requirements might be more easily answered.
MATERIALS AND METHODSCollection of embryos. Embryos were obtained from eight-week-old Swiss female mice, which were hormonally superovulated and mated with Swiss males. The embryos were collected by removing the fallopian tube, for early stages, or the tube plus uterus for later stages,...
