Molecular and cellular aspects of facultative delayed implantation in the mouse.

Various aspects of RNA, DNA and protein synthesis, as well as cellular fine structure, were examined in mouse embryos during the developmental diapause associated with delayed implantation, and during the reactivation of the embryo either by hormonal administration or by culture in vitro. The findings from these studies demonstrate that a cessation of DNA synthesis and mitosis, and a marked decline in the level of protein synthesis, but not of RNA synthesis, accompany diapause. Reactivation of the blastocyst results in the resumption of DNA synthesis and cell division, as well as in quantitative and qualitative changes in protein synthesis. At the fine-structural level diapause is indicated by the accumulation of lipid-like vacuoles, microfilaments, a basal lamina coating trophectodermal cells and the disassembly of polysomes into ribosomes. Significantly, nucleolar morphology remains unaltered during diapause and cisternae of the rough-surfaced endoplasmic reticulum persist at least for the first five days of delay. Reactivation of diapausing blastocysts is associated with the reassembly of polysomes and the accumulation of large quantities of an amorphous material within the cisternae of the rough-surfaced endoplasmic reticulum. Studies of blastocyst growth and development in vitro suggest experimental approaches to the question of the regulation delayed implantation. Finally, the role of ionic balances and concentrations in the control of the onset, maintenance and termination of delayed implantation in the mouse is discussed.