Evidence for the Requirement of Autocrine Growth Factors for Development of Mouse Preimplantation Embryos in Vitro1

Abstract

The mitotic stimuli in the early mammalian embryo have not been unequivocally identified. One hypothesis is that the embryo releases autocrine growth factors (GFs) that have a role in such growth. To determine whether such putative GFs were limited by dilution, and hence secreted, development was observed at various embryo concentrations in culture. Embryos were collected at the zygote or 2-cell stage. Zygotes were produced by fertilization in situ (ISF) or in vitro (IVF). Two-cell-stage embryos had a high rate of development to the blastocyst stage across an embryo concentration range of 1/microl-0.001/microl. By contrast, zygotes produced by either ISF or IVF were adversely affected by reducing the embryo concentration over this range (p < 0.001), with approximately 80% of ISF zygotes developing to blastocysts at the highest concentration but only 26% at the lowest. For IVF zygotes the corresponding results were 64% and 6%. For all three embryo types, the number of cells in each blastocyst was significantly lower with reduced embryo concentration. The major determinant of zygote development was the concentration of embryos in culture rather than the absolute volume of culture medium or the actual number of embryos present. A concentration of 1 embryo/microl (in the form of 10 embryos/10microl) gave the best development rates and highest cell numbers per blastocyst. Varying the albumin concentration influenced development rates; a 10-fold reduction in BSA concentration (to 0.3 mg/ml) resulted in significantly more IVF zygotes developing to the blastocyst stage. Platelet-activating factor (PAF) is released by embryos, and albumin can act as a competitive inhibitor of PAF's action on cells. ISF embryos released more PAF (p < 0.05) into media than did similarly treated IVF embryos. There was no difference in the amount of PAF remaining associated with the resulting 2-cell embryos. The amount of PAF released by both these groups was markedly less (p < 0.001) than the amount released by 2-cell embryos collected fresh from the reproductive tract and cultured for 24 h. PAF supplementation of media caused a significant increase in the rate of blastocyst development of IVF zygotes at embryo concentrations of 0.1/microl (1 ng/ml) and 0.01/microl (100 ng/ml). Insulin-like growth factor (IGF)-I (30 ng/ml) and IGF-II (1 ng/ml) also stimulated development of IVF zygotes when cultured at an embryo concentration of 1/10 microl. Epidermal growth factor was without effect over the range 0.2-2000 ng/ml. Supplementation of media with both PAF and IGF-II gave no additional benefit over that caused by IGF-II alone, but this treatment was marginally better (p < 0.05) than PAF treatment alone. The results show that factors necessary for normal embryo development are diluted to suboptimal levels during culture at low embryo concentration. The ability of PAF, IGF-I, and IGF-II to partially compensate for the adverse effects of low embryo concentration during culture is consistent with their having roles as autocrine embryotrophic factors. The use of IVF and low embryo concentrations in culture may provide a functional multiple ablation model that will help to define the range of GFs required for normal embryo development.