Modifying a chemically defined medium for hamster embryos (HECM-1) has improved the early embryonic development of rat 1-cell embryos to the blastocyst stage. Currently, in a chemically defined medium supplemented with 20 amino acids, designated as modified rat 1cell embryo culture medium (mR1ECM), 80-90% of 1-cell rat embryos can develop to the blastocyst stage. Furthermore, cumulus-enclosed rat oocytes, fertilized during 10-h coculture with epididymal spermatozoa in modified Krebs-Ringer bicarbonate solution (mKRB), developed in mR1ECM to the blastocyst stage. Normal offspring have been produced after transfer of morulae/blastocysts fertilized and developed in vitro. However, recent studies indicated that early development of 1-cell rat and hamster embryos in mR1ECM was reduced when the embryos were collected from oviducts before pronuclear formation or were produced by in vitro fertilization. Their development was maintained by preincubation of embryos in mKRB or by delaying the time of transfer from mKRB to mR1ECM. buy cheap antibiotics
Therefore, one or more factors in mKRB seem to be required during pronuclear formation to achieve successful early development of rat embryos. However, rat embryos fertilized in vitro show a complete developmental block at the 2- to 4-cell stage in mKRB. Only 45-64% of embryos fertilized in vitro could develop to the blastocyst stage even when transferred from mKRB to mR1ECM at 5-30 h after in vitro insemination. Therefore, although mKRB seems to contain critical factor(s) required for sperm penetration and pronuclear formation, mKRB may also contain factor(s) detrimental to early embryonic development. If sperm penetration and pronuclear formation can be achieved during coculture of cumulus-oocyte complexes with spermatozoa for 10 h in modified mR1ECM, the developmental ability of rat embryos fertilized in vitro may be improved. In the present studies, we compared mR1ECM, mKRB, and modified mR1ECM with respect to sperm penetration and developmental ability of rat embryos.