Preimplantation embryodevelopment invitro: Cooperative interactions amongembryos androle ofgrowth factors (morula/blastocyst/epidermal factor receptor/autocrine/paracrine)

We haveestablished a modelthatshows cooperative interaction amongpreimplantation embryos and therole ofgrowth factors ontheir development andgrowth. Two-cell mouseembryos cultured singly in25-ed microdrops hadinferior development toblastocysts andlower cell numbers perblastocyst compared with those cultured ingroups of5or 10.Theinferior development ofsingly cultured embryos was markedly improved byaddition ofepidermal growth factor (EGF)ortransforming growth factor a or31(TGF-aor TGF-131) totheculture medium. Thestage ofembryonic development, primarily affected bythese treatments, was between eight-cell/morula andblastocyst. Furthermore, bIas- tocysts developed fromeight-cell embryos cultured ingroups or singly inthepresence ofEGFshowed ahigher incidence ofzona hatching compared withthose cultured singly intheabsence of EGF.Detection ofEGFreceptors ontheembryonic cell surface ateight-cell/morula andblastocyst stages suggests beneficial effects ofEGForTGF-aonpreimplantation embryo develop- mentandblastocyst functions. Insulin-like growth factor I (IGF-I) hadnoinfluence onembryo development. Tofurther A'scuMent thecooperative interactions amongembryos, the wvhmeoftheculture mediumwasdoubled to50A.LThis imeaseinculture volume wasevenmoredetrimental tothe development ofsingly cultured embryos. However, this detri- mental effect wassignificantly reversed byEGFandreversed evenmoremarkedly byacombination ofEGFandTGF-131 but notbyTGF-131 alone. Although TGF-131 plus IGF-I caused a modest improvement ofembryo development, theresponse was notasgreat asshownbyEGFalone. Furthermore, IGF-I had noadditive effect onEGF-induced embryonic development. Thestudy presents clear evidence that specific growth factors ofembryonic and/or reproductive tract origin participate in preimplantation embryo development andblastocyst functions inanautocrine/paracrine manner. Thedevelopment ofmousepreimplantation embryos tothe bastocyst stage is, atleast partially, afunction ofautoreg- ulation. Twodistinct features ofthepreimplantation embryo development areactivation andcleavage ofthefertilized egg anddifferentiation oftheembryonic cells into inner cell mass (ICM) andtrophectoderm attheblastocyst stage. Themech- anismbywhichproliferation anddifferentiation arecon- trolled inthepreimplantation embryo isunclear. Although fertilized eggscandevelop into blastocysts within therepro- ductive tract ofsteroid hormone-depleted mice, asubstantial loss inthenumber ofembryos andcells perembryo occurs under this condition. Treatment with anappropriate combi- nation ofprogesterone (P4) andestrogen (E)reverses these defects (1,2).Theseobservations suggest thatalthough certain factors ofembryonic origin participate inanautocrine regulation ofembryonic development, thefull complement of