Influence of Uterine Growth Factors on Blastocyst Expansion and Trophoblast Knob Formation in the Rabbit

The effects of insulin-like growth factors (IGF-1 and IGF-2) on blastocyst expansion, and of basic fibroblast growth (FGF-2) on trophoblast knob formation were studied by in vitro culture of rabbit blastocysts. Both growth factors are present in the uterine secretions. Embryo culture was carried out in Ham's F10 supplemented with polyvinylpyrrolidone in the presence or absence of human recombinant growth factors in concentrations ranging from 1 to 100 ng/ml. IGF-1 stimulated expansion of late preimplantation blastocysts to levels found in vivo; after addition of 10 ng/ml, day-6.0 blastocysts increased their diameter within 24 h to 103% of that of day-7.0 blastocysts expanded in vivo (in comparison to 84% without growth factor), and day-7.0 blastocysts expanded to 108% (in comparison to 76% without growth factor). IGF-1 suppressed the synthesis of a pH 6.3/35-kDa protein. Addition of IGF-2 had no effects. FGF-2 effected formation of trophoblast knobs in day-7.0 blastocysts. After addition of 10 ng/ml, the trophoblast knobs appeared within 12 h of culture. The controls without FGF-2 were negative. The striking increase of FGF-2 concentration in the uterine secretion at day 7.0 is perhaps connected with the formation of trophoblast knobs in vivo. FGF receptor-1 was localized in the trophoblast knobs of day-7.5 blastocysts by the use of immunostaining.