When placed in serum-free medium on reconstituted collagen surfaces re-implantation mouse embryos are capable of producing characteristic trophoblast outgrowths. Previously this pattern of differentiation has been considered to be essentially dependent on the presence of serum macromolecules. Such activity is expressed only at the late blastocyst stage and is qualitatively different from the adhesive interactions between blastomeres earlier in development. The development of the properties responsible for outgrowth is intrinsic to the blastocyst, being independent of stimulation by exposure either to the uterine environment or to whole serum. The significance of these observations related to implantation control in vivo is discussed.

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