Pii: S0925-4773(99)00082-9

We report that the IIIc transcriptional alternative of Fgfr2 is transcribed in the unfertilized egg and that during early zygotic transcription, messages encoded by both Fgfr2 alternatives (IIIc and IIIb) are present. The Fgfr2 protein was ®rst detected in peripheral blastomeres of compacted morulae. Trophectoderm speci®city of Fgfr2 became obvious in the early blastocyst and with maturation its localization underwent further speci®cation, Fgfr2 concentration increased at the abembryonic pole and decreased at the embryonic pole. Moreover Fgfr2 expression became markedly asymmetrical along the animal-vegetal axis of the mature blastocyst. Our observations indicate a role for Fgfr2 in trophectoderm growth and speci®cation and in the orientation and polarity of the preimplantation conceptus. q 1999 Elsevier Science Ireland Ltd. All rights reserved.