The murine Foxf1 gene encodes a forkhead transcription factor expressed in extra-embryonic and lateral plate mesoderm and later in splanchnic mesenchyme

The body plan of the vertebrate embryo is established during gastrulation by the formation of mesoderm, separating the ectodermal and endodermal germ layers. Embryonic mesoderm, formed in the primitive streak, migrates anteriorly and is divided along the mediolateral axis into distinct populations including axial, paraxial, intermediary and lateral plate mesoderm. The antagonistic activities of BMP4 and noggin control mediolateral differentiation of mesoderm and high BMP4 signaling promotes lateral plate formation (Tonegawa et al., 1997; Tonegawa and Takahashi, 1998). The lateral plate is further subdivided into somatopleure and splanchnopleure by a split in the mesoderm which creates the coelom, or body cavity. Cues provided by the ectoderm and endoderm induce subdivision of the lateral plate and ectodermal BMP signaling (BMP2 and/or BMP7) has been implicated in this process (Funayama et al., 1999). Mesoderm also contributes to extra-embryonic structures such as amnion, allantois and yolk sac. The amnion, which is the extra-embryonic extension of the somatopleure, is a thin membrane that encloses the embryo. It consists of two connected monolayers of flattened cells, one mesodermal and one ectodermal. The composition of the amniotic fluid is determined by active transport of metabolites across the amnion and damage to this barrier will disturb embryonic development (Chang et al., 1996). The allantois is a mesodermal structure formed as an outgrowth from the posterior primitive streak. Originally an adaptation to terrestrial life in oviparous animals, the allantois in mammals forms the blood vessels of the umbilical cord and provides the embryonic link to the placenta through fusion with the chorion (Downs, 1998). Vascularization of the allantois is the result of de novo blood vessel formation (vasculogenesis), which begins in its distal part (Downs et al., 1998) and requires VEGF signaling through the receptor, Flk1 (Kdr; Shalaby et al., 1995). The fusion of chorion and allantois is essential for placentation and is mediated by α4-integrin on chorion cells binding to vascular cell adhesion molecule (VCAM1) on the distal allantoic cells (Gurtner et al., 1995; Kwee et al., 1995; Yang et al., 1995). The mammalian yolk sac is another evolutionary legacy from embryonic development inside an egg. It consists of an endodermal and a mesodermal layer, and is continuous with 155 Development 128, 155-166 (2001) Printed in Great Britain © The Company of Biologists Limited 2001 DEV3232