Study of the murine allantois by allantoic explants

The murine allantois will become the umbilical artery and vein of the chori oallantoic placenta. In previous studies, growth and differentiation of the allantois had been elucidated in whole embryos. In this study, the extent to which explanted allantoises grow and differentiate outside of the concep tus was investigated. The explant model was then used to elucidate cell and growth, factor requirements in allantoic development. Early headfold-stage murine allantoises were explanted directly onto tissue culture Drastic or suspended in test tubes. Explanted allantoises vascularized with distal-to- proximal polarity, they exhibited many of the same signaling factors used b y the vitelline and cardiovascular systems, and they contained at least thr ee cell types whose identity, gene expression profiles, topographical assoc iations, and behavior resembled those of intact allantoises. DiI labeling f urther revealed that isolated allantoises grew and vascularized in the abse nce of significant cell mingling, thereby supporting a model of mesodermal differentiation in the allantois that is position- and possibly age-depende nt. Manipulation of allantoic explants by varying growth media demonstrated that the allantoic endothelial cell lineage, like that of other embryonic vasculatures, is responsive to VEGF(164). Although VEGF(164) was required f or both survival and proliferation of allantoic angioblasts, it was not suf ficient to induce appropriate epithelialization of these cells. Rather, oth er VEGF isoforms and/or the outer sheath of mesothelium, whose maintenance did not appear to be dependent upon endothelium, may also play important ro les. On the basis of these findings, we propose murine allantoic explants a s a new tool for shedding light not only on allantoic development, but for elucidating universal mechanisms of blood vessel formation, including vascu lar supporting cells, either in the intact organism or in existing in vitro systems. (C) 2001 Academic Press.