Dlk1 expression marks developing endothelium and sites of branching morphogenesis in the mouse embryo and placenta

The protein product of the Delta-like 1 (Dlk1) gene belongs to the Delta-Notch family of signaling molecules, proteins involved in cell fate determination in many tissues during development. The DLK1 protein is believed to function as a growth factor, maintaining the proliferative state of undifferentiated cells, and is usually down-regulated as immature cells differentiate. The expression pattern of the DLK1 protein has been described in certain human tissues; however, Dlk1 expression is not well understood in the mouse, the most tractable mammalian genetic model system. To better understand the role of Dlk1 in embryonic development, the tissue-specific expression pattern of Dlk1 mRNA during mouse embryogenesis was analyzed by in situ hybridization. In embryonic day 12.5 (e12.5) embryos, high levels of Dlk1 were found in the developing pituitary, pancreas, lung, adrenal, and many mesodermally derived tissues. Strikingly, Dlk1 expression also marks the growing branches of organs that develop through the process of branching morphogenesis. At e16.5, Dlk1 expression is down-regulated in most tissues but remains in the pituitary, the adrenal gland, and in skeletal muscle. In the placenta, expression of Dlk1 is detected in endothelial cells lining the fetal blood vessels of the labyrinth. This pattern is distinct from that seen in the human placenta and suggests a role for Dlk1 in regulating maternal-fetal interactions.