Paraffin sections were stained with hematoxylin and eosin (H&E) or used for immunohistochemical staining as described26 (link), 27 (link). Primary antibodies and detection systems are listed in Supplemental Material. For co-localization experiments, secondary antibodies conjugated with Alexa Fluor 488 or Alexa Fluor 594 (Invitrogen) were used as previously described28 (link), 29 (link). Slides were counterstained with DAPI (Vector Lab). Fluorescent images were obtained using a Zeiss Axioplan2 microscope equipped with an AxioCam MRm digital camera and AxioVision 4.3 Software (Carl Zeiss Microimaging, Thornwood, NY). Flow cytometry was performed using cells isolated from yolk sacs and lungs as described27 (link), 30 (link). Antibodies used for flow cytometry are listed in Supplemental Materials. BrdU was injected i.p. into pregnant females 2 hr prior to embryo harvest. Annexin V kit was from eBioscience. Stained cells were separated using cell sorting (Five-laser FACSAria II, BD Biosciences). Purified cells were used for RNA preparation and qRT-PCR analysis.