ROSA26 reporter mouse lines and image analyses reveal the distinct region-specific cell behaviors in the visceral endoderm.

The early post-implantation mouse embryo changes dramatically in both size and shape. These morphological changes are based on characteristic cellular behaviors, including cell growth and allocation. To perform clonal analysis, we established a Cre/loxP-based reporter mouse line, R26R-ManGeKyou, that enables clonal labeling with multiple colors. We also developed a novel ImageJ plugin, LP-Clonal, for quantitative measurement of the tilt angle of clonal cluster shape, enabling identification of the direction of cluster expansion. We carried out long-term and short-term lineage tracking. We also performed time-lapse imaging to characterize cellular behaviors using R26-PHA7-EGFP and R26R-EGFP. These images were subjected to quantitative image analyses. We found that the proximal visceral endoderm overlying the extra-embryonic ectoderm shows coherent cell growth in a proximal-anterior to distal-posterior direction. We also observed that directional cell migration is coupled with cell elongation in the anterior region. Our observations suggest that the behaviors of visceral endoderm cells vary between regions during peri-implantation stages.