Embryonic development generates a variety of complex organisms using diverse and contrasting developmental strategies. For instance, adaptation to seasonal drought by annual killifish leads to a unique developmental pattern in which embryogenesis occurs only after undifferentiated embryonic cells have completed epiboly and dispersed at low density on the egg surface (Fig 1A). Hence, the first stage of embryogenesis requires congregation of embryonic cells to form a single aggregate that gives rise to the embryo proper. How embryonic cells do this is currently unknown. To address this question, we developed a biophysical model of the aggregation dynamics of early killifish embryogenesis. Dispersed cells that initially move by random walk form a circular cell aggregate at a pole of the embryo. However, directed cell motion and changes in migration persistence - suggestive of chemotaxis – are not experimentally observed. Hence, we propose that cell aggregation is driven by self-organizing processes due to the intrinsic motility and polarity of the cells and the interplay between cell adhesion and contact inhibition of locomotion (Fig 1). Our numerical simulations demonstrated that aggregation can indeed spontaneously emerge under a limited set of conditions (Fig 1C). However, the dynamics of aggregation as well as the resulting morphology depart from the experimental data (Fig 1B). This suggests a role for other factors either intrinsic to the self-organizing system or provided by external cues. We are currently exploring these mechanisms. The aggregation phase of annual Killifish is unique among vertebrates and presents the opportunity to dissect the self-organizing principles involved in early organization of embryonic stem cells.
