Individual Epiblast Cells Acquired Invasiveness Precedent to the Primitive Streak Formation in the Chick Embryo

We developed a serum-free culture technique for chick epiblast explants in low density. In the condition free of unknown serum factors, we examined the invasiveness of chick epiblast cells by dissociating the cells into single cells and seeding them onto basement membrane analogues. At stage X–XI of Eyal-Giladi and Kochav's table (1976), 5–11% of the epiblast cells invaded laminin gel or the reconstituted basement membrane (EHS gel). The proportion of invasive cells increased quickly up to 50% at stage XII. Single prestreak epiblast cells on laminin gel still showed invasive behavior even in the simple medium without any growth factors or vitamins. They were also invasive on fibronectin gel, but not on type I collagen gel or a mixture of EHS gel and type I collagen gel. As for regionality, invasive epiblast cells were more densely distributed in the posterior region of area pellucida than in the anterior region. The early commitment of invasiveness in individual epiblast cells precedent to gastrulation was proved in our serum-free culture. In addition, we confirmed that prestreak epiblast cells are already heterogeneous in the invasive potency; they can be classified into two groups, invasive cells and non-invasive ones.