Expression of Notch genes and their ligands during gastrulation in the chicken embryo.

Notch signalling is an important evolutionary conserved mechanism known to control cell fate choices through local interactions. Here, the patterns of expression of Notch-1 and -2 genes and their ligands Delta-1, Serrate-1 and -2, were established in the early blastodisc of the chicken embryo from the pre-streak to the first somite stages. Delta-1 was detected as early as the pre-streak stage in the posterior part of the embryo shortly followed in the same region by Notch-1 at the initial streak stage. Thereafter both were strongly expressed in the posterior part of the primitive streak until HH4. Notch-2 was also found at the level of the streak although at low levels. Notch-1 was homogeneously expressed by the epiblast and by mesodermal cells ingressing at the level of the streak whereas Delta-1 expression formed a 'salt and pepper' pattern. The difference between the two was clearly detected by double in situ hybridisation. From the mid-streak to the first somite stages, Notch-1 and Delta-1 expressions appeared in the anterior part of the embryo. Serrate-1 and -2 were not detected at these stages. Taken together, these results constitute a framework for analysing the role(s) for Notch signalling during gastrulation.