A PRIMARY REQUIREMENT FOR NODAL IN THE FORMATION AND MAINTENANCE OF THE PRIMITIVE STREAK IN THE MOUSE

The 413.d insertional mutation arrests mouse development shortly after gastrulation. nodal, a novel TGF beta-related gene, is closely associ ated with the locus. The present study provides direct evidence that t he proviral insertion causes a loss of function mutation. nodal RNA is initially detected at day 5.5 in the primitive ectoderm. Concomitant with gastrulation, expression becomes restricted to the proximal poste rior regions of the embryonic ectoderm. nodal RNA is also expressed in the primitive endoderm overlying the primitive streak. A few hours la ter, expression is strictly confined to the periphery of the mature no de. Interestingly 413.d mutant embryos show no morphological evidence for the formation of a primitive streak. Nonetheless, about 25% of mut ant embryos do form randomly positioned patches of cells of a posterio r mesodermal character. Data presented in this report demonstrate the involvement of a TGF beta-related molecule in axis formation in mammal s.