EXPERIMENTAL CONTROL OF AXIAL PATTERN IN THE CHICK BLASTODERM BY LOCAL EXPRESSION OF WNT AND ACTIVIN - THE ROLE OF HNK-1 POSITIVE CELLS

Small grafts from transfected mammalian cell lines that secrete activi n or express Wnt-1 RNA were made to the marginal zone of entire chick blastoderms in culture. Grafts from appropriate control cell lines pro duced no effects on development, The activin-secreting grafts, implant ed before streak formation, could cause the streak to form opposite th eir marginal position even when this was 180 degrees distant around th e blastoderm from the original presumptive streak site. Alternatively, opposed twin streaks were observed, one at the original presumptive s ite and one in relation to the graft. Witt-expressing grafts implanted early could also reposition axis formation, but only to graft sites w ithin approximately 100 degrees of angular distance from the host's pr esumptive streak origin. No Wnt-induced twinning was observed, Grafts of both experimental cell types intermixed were the most effective in reorientating, twinning, or globally disturbing the axial pattern and led to second axes with the least delay, relative to normal developmen t, in reaching headfold stages. The incidence and distribution of cell s positive for the epitope HNK-1 was investigated during early stages of normal and of experimentally twinned development. Only two nonhypob last regions of HNK-1 expression were consistently observed in normal early development; a sector in the germ wall area opaca, behind the si te of streak formation, and then a localised region of intensely, newl y expressing cells arising in epiblast and in anteriormost parts of th e (epiblast-derived) streak at the half-length streak stage. Both ''ac tivin only'' and ''activin/Wnt'' mixed grafts, although not control gr afts, became surrounded by new sectors of ''germ wall'' HNK-1 positivi ty. Such positivity may therefore mark a cell group with a signaling r ole (but no anatomical participation) in streak initiation. However, t here was no change of the local background incidence of epiblastic HNK -1 positivity in the structure of streaks induced by ''activin only'' grafts. This indicates that most cells of the streak are specified by relatively local induction, rather than deriving from selective aggreg ation. Only grafts including the Wnt-expressing cells gave rise to obv ious new HNK-1 expression within epiblast-derived cells anteriorly, as does the complete normal streak, This suggests that the Wnt class of response pathway can complement the activin one in producing rostrocau dally complete axial pattern, as has been suggested for amphibian deve lopment. 1994 Academic Press, Inc