AVIAN MARGINAL ZONE CELLS FUNCTION AS PRIMITIVE STREAK INDUCERS ONLY AFTER THEIR MIGRATION INTO THE HYPOBLAST

Hypoblast cells of posterior marginal zone origin have been shown prev iously to be the inducers of primitive streak in the avian embryo. Her e we checked: (1) whether the above cells acquire their inductivity wh ile still whithin the marginal zone; (2) can inductivity be found in s upernatants of defined blastodermic regions; (3) can differences in th e electrophoretic pattern be shown between inducing and non-inducing t issue fragments and their conditioned media, which might give a clue a s to what the inductive substance is. The following observations were made: 1. (a) Stage X chick posterior marginal zone cells prior to thei r migration into the hypoblast do not induce a primitive streak, when applied to a stage XIII competent epiblast central disc. (b) A posteri or marginal zone fragment, when applied to an epiblast central disc, e ven after being preincubated for up to 9 hours in vitro, is still non- inductive. (c) Mechanically fragmented stage X posterior marginal zone s when applied as a layer to epiblast central discs are non-inductive. (d) Hypoblastic tissue in strip form induces a primitive streak. 2. C ompetent stage XIII epiblast central discs (chick) were incubated for 2 hours in supernatants of stage XIII epiblasts or hypoblasts. Whereas no inductive effect was exerted by the epiblast supernatant, primitiv e streaks developed in about 50% of the epiblast central discs incubat ed in the hypoblast supernatant. 3. Electrophoretic analysis (quails) reveals a protein of 28x10(-3) M(r) that is enriched in both hypoblast ic tissue and its incubation medium and not in the epiblast + marginal zone + area opaca and their incubation medium. These findings suggest a possible correlation between this protein and the induction process .