Graded changes in dose of a Xenopus activin A homologue elicit stepwise transitions in embryonic cell fate.
The protein XTC-MIF, a Xenopus homologue of activin A and a potent mesoderm-inducing factor, can induce responding animal pole explants to form several different cell types in a dose-dependent manner, higher doses eliciting more dorso-anterior tissues. This graded response, characteristic of classically postulated morphogens, may underlie pattern formation, but the response of intact animal caps to XTC-MIF provides only a crude indication of trends. Here we report the effects of XTC-MIF on dispersed blastomeres rather than intact animal caps. Under these conditions, responding cells distinguish sharply between doses of pure XTC-MIF differing by less than 1.5-fold. Two different response thresholds have been found, defining three cell states. This suggests that XTC-MIF has an instructive effect. Notochord and muscle are both induced in the same narrow dose-range. Mixing treated with untreated cells does not seem to shift the dose thresholds, showing that at least some cells can stably record the received dose of inducing factor.
Green
J B
JB
Laboratory of Embryogenesis, National Institute for Medical Research, Mill Hill, London, UK.
Smith
J C
JC
D000199:Actins / Q000235:genetics; D028341:Activins; D000818:Animals; D001757:Blastomeres / Q000166:cytology* / Q000187:drug effects; D002449:Cell Aggregation; D002454:Cell Differentiation; D004305:Dose-Response Relationship, Drug; D000078404:Epidermal Cells; D004817:Epidermis / Q000196:embryology; D005455:Fluorescent Antibody Technique; D006133:Growth Substances / Q000008:administration & dosage / Q000494:pharmacology*; D007265:Inhibins; D007633:Keratins / Q000235:genetics; D008648:Mesoderm / Q000166:cytology; D009132:Muscles / Q000166:cytology / Q000196:embryology; D009672:Notochord / Q000166:cytology / Q000196:embryology; D012333:RNA, Messenger / Q000096:biosynthesis; D012260:Ribonucleases; D014981:Xenopus / Q000196:embryology* / Q000378:metabolism
D000199:Actins; D006133:Growth Substances; D012333:RNA, Messenger; D028341:Activins; D007265:Inhibins; D007633:Keratins; D012260:Ribonucleases