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.
Laboratory of Embryogenesis, National Institute for Medical Research, Mill Hill, London, UK.
J B
JB
Green
J C
JC
Smith
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