More than 95% reversal of left-right axis induced by right-sided hypodermic microinjection of activin into Xenopus neurula embryos.
In recent years, genes that show left-right (L-R) asymmetric expression patterns have been identified one after another in vertebrate gastrula-neurula embryos. However, we still have little information about when the irreversible L-R specification is established in vertebrate embryos. In this report, we show that almost 100% of the embryos develop to be L-R-inverted larvae after microinjection of activin molecules into the right lateral hypodermic space of Xenopus neurula embryos. After right-side injection of 10-250 pg activin protein, both early neurulae just after gastrulation movement (stage 13-14) and late neurulae just before neural tube closure (stage 17-18) showed almost 100% reversal of the heart and gut L-R axes. At higher doses of activin, more than 90% of the L-R-inverted embryos showed L-R reversal of both heart and gut. The survival ratio of the right-injected 4-day embryos was 90% on average. In the left-injected embryos, the occurrence of L-R inversion was less than 2% as observed in normal untreated siblings (1.7%). When the same amount of activin (1-50 pg) was microinjected into both sides of neurula embryos, the incidence of L-R inversion was reduced to 58%. The injection of activin along the dorsal midline in the trunk region also randomized the visceral L-R axis. Injection of activin into the right side changed normal left-handed expression of Xnr-1 to right-handed or bilateral expression. In contrast, left-handed expression of Pitx2 was switched to the right side by right activin injection. This is the first report of a method that achieves complete inversion of the visceral L-R axis by treatment of embryos at the neurula stage. Activin not only acts on the neurulae to cancel the original L-R specification up to the late neurula stage, but also rebuilds a new L-R axis whose left side coincides with the injection side. It is suggested that the left and right halves of neurulae have equal potential for L-R differentiation.
Toyoizumi
R
R
Department of Biological Sciences, Faculty of Science, Kanagawa University, Tsuchiya 2946, Hiratsuka, 259-1293, Japan. toyosun@info.kanagawa-u.ac.jp
Mogi
K
K
Takeuchi
S
S
D028341:Activins; D000818:Animals; D019521:Body Patterning / Q000187:drug effects / Q000502:physiology*; D004625:Embryo, Nonmammalian / Q000002:abnormalities / Q000502:physiology*; D005260:Female; D005775:Gastrula / Q000187:drug effects / Q000502:physiology; D006133:Growth Substances / Q000494:pharmacology; D007265:Inhibins / Q000008:administration & dosage / Q000494:pharmacology*; D008297:Male; D008845:Microinjections; D014982:Xenopus laevis / Q000196:embryology*
D006133:Growth Substances; D028341:Activins; D007265:Inhibins