All components required for the eventual activation of muscle-specific actin genes are localized in the subequatorial region of an uncleaved amphibian egg.

Abstract

Fertilized Xenopus eggs have been ligated with a hair loop into separate fragments before the first cleavage. The plane of the ligation was varied in relation to the animal-vegetal and dorso-ventral axes. The fragments that contained a nucleus were cultured for 24 hr until controls reached the neurula stage; they were then analyzed by S1 nuclease protection for their content of muscle-specific actin mRNA, using a gene-specific probe. We find that all egg components required for the eventual activation of these actin genes are localized, already at the 1-cell stage, in a region below the equator, and mostly on the dorsal (grey crescent) side. This material subsequently occupies the equivalent position in 8-cell and 32-cell embryos. We interpret our results, in combination with the previous work of others, to mean that mesoderm (including muscle) formation in Amphibia depends both on cytoplasmic substances already localized in the egg as well as on inductive cell interactions during cleavage.An important question in embryology is whether a fertilized but uncleaved egg contains substances already localized in a way that is related to future cell differentiation, or whether such substances first appear and become localized at a later multicellular stage. The question is significant because the localization of materials in a single cell, the egg, is most likely to depend on large molecules or complexes, whereas cell interactions may well involve small molecules capable of passing rapidly from cell to cell.There are several examples in invertebrate animals in which one region of egg cytoplasm seems to be related to a particular type of differentiation. The most celebrated cases are insect pole plasm (1) (related to egg and sperm formation) and the yellow cytoplasm in ascidian eggs (related to tail muscle formation) (reviewed in refs. 2 and 3). A more common situation is that in which a particular region of egg cytoplasm becomes localized within a certain cell lineage by asymmetric early cleavage divisions, as in the photocyteand cilia-forming cells of Ctenophores (4). In the vertebrates, the only definite example of either kind is the germ plasm of Amphibia (5), which appears to be functionally equivalent to the pole plasm of insects. Here we describe egg ligation experiments in Xenopus and the use of a gene-specific probe to investigate the localization of materials required for the activation of muscle-specific a-actin genes. Rather surprisingly, we find that these materials are already localized in a particular region of the uncleaved egg. MATERIALS AND METHODSEggs and Embryos. Eggs laid after hormone injection were artificially fertilized as described (6). Developmental stages are those of Nieuwkoop and Faber (7).Hair-Loop Ligations. Eggs were dejellied in saline containing 2% cysteine (pH 8.0), washed, and the vitelline membrane was removed manually with forceps. Eggs were ligated with a thin brunette hair-loop in the desired orientation in modified Barth saline (MBS) (8), between 30 and 60 min after fert...

Anteroposterior neural tissue specification by activin-induced mesoderm

“…An example of this is the torpedo͞gurken system in Drosophila (43) but there is ample experimental evidence for fixing of pattern in one axis (D-V) before the other (A-P) in amphibians. Maternal cytoplasmic components are crucial for formation of at least some part of ventral and dorsal mesoderm (44,45). Induction of the remaining dorsal mesoderm by the Nieuwkoop Center in dorsal vegetal cells is initiated during cleavage stages and so presumably involves maternal components as well (46) though the final D-V patterning of mesoderm is not thought to be fixed until gastrula stages (47).…”

Xenopus endo B is a keratin preferentially expressed in the embryonic notochord.

“…Much has already been leamed about early embryonic events required for the induction of the mesoderm in Xenopus and the subsequent formation of the notochord (Gimlich and Gerhart 1984;Dale et al 1985;Gurdon et al 1985;Kimelman and Kirschner 1987;Slack et al 1987;Smith 1987;Rosa et al 1988). However, little is known about the function of the notochord during early embryogenesis.…”

Dorsal Blastomeres in the Equatorial Region of the 32‐Cell Xenopus Embryo Autonomously Produce Progeny Committed to the Organizer

“…Gurdon and his colleagues (5) have shown that all egg components required for eventual activation of muscle-specific actin genes are localized in the dorsal equatorial region of the fertilized egg and subsequently occupy the equivalent position in the 32-cell embryos which includes the B1 C1 blastomeres. These results could be interpreted by supposing that the origin of the organizer is the allocation of morphogenetic determinants to the dorsal equatorial region of the fertilized but uncleaved egg.…”