Lettice LA and Slack JM (1993), Properties of the dorsalizing signal in gastrul...

XB-ART-39391

Development 1993 Jan 01;1171:263-71.

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Properties of the dorsalizing signal in gastrulae of Xenopus laevis.

Lettice LA , Slack JM .

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According to the 'three signal model', the regional specification of tissue type within the mesoderm of Xenopus laevis occurs in a process called 'dorsalization'. We have studied the timing and transmission characteristics of this signal, and assessed the dorsalizing activity of the lithium ion and a panel of cytokines.The marginal zone has been fate mapped during gastrulation by colloidal gold labelling and it is shown that the ventral tissue undergoes substantial circumferential expansion. The fate map information is used to provide tissues of constant cellular composition for experiments conducted at different stages.The stage at which dorsalization can occur has been investigated by means of heterochronic dorsal-ventral combinations. The results indicate that the interaction occurs during gastrulation, with a decline in both signal strength and competence of the ventral marginal zone to respond as gastrulation proceeds.The signal is capable of passing through arrangements of membranes that exclude the possibility of cytoplasmic contact, implying that it can be carried by a diffusible morphogen.The effect on the ventral marginal zone of lithium and a number of cytokines has also been studied. While none appears to function as a dorsalizing signal, lithium acts during blastula stages to alter the response to the mesoderm-inducing signal such that the inductions are of a more dorsal character.These data confirm that the dorsalizing signal is independent of and operates later than the signal(s) from the vegetal hemisphere that induce mesoderm during the blastula stages.

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Species referenced: Xenopus laevis
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	Fig. 1. This shows the areas that were excised and grafted in the fate mapping experiments. A shows the view from the vegetal pole and B the side view when the embryo is bisected along the dotted line. (Adapted from Smith and Slack, 1983.)
	Fig. 2. Histogram of the amount of muscle in explants from the lithium treatments. The open bars represents untreated explants from stages 7, 9 and 10, while the hatched bars represent those explants that were treated with lithium at stage 7 (horizontal hatching) and stage 9 (diagonal hatching).
	Fig. 3. Summary of ventral marginal zone expansion during gastrulation. (A) A ventral graft (hatched) when it is first made at stage 10. This 60° area expands during gastrulation, so that, by stage 11 (B), it measures 90°and, by stage 12 (C), it has expanded to 130°. By the end of gastrulation (D), one half of the circumference of the blastopore is surrounded by labelled cells.
	Fig. 4. Dorsalization of ventral tissue. A section of a stage 10 VMZ was labelled with FDA was cultured in combination with an unlabelled stage 12 DMZ. A shows nuclei in the entire combination stained with DAPI. B shows those areas that contain FDA. The epidermis, some mesenchyme and a substantial amount of muscle are labelled, surrounding an unlabelled notochord. C illustrates staining with the anti-muscle antibody, 12-101. The large muscle block encompasses not only those cells FDA labelled, but also those cells immediately adjacent to the notochord. Bar: 10 mm.
	Fig. 5. Explants from transfilter experiments. (A) An explant from a control transfilter combination. Typical mesenchyme (ms), mesothelium (mt), blood (bl) and endoderm (en) are clearly visible. (B) Section of a ventral explant cultured in combination with a dorsal marginal zone. The explants were separated by a 3 mm Nuclepore filter until stage 13, when the apparatus was dismantled. The influence of the dorsalizing signal is seen by the presence of organized blocks of muscle (mu) among the more ventral tissues. Bar: 10 mm.
	Fig. 6. Effects of lithium on ventral marginal zone. The control untreated explant (A) contains the ventral tissues blood, mesenchyme (ms) and mesothelium (mt), while those explants treated at stage 7 (B), around the time of mesoderm induction, have large blocks of muscle (mu) and notochord (no). Explants treated at stage 9 (C) show large blocks of muscle (mu) but are never found to contain notochord, while explants treated from stage 10 (D) through to the end of gastrulation are indistinguishable from the controls. Bar:10 mm.