Kageura H (1997), Activation of dorsal development by contact bet...

XB-ART-16699

Development 1997 Apr 01;1248:1543-51. doi: 10.1242/dev.124.8.1543.

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Activation of dorsal development by contact between the cortical dorsal determinant and the equatorial core cytoplasm in eggs of Xenopus laevis.

Kageura H .

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In eggs of Xenopus laevis, dorsal development is activated on the future dorsal side by cortical rotation, after fertilization. The immediate effect of cortical rotation is probably the transport of a dorsal determinant from the vegetal pole to the equatorial region on the future dorsal side. However, the identity and action of the dorsal determinant remain problematic. In the present experiments, individual isolated cortices from various regions of the unfertilized eggs and embryos were implanted into one of several positions of a recipient 8-cell embryo. The incidence of secondary axes was used not only to locate the cortical dorsal determinant at different times but also to locate the region of the core competent to respond to the dorsal determinant. The dorsal axis-inducing activity of the cortex occurred around the vegetal pole of the unfertilized egg. During cortical rotation, it shifted from there to a wide dorsal region. This is apparently the first evidence for the presence of a dorsal determinant in the egg cortex. The competence of the core of the 8-cell embryo was distributed in the form of gradient with the highest responsiveness at the equator. These results suggest that, in the normal embryo, dorsal development is activated by contact between the cortical dorsal determinant and the equatorial core cytoplasm, brought together through cortical rotation.

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Species referenced: Xenopus laevis

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	Fig. 1. Localization of dorsal activity in the egg and embryos. Each isolated cortex (0.4´0.4 mm-0.4´0.9 mm) was implanted into the ventroequatorial region of a ventrovegetal cell of an 8-cell embryo. For each region, 50 recipients were prepared by cortical implantation. The incidence of secondary axes is shown by a percentage written above the line, and the average relative length of the secondary axis is indicated by a number written below the line. Ap, animal pole; Vp, vegetal pole; D, dorsal; V, ventral. (A) A pricked unfertilized egg. (B) A 2-cell embryo. (C) A 32-cell embryo.
	Fig. 2. Embryos derived from the 8-cell embryos into which a piece of cortex of eggs or embryos was implanted. The implantation site was the ventroequatorial region of a ventrovegetal cell. (A) Thirteen pairs of unequal twins derived from the recipients into which a piece of the VgIII cortex of the pricked unfertilized egg was implanted (stage 41; lateral view). (B) Unequal twins derived from the recipient into which a piece of the VgIII cortex of the pricked unfertilized egg was implanted (stage 41; lateral view). This secondary axis is one of the largest induced by cortex implantation. (C ) Unequal twins derived from the recipient into which two D1 cortices of the 32-cell embryo was implanted (stage 31; lateral view). (D) A normal embryo derived from the recipient into which two D4 cortices of the 32-cell embryo was implanted (stage 31; lateral view).
	Fig. 3. Localization in the 8-cell embryo of the core competence to respond to the dorsal determinant. For each position, 50 recipients were prepared by implantation of the vegetal cortex of the 2-cell embryo. The incidence of secondary axes is shown by a percentage written above the line and the average relative length of the secondary axis is indicated by a number written below the line. Ap, animal pole; Vp, vegetal pole; D, dorsal; V, ventral. VP, at the vegetal pole of the vegetal ventral cell; V30°, at latitude 30° south of the vegetal ventral cell; V45°, at latitude 45° south of the vegetal ventral cell; V60°, at latitude 60° south of the vegetal ventral cell; VV, at the 90° south of the vegetal ventral cell; AV, at latitude 90° north of the animal ventral cell; AP, at the animal pole of the animal ventral cell; VD, at the latitude 90° south of the vegetal dorsal cell.
	Fig. 4. Embryos derived from 8-cell embryos into which the vegetal cortex of the 2-cell embryo was implanted (stage 41; lateral view). (A) Unequal twins. The implanted position is the VV. (B) Normal embryos. The implanted position is the VD.
	Fig. 5. Isolated cortices. The isolated cortices are 4-8 mm thick and consist of two layers. The outer layer is a rigid, parallel peripheral unit, about 2 mm thick. The inner layer is a yolky layer, 2-6 mm in thickness. Bar, 10 mm. (A) The isolated animal cortex. The outer layer contains many pigment granules. The inner layer contains small yolk platelets. (B) The isolated dorsovegetal cortex. In the outer layer, pigment granules are sparse. The inner layer contains large yolk platelets.
	Fig. 6. A model for activation of Before cortical rotation After cortical rotation dorsal development. Ap, animal pole; Vp, vegetal pole; D, dorsal; V, ventral. (A) An unfertilized egg (before cortical rotation). The dorsal determinant is strongly localized in the cortical peel around the vegetal pole. The cortical peel was remarkably thin and 4-8 mm thick. A ‘core factor’, which interacts with the dorsal determinant for activating dorsal development, occurs within the equatorial zone of the core in the form of a gradient and its maximum is centered on the equator. (B) An early embryo (after cortical rotation). The dorsal determinant has been transported from the vegetal pole to the dorsovegetal region. Although the dorsal determinant mainly occurs in the dorsovegetal region, it also extends into the dorsoequatorial and dorsoanimal regions, in some cases. On the other hand, location of the core factor is not changed by cortical rotation. A hypothetical active factor, that is, an ‘ actual dorsal determinant’ is formed in dorsoequatorial region of the core by the interaction between the dorsal determinant and the core factor. Finally dorsal development is activated on the future dorsal side by action of the actual dorsal determinant.