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Figure 1.
Injection of an Oligo Antisense to VegT mRNA Depletes the Endogenous Maternal VegT mRNA, but Does Not Prevent the Transcription of Zygotic VegT
(A) Northern blot, probed for VegT RNA from embryos derived from one batch of oocytes frozen at the stages indicated. O, 4 ng VT9M oligo injected; U, uninjected. The blot was stripped and reprobed for Xbra and EF1α. The arrowhead points to residual VegT signal that was incompletely stripped before adding Xbra probe.
(B) Northern blot of sibling embryos to those in Figure 1A that were injected with oligo VT9M at the same time as those in Figure 1A and, 24 hr later, were also injected with synthetic VegT mRNA (300 pg) before fertilization (O PLUSPUSSIGN R). The injected mRNA is indicated by the arrow. The appearance of sibling gastrulae treated in the same fashion from this experiment is shown in Figure 5A.
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Figure 5.
The Expression of Mesodermal Markers Is Delayed in VegT-Depleted Embryos, and This Is Rescued by the Injection of VegT mRNA
(A) The appearance of oligo-injected (V), oligo PLUSPUSSIGN VegT mRNA–injected (V PLUSPUSSIGN R), and uninjected sibling control embryos (U) at the midgastrula stage.
(B) Sibling embryos of these were frozen and assayed for mesodermal markers by RT-PCR at the late blastula (stage 9), midgastrula (stage 11), and early neurula stage (stage 13). Relative levels of expression were determined normalized to EF1α
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Figure 2.
The Phenotype of Maternal VegT-Depleted Embryos
The appearance of embryos depleted of VegT mRNA as oocytes at the late tailbud stage (stage 34). Oocytes were uninjected (U), injected with 4 ng of oligo (L), or with 5 ng of oligo (H). The arrow points to the cement gland.
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Figure 3.
The Development of VegT-Depleted Embryos
(A) Gastrulae derived from oocytes depleted of maternal VegT are unable to form normal blastopores, compared to sibling uninjected control embryos. Arrow points to the ectopic ingression site that appears at the late gastrula stage in VegT-depleted embryos.
(B) Neurulae from the same batch of embryos shown in (A). The neural folds of VegT-depleted embryos are nonpigmented.
(C) Tailbud embryos from the same batch of embryos as shown in (A). V, VegT-depleted using 5 ng oligo; U, uninjected control.
(D) (stage 12) and (E) (stage 16), VegT-depleted embryos. Dye marks were placed on the vegetal poles of VegT-depleted embryos at the late blastula stage and photographed here at sibling late gastrula stage (D) and midneurula stage (E). The dye marks pass inside the embryos during this time.
(F) Whole-mount in situ hybridization to show the location of the neural marker NCAM within the unpigmented cells in a VegT-depleted tailbud stage embryo.
(G–J) Histological sections of control (G and I) and VegT-depleted embryos (H and J). A comparison between (G) and (H) shows the abnormal ventral vesicle in (H). In (H), one animal cell (A1) of the embryo was injected with β-galactosidase mRNA at the 32-cell stage, and descendant blue cells lie in the wall of the ventral vesicle. (I) and (J) show a higher power comparison of the dorsal axial structures of a control (I) and VegT-depleted (J) embryo. In (J), a single vegetal cell was injected with β-galactosidase mRNA at the 16-cell stage, and blue progeny were located in the neural tube and notochord.
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Figure 4.
Zygotic Endodermal Differentiation Markers Are Not Expressed in Maternal VegT-Depleted Embryos and Are Rescued by the Injection of VegT mRNA
(A) and (B) show RT-PCR analysis of the levels of expression of the endodermal markers endodermin (edd), Xsox17α, Xlhbox8, insulin, and IFABP. EF1α was assayed as a loading control. (A) Pairs of control (uninjected), VegT-depleted (oligo), and Oligo PLUSPUSSIGN VegT RNA (oligo PLUSPUSSIGN VegT) injected embryos frozen at stage 33, and (B), at stage 38 for analysis.
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Figure 6.
In VegT-Depleted Embryos, Mesoderm Forms in the Vegetal Mass and Not in the Equatorial Zone
(A) shows the dissection carried out on control, uninjected and VegT-depleted midblastulae to explant the three regions, the animal caps, equatorial zones, and vegetal masses.
(B) shows the appearance of the equatorial explants from control (U), 3, 4, and 5 ng oligo-injected blastulae after culture to sibling early tailbud stage.
(C and D) Vegetal masses derived from control (C) and 3 ng oligo-injected (D) blastulae after culture to sibling early tailbud stage. These are from the same experiment as shown in Figure 6B and Figure 6F.
(E) RT-PCR analysis of mesodermal markers from animal (cap), equatorial (equator), and vegetal (base) explants from uninjected and VegT-depleted embryos cultured to the midneurula stage (stage 16). The markers VegT, Xwnt 8, goosecoid (gsc), and Xbra are expressed strongly in the equatorial regions of controls and in the vegetal masses of VegT-depleted embryos. EF1α is used as a loading control.
(F) Oocytes were uninjected or injected with 3 or 5 ng oligo, and equatorial zones (Eq.) and vegetal masses (Bs.) were dissected at the midblastula stage. The explants were cultured until the early tailbud stage and then analyzed by RT-PCR for ectodermal and mesodermal markers.
(G) Whole embryos from the same experiment as Figure 6B–D and Figure 6F analyzed by RT-PCR for the expression of muscle markers at the early tailbud stage.
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Figure 7.
Maternal VegT Expression Is Required for Mesoderm-Inducing Signals to Be Released by Vegetal Cells
(AâD) Nieuwkoop recombinants cultured to the early neurula stage. (A) Uninjected animal cap/ uninjected vegetal mass. (B) VegT-depleted animal cap/uninjected vegetal mass. (C) Uninjected animal cap/VegT-depleted vegetal mass. (D) VegT-depleted animal cap/VegT-depleted vegetal mass. (E) Animal caps cultured on vegetal masses for 2 hr and then separated and cultured to sibling early tailbud stage. These caps were analyzed by RT-PCR in (F). Upper left are uninjected caps that were cultured with wild-type bases for 2 hr (wt/wt). Upper right are animal caps from VegT-depleted embryos cultured with wild-type bases (VgT/wt). Lower left are wild-type caps cultured with VegT-depleted bases (wt/VgT), and lower right are VgT-depleted caps cultured with VegT-depleted bases (VgT/VgT).
(F) RT-PCR analysis on animal caps shown in (E) for mesodermal and epidermal markers.
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Figure 8.
The Role of Maternal VegT in Tissue Specification
(A) The fates of the three regions of the blastula are altered in VegT-depleted embryos. The vegetal mass forms endoderm in wild-type embryos; in VegT-depleted embryos, it forms mesoderm, epidermis, and neural tissue and undergoes convergence extension and ingression movements. The equatorial zone forms mesoderm in controls, but it forms epidermis and CNS in VegT-depleted embryos and does not undergo gastrulation movements. The animal cap forms both neural and epidermal derivatives of ectoderm in wild-type embryos, but forms only epidermal tissue in VegT-depleted embryos.
(B) Models for mesoderm and endoderm tissue formation. In Model 1, maternal VegT is required only in the vegetal mass at MBT for the formation of endoderm. Vegetal mass cells then induce overlying equatorial cells to form mesoderm. In Model 2, maternal VegT is required separately in both the equatorial zone and the vegetal mass at MBT. In the equatorial region, it acts with another factor (X) to activate mesoderm formation. In the vegetal mass, a second factor (Y) is present that, together with VegT and X, activates endodermal differentiation. In Model 3, there is a gradient of maternal VegT activity at MBT. At low concentration, VegT activates mesodermal differentiation pathways, while at high concentration, it activates endodermal differentiation pathways.
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