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The establishment of heartmesoderm during Xenopus development has been examined using an assay for heart differentiation in explants and explant combinations in culture. Previous studies using urodele embryos have shown that the heartmesoderm is induced by the prospective pharyngeal endoderm during neurula and postneurula stages. In this study, we find that the specification of heartmesoderm must begin well before the end of gastrulation in Xenopus embryos. Explants of prospective heartmesoderm isolated from mid- or late neurula stages were capable of heart formation in nearly 100% of cases, indicating that the specification of heartmesoderm is complete by midneurula stages. Moreover, inclusion of pharyngeal endoderm had no statistically significant effect upon either the frequency of heart formation or the timing of the initiation of heartbeat in explants of prospective heartmesoderm isolated after the end of gastrulation. When the superficialpharyngeal endoderm was removed at the beginning of gastrulation, experimental embryos formed hearts, as did explants of prospective heartmesoderm from such embryos. These results indicate that the inductive interactions responsible for the establishment of heartmesoderm occur prior to the end of gastrulation and do not require the participation of the superficialpharyngeal endoderm.
Fig. 1. Explants of heartmesoderm during neurulation. Anterior is at the left. (A) Tissue explanted from embryos at stages 12·5 to 13. Explants include anteriorlateraltissue up to anterior midline. (B) Tissue explanted from embryos at stages 14 to 19. Explants include anterior lateroventral tissue up to ventral midline. Explants shown in (A) and (B) include the same region of tissue. The difference between (A) and (B) reflects the morphogenetic movements occurring throughout the embryo during neurulation, especially the ventrally-directed movement of the prospective heartmesoderm at the anteriorlateral edge of the mesodermal mantle. All explants included mesoderm and epidermis in the presence or absence of the underlying endoderm.
Fig. 2. Explants of heart-forming regions from stage 19 embryos after seven days in culture. Explants are enclosed within epithelial vesicles. (A) Mesoderm + endoderm. (B) Mesoderm only. Both explants have formed beating hearts (indicated by arrows).
Fig. 3. Explant of mesoderm from a stage 13 embryo after 5 days in culture. Arrows point to foci of beating tissue; bar, 0·4 mm.
Fig. 4. The frequency of heart formation in mesodermal explants in the presence and absence of endoderm. Explants were removed at stages ranging from the end of gastrulation to the end of neurulation. A chi-squared test was used to determine that differences in the frequency of heart formation in the presence and absence of endoderm are statistically insignificant at all stages examined. Sample sizes for explants of mesoderm + endoderm: stage 12·5, n = 18; stage 13, n = 23; stage 14, n = 32; stage 15, n = 17; stage 16, n = 27; stage 17, n = 21; stage 18, n = 23; stage 19, n = 20. Sample sizes for explants of mesoderm: stage 12-5, n = 22; stage 13, n = 37; stage 14, n = 29; stage 15, n = 17; stage 16, n = 25; stage 17, n = 20; stage 18, n = 39; stage 19, n = 24.
Fig. 5. Extirpation of superficial pharyngeal endoderm at the beginning of gastrulation. Broken lines delimit excised tissue. (A) Extirpation of dorsal bottle cells. (B) Extirpation of bottle cells and suprablastoporal endoderm from the entire dorsal half of the embryo.
Fig. 6. Midsagittal view of intact embryos and embryos lacking derivatives of the dorsal bottle cells and suprablastoporal endoderm at stage 14. The blastopore is to the right. (A) Intact embryo. The archenteron has expanded (indicated by arrow). (B) Operated embryo. The archenteron is not visible; B, blastocoel; bar, 0·5 mm.
Fig. 7. Midsagittal sections of operated and intact embryos at stage [in eq]. (A) Intact embryo. (B) Operated embryo; bar, 0·8 mm. (C) Anterior region of intact embryo. The pharyngeal cavity, P, is visible; bar, 0·5 mm. (D) Anterior region of operated embryo. The pharyngeal cavity is occluded by endoderm, E, from more posterior regions.
