Gastrulation

During gastrulation, the blastula, a hollow ball of cells with radial symmetry, is converted into a three-layered structure with a central midline and bilateral symmetry. The highly dramatic movements of gastrulation are preceded, during late blastula stages, by pregastrulation movements of which the most

A AP

A AP

Fig. 2. Fate maps of Xenopus embryos, including tracing the outer layer of late blastula embryos by marking the cells with Bolton-Hunter Reagent (11). (A) Fate map of the 32-cell embryo (8). (B) Fate maps of the superficial (left) and deep layers (right) of an early gastrula embryo. Reproduced from Keller (12,13). (C) Transverse section of a neurula stage embryo that was dipped in Bolton- Hunter Reagent at a blastula stage to mark the outer layer of cells. The darkly marked cells have migrated during gastrula stages and can be seen lining the archenteron, in the neural tube and in the epidermis. Dorsal is to the top. (D) A sagittal section of a similarly marked embryo showing darkly marked cells in the same tissues. Anterior is to the left and dorsal upward in this photo. AP, animal pole; VP, vegetal pole; V, ventral; D, dorsal; NT, neural tissue; AR, archenteron roof; AF, archenteron floor; EPI, epidermis; BLC, blastocoel; NOT, noto-chord; SOM, somite; LP, lateral plate; PE, pharyngeal endorlerm; HM, head meso-derm; H, heart.

Fig. 2. Fate maps of Xenopus embryos, including tracing the outer layer of late blastula embryos by marking the cells with Bolton-Hunter Reagent (11). (A) Fate map of the 32-cell embryo (8). (B) Fate maps of the superficial (left) and deep layers (right) of an early gastrula embryo. Reproduced from Keller (12,13). (C) Transverse section of a neurula stage embryo that was dipped in Bolton- Hunter Reagent at a blastula stage to mark the outer layer of cells. The darkly marked cells have migrated during gastrula stages and can be seen lining the archenteron, in the neural tube and in the epidermis. Dorsal is to the top. (D) A sagittal section of a similarly marked embryo showing darkly marked cells in the same tissues. Anterior is to the left and dorsal upward in this photo. AP, animal pole; VP, vegetal pole; V, ventral; D, dorsal; NT, neural tissue; AR, archenteron roof; AF, archenteron floor; EPI, epidermis; BLC, blastocoel; NOT, noto-chord; SOM, somite; LP, lateral plate; PE, pharyngeal endorlerm; HM, head meso-derm; H, heart.

obvious is epiboly, a vegetally directed movement of animal hemisphere cells. This results in a thinning of the blastocoel roof and the accumulation of prospective mesodermal cells from a position above the equator of the embryo to a subequatorial location.

The first sign of gastrulation proper is the appearance of a pigmented depression in the dorsal—vegetal quadrant of the embryo—the dorsal lip of the blastopore (Fig. 3). Formation of this blastopore lip, and the associated line of pigment, reflects the formation of the so-called bottle cells. These are a

Fig. 3. Blastopore regions of gastrulae and a dorsal view of an early neurula stage embryo. (A) Dorso-vegetal view of an early gastrula (stage 10.5). Note the darkly pigmented, crescent-shaped dorsal blastopore lip. The animal hemisphere is upward. (B) Vegetal view of a midgastrula (stage 11). The blastopore is now circular. Dorsal is to the top. (C) A postero-ventral view of a late gastrula embryo (stage 12.5). The closing circular blastopore is now smaller than at earlier stages. (D) Dorsal view of an early neurula (stage 15). The more darkly pigmented closing neural tube is evident in the midline. Anterior is to the upper left.

Fig. 3. Blastopore regions of gastrulae and a dorsal view of an early neurula stage embryo. (A) Dorso-vegetal view of an early gastrula (stage 10.5). Note the darkly pigmented, crescent-shaped dorsal blastopore lip. The animal hemisphere is upward. (B) Vegetal view of a midgastrula (stage 11). The blastopore is now circular. Dorsal is to the top. (C) A postero-ventral view of a late gastrula embryo (stage 12.5). The closing circular blastopore is now smaller than at earlier stages. (D) Dorsal view of an early neurula (stage 15). The more darkly pigmented closing neural tube is evident in the midline. Anterior is to the upper left.

group of superficial cells that undergo dramatic changes in shape in which their apices contract and cytoplasm is forced away from the surface of the embryo. This causes the cells to adopt their eponymous bottle-like shape, and the accumulation of pigment granules in the apices of the cells results in the formation of the blastoporal pigment line. The first bottle cells to form do so on the dorsal side of the embryo, but they are soon joined by more lateral and ventral cells, so that the lip becomes an arc, then a semicircle, and finally a complete circle. As development proceeds, the bottle cells are propelled to the interior of the embryo by the mesodermal cells. Eventually, by the midgastrula stage, they respread on the roof of the blastocoel and form part of the archenteron, or gut.

At the same time that the blastopore lip becomes visible on the outside of the embryo, prospective mesodermal cells on the inside (11) begin to migrate across the roof of the blastocoel toward the animal pole. As with the formation of the blastopore lip, the first mesodermal cells to migrate are those at the dorsal side of the embryo, and the movement then spreads laterally and ven-trally, reaching the ventral side by the late gastrula stage. It is only the leading mesodermal cells that migrate; those that follow undergo convergence and extension, processes that occur most vigorously in the prospective notochord and somites. Convergence and extension both result from the directed intercalation of mesodermal cells in such a way that they converge toward the dorsal midline of the embryo and in doing so cause the embryo to extend. An analogy would be to squeeze a tube of toothpaste (not from the bottom, the approach we prefer in a domestic situation, but rather by gripping the whole tube in one's fist). As the toothpaste converges toward the center of the tube, it extends from the hole at the end.

As convergence and extension proceed, the circumference of the blasto-pore becomes smaller, and eventually is reduced to a slit. The cells of the animal hemisphere now cover the entire embryo, and constitute the ectoderm. The mesoderm lies beneath the ectoderm, having reached this position through active movements, such as migration, convergence, and extension. Finally, the endoderm lies within the mesoderm. The endoderm occupies this position partly by default, because the yolk mass is dragged passively into the middle of the embryo by the mesoderm. However, the superficial cells of the early gastrula that go on to form the endoderm (remember that in Xenopus, all the mesoderm is formed from deep tissue [11]) also undergo convergent extension, and these foim the roof and walls of the archenteron, or primitive gut (Fig. 2).

The closure of the blastoporc marks the end of gastrulation proper, and by this stage the three germ layers have reached their definitive positions. The animal-vegetal and dorso-ventral axes of the blastula-stage embryo are no more, and although at first sight it might seem that they can be translated directly into the antero-posterior and dorso-ventral axes of the late gastrula/early neurula stage embryo, this is not so. The earlier, and more dramatic, gastrulation movements on the "dorsal" side of the early gastrula ensure that these cells form most of the anterior tissues of the embryo as well as the entire notochord and much of the somites. The "ventral" side of the early gastrula forms much of the posterior of the embryo, and, especially toward the tail, gives rise even to substantial amounts of somitic tissue.

Although many region-specific markers are expressed during gastrula stages (which can be used in experiments on inductive interactions), little obvious cytodifferentiation has occurred. The most obvious tissue at this stage is the notochord, which has physically separated from the somites that flank it, and has acquired a "stack-of-coins" appearance.

Fig. 4. A late stage tadpole and sections of a similar stage highlighting the tissue organization that results after gastrulation and neurulation are complete. (A) A tadpole (stage 40). Anterior is to the left and dorsal to the top. (B) A section through the anterior region of the tadpole. The neural tube is stained a magenta color owing to the high concentration of nuclei and is the uppermost structure. Beneath the neural tube is the vacuolated notochord, which is flanked on each side by somitic tissue (stained green). Nephtic tubules are evident lateral and ventral to the somites, and the large yolk-filed gut (endoderm) is stained yellow. (C) A slightly more posterior section than (B). The neural tube, notochord, somites, and endoderrn are evident. (D) An even more posterior section than (B) or (C). (E) A section through the tail region. Note the proportionally smaller neural tube and larger somites than in more anterior regions. The notochord is a similar size along the anterior-posterior axis. The magnification in (E) is slightly greater than in (B)-(D).

Fig. 4. A late stage tadpole and sections of a similar stage highlighting the tissue organization that results after gastrulation and neurulation are complete. (A) A tadpole (stage 40). Anterior is to the left and dorsal to the top. (B) A section through the anterior region of the tadpole. The neural tube is stained a magenta color owing to the high concentration of nuclei and is the uppermost structure. Beneath the neural tube is the vacuolated notochord, which is flanked on each side by somitic tissue (stained green). Nephtic tubules are evident lateral and ventral to the somites, and the large yolk-filed gut (endoderm) is stained yellow. (C) A slightly more posterior section than (B). The neural tube, notochord, somites, and endoderrn are evident. (D) An even more posterior section than (B) or (C). (E) A section through the tail region. Note the proportionally smaller neural tube and larger somites than in more anterior regions. The notochord is a similar size along the anterior-posterior axis. The magnification in (E) is slightly greater than in (B)-(D).

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