Early Cleavage Stages

It is important to note that early stages of Xenopus development rely completely on maternal stores of RNA and protein; transcription does not begin until the so-called midblastula transition, about 7 h after fertilization and when there are 4096 cells (that is, after 12 cleavages) (7).

The first Xenopus cell cycle occupies about 90 min at 21 °C; subsequent cycles last about 30 min. One of the greatest advantages of Xenopus as a tool for developmental biology is that cleavage planes are regular and result in the

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Fig. 1. Xenopus eggs and embryos at cleavage and blastula stages. (A) Unfertilized eggs. The darkly pigmented animal hemisphere is easily distinguishable from the lighter vegetal hemisphere. The swollen translucent jelly coat is evident around each. (B) Animal view of a one-cell embryo after removal of the jelly coat. (C) A two-cell embryo. The cleavage plane divides the embryo into the future right and left halves along the animal-vegetal axis. (D) Animal view of a four-cell stage embryo. Note the pigmentation differences between the smaller, more lightly pigmented presumptive dorsal blastomeres (the upper two) and the presumptive ventral blastomeres. (E) A 16-cell embryo. The smaller animal blastomeres are resting on the larger vegetal cells. (F) Animal view of 32- to 64-cell embryos. The lightly pigmented presumptive dorsal side (to the left) is easily recognized in the embryo in the center. (G) The same embryos as in (F) at a midblastula stage (stage 8). The embryos consist of approx 4000 cells. (H) The same embryos at a later blastula stage (stage 9). Note the smaller size of the cells than in (F) and (G). (I) Embryos at a very late blastula stage, just prior to the beginning of gastrulation. The images in (B)-(E) are photographed at the same magnification. The embryos are approx 1.3 mm in diameter.

formation of identifiable blastomeres whose fates may be predicted. It is important to emphasize, however, that the fate maps described here, and by other workers, apply only to embryos in which the cleavages are archetypical. Sometimes (indeed, quite often), cleavages are inegular, and although the embryo develops perifectly normally, it is not possible in these cases to predict the fates of particular cells.

Bearing these provisos in mind, the first cleavage in Xenopus separates the future leftand right-hand sides of the embryo. The second, 30 min later, is at right-angles to the first and separates the future dorsal and ventral halves. At this four-cell stage, it is often possible to distinguish the two dorsal blastomeres from their ventral counterparts: if the first two cleavages have occurred in a regular pattern, the dorsal blastomeres are usually slightly smaller and more lightly pigmented than the ventral cells.The third cleavage, which occurs after another 30 min, is orthogonal to the first two, and separates the animal and vegetal poles. Unlike the first two divisions, however, which divide the embryos into (roughly) equal pieces, the plane of the third cleavage is well above the equator of the embryo, reflecting a rule that all cells cleave away from the side with more yolky cytoplasm.

After the fourth (meridional) and fiifth (equatorial) cleavages, the embryo comprises 32 cells, arranged in four tiers of eight cells (stage 6 of Nieuwkoop and Faber). The fates of these cells have been determined by microinjection of cell lineage markers (8,9), and the fate map of Dale and Slack is shown in Fig. 2. Notice that individual cells at this stage are far from restricted to a single fate, and that there may be considerable variation from embryo to embryo.

During the early cleavage stages, and beginning with the first, a small space forms becomes larger as cleavages procccd, and eventually becomes the blastocoel.

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