Stages During the Cleavage Period
Kimmel et al., 1995.
Developmental Dynamics 203:253-310. Copyright © 1995 Wiley-Liss, Inc.
Reprinted only by permission of Wiley-Liss, a subsidiary of John Wiley &
2-cell stage (3/4 h):
The first cleavage furrow, ending the first zygotic cell cycle, is vertically oriented, as is usual until the 32-cell stage. The furrow arises near the animal pole and progresses rapidly towards the vegetal pole, passing through only the blastodisc and not the yolky region of the egg (Fig. 4A). Near the bottom of the blastodisc the furrow changes to a horizontal orientation to undercut the blastodisc in the fashion described originally by Wilson (1889) for the sea bass, but still leaves the cells only partly cleaved from the underlying yolky region. The two blastomeres are of equal size and appear otherwise undistinguished from one another.
The following several cleavages are strictly oriented relative to the first one. However, the eventual axes of body symmetry (i.e. the dorsal-ventral and anterior-posterior axes) apparently cannot be predicted with any certainty from the orientation of the cleavage (Kimmel and Warga, 1987, Abdelilah et al., 1994, Helde et al., 1994), despite some reports to the contrary (Strehlow and Gilbert, 1993; Strehlow et al., 1994).
4-cell stage (1 h):
The two blastomeres cleave incompletely (Fig. 4B) and in a single plane that passes through the animal pole at right angles to the plane of the first cleavage. Hence, cycle 3 begins with 4 blastomeres in a 2x2 array. A view from the animal pole ("animal polar view", cartooned in Fig. 6) reveals that the blastodisc is ellipsoidal in shape. The second cleavage plane is oriented along the longer axis.
8-cell stage (1 1/4 h):
Cleavages ending cycle 3, still incomplete, occur in two separate planes, parallel to the first one, and on either side of it. They cut the blastodisc into a 2x4 array of blastomeres. As the dechorionated embryo usually lies in a dish, the 4-cell aspect, rather than the 2-cell aspect, faces the observer. This "face" view (Kimmel and Law, 1985a) is along the odd numbered cleavage planes (furrows 1 and 3 are visible; Fig. 7). The dechorionated embryo tends to lie in the same orientation through late blastula stages and this view is the one shown in the first part of Fig. 1 (through the high stage), Fig. 4 (except for Fig. 4B), and Fig. 7.
16-cell stage (1 1/2 h):
The fourth set of cleavages also occur along two planes, parallel to and on either side of the second one, and produces a 4x4 array of cells. Use care to distinguish this stage from the 8-cell stage, because they look similar in face view (Fig. 4C, D).
For the first time some of the cells now become completely cleaved from the others. These 'complete' cells are the 4 most central blastomeres, the quartet that is entirely surrounded by other cells in Fig. 6E. Their complete cleavage occurs near the end of the 16-cell stage because of the way the cleavage furrows undercut the blastodisc from the center, going outwards towards the blastodisc margin. Indeed, the undercutting furrows still do not reach the margin, and the 12 cells surrounding these 4 central ones, the so-called marginal blastomeres, remain connected to the yolk cell by cytoplasmic bridges (Kimmel and Law, 1985a). From this stage onwards until the midblastula period the cleavages completely partition most or all of the nonmarginal blastomeres, but still incompletely partition the marginal ones.
32-cell stage (1 3/4 h):
The cleavages ending cycle 5 often occur along four parallel planes, rather than two, lying between those of the first and third cycles. However, oblique orientations of the furrows are now common. Frequently the 32 blastomeres of this stage are present in a 4x8 array, but other regular patterns, as well as irregular ones involving one or more of the blastomeres also occur (Kimmel and Law, 1985b). In a side view one usually sees two tiers, or horizontal rows, of blastomeres between the margin and the animal pole (Fig. 4E and Fig. 7). This is because the plane of the blastodisc is curved; marginal cells are more vegetal, and they lie partly in front of the nonmarginal ones positioned closer to the animal pole.
64-cell stage (2 h):
Cleavages ending the sixth cycle pass horizontally, so that in an animal polar view the blastomere array may look similar to the 32-cell stage, although the cells entering cycle 7 are smaller. From the side the cell mound looks distinctly higher (Fig. 1, Fig. 4F, and Fig. 7). For the first time some of the blastomeres completely cover other ones. The buried cells, or deep cells, each arise as one of the two daughters of the 4 central blastomeres that were present at the 32-cell stage. The other daughter remains superficial, in the top-most tier of what is now the enveloping layer (EVL) of the blastodisc. During the same cleavage the horizontal divisions of marginal blastomeres present at cycle 6 produce two EVL sister cells, and in a face view of the 64-cell stage one sees three tiers of EVL cells (Fig. 7).
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