Question

1. After fertilization, all embryos undergo cleavage. In the Xenopus embryo, what is the relationship of the animal-vegetal axis to the plane of the first three cleavages? Describe how cleavage in the chick embryo differs from cleavage in frogs and mice; why do these differences occur?

Answer 1

After fertilization, Xenopus embryos undergo cell cycles that have characteristic features. During the first, 90-minute cell cycle, cortical cytoplasmic movements and male and female pronuclear fusion occur. The next eleven divisions occur at 20- to 30-minute intervals with no gap phases, while the embryo forms a ball of 4000 cells, which encloses a fluid-filled blastocoel cavity. This mid-blastula embryo has three regions, the animal cap (which forms the roof of the blastocoel), the equatorial or marginal zone (the walls of the blastocoel) and the vegetal mass (the blastocoel floor) . Although all mid-blastula cells are pluripotent (Heasman et al., 1984), explants of the animal cap form ectodermal derivatives in culture, while equatorial explants form mesoderm and vegetal explants form endoderm. At the end of the twelfth cycle, gap phases reappear, the cell cycle lengthens to 50 minutes and zygotic transcription starts (this is called the mid-blastula transition, MBT). In the 15th cycle, the dorsal lip of the blastopore forms, the cell movements of gastrulation begin and mitosis stops. Gastrulation converts the embryonic ball into three layers, and establishes definitive anteroposterior and dorsoventral axes. In this review, I retrace this developmental pathway and ask how cells become committed to specific fates.

Most organs were populated largely by the descendants of particular sets of blastomeres, the progeny of each often being restricted to defined spatial addresses. Thus, the descendants of any one blastomere were distinct and predictable when embryos were preselected for stereotypic cleavages. However, variations among embryos were common and the frequencies with which one may expect organs to contain progeny from any particular blastomere are reported. The differences in the fates of the 16-cell-stage blastomeres and their 32-cell-stage daughter blastomeres are outlined and can be grouped into three general categories.

Depending mostly on the amount of yolk in the egg, the cleavage can be holoblastic (total or entire cleavage) or meroblastic (partial cleavage). The pole of the egg with the highest concentration of yolk is referred to as the vegetal pole while the opposite is referred to as the animal pole.

Cleavage differs from other forms of cell division in that it increases the number of cells and nuclear mass without increasing the cytoplasmic mass. This means that with each successive subdivision, there is roughly half the cytoplasm in each daughter cell than before that division, and thus the ratio of nuclear to cytoplasmic material increases.

Cleavage in most frog and salamander embryos is radially symmetrical and holoblastic, just like echinoderm cleavage. The amphibian egg, however, contains much more yolk. This yolk, which is concentrated in the vegetal hemisphere, is an impediment to cleavage. Thus, the first division begins at the animal pole and slowly extends down into the vegetal region . In the axolotl salamander, the cleavage furrow extends through the animal hemisphere at a rate close to 1 mm per minute. The cleavage furrow bisects the gray crescent and then slows down to a mere 0.02–0.03 mm per minute as it approaches the vegetal pole .

Although mouse development is regulative, the cleavage pattern of the embryo is not random

The first cleavage tends to relate to the site of the previous meiosis . Sperm entry might provide a second cue, but evidence for and against this is indirect and has been debated

To resolve whether sperm entry position relates to the first cleavage, we have followed development from fertilization by time-lapse imaging. This directly showed cytokinesis passes close to the site of the previous meiosis and to both the sperm entry site and trajectory of the male pronucleus in a significant majority of eggs. We detected asymmetric distribution of Par6 protein in relation to the site of meiosis, but not sperm entry. Unexpectedly, we found the egg becomes flattened upon fertilization in an actin-mediated process. The sperm entry position tends to lie at one end of the short axis along which cleavage will pass. When we manipulated eggs to change their shape, this repositioned the cleavage plane such that eggs divided along their experimentally imposed short axis. Such manipulated eggs were able to develop to term, emphasizing the regulative nature of their development.

Cleavage differs from other forms of cell division in that it increases the number of cells and nuclear mass without increasing the cytoplasmic mass. This means that with each successive subdivision, there is roughly half the cytoplasm in each daughter cell than before that division, and thus the ratio of nuclear to cytoplasmic material increases.