Cleavage results in a repackaging of the egg cytoplasm into a large number of small cells surrounding the fluid-filled blastocoel. Except in mammals, there is little gene expression during early cleavage divisions. Cells in different regions of the blastula do, however, possess different complements of the nutrients and cytoplasmic determinants that were present in the egg. For example, Figure 43.2 illustrated the processes by which β-catenin becomes localized in the region of the amphibian zygote that will become the dorsal side of the embryo.

The blastocoel prevents cells from different regions of the blastula from coming into contact and interacting, but that will soon change. During the next stage of development, the cells of the blastula move around and come into new associations with one another, communicate instructions to one another, and begin to differentiate. In many animals, these movements of the blastomeres are so regular and well orchestrated that it is possible to label specific blastomeres with a dye, thus producing fate maps that identify the tissues and organs formed from each blastomere’s progeny (Figure 43.6).

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Blastomeres become determined—committed to specific fates—at different times in different species. In some species, such as roundworms, the fates of blastomeres are restricted as early as the two-cell stage. This early determination depends on informational molecules that were originally present in the egg cytoplasm and is called autonomous specification. If one of these blastomeres is experimentally removed, a particular portion of the embryo will not form. The resulting developmental pattern has been called mosaic development because each blastomere seems to contribute a specific set of “tiles” to the final “mosaic” that is the adult animal. Mosaic development is the common form of development in invertebrates.

The fates of blastomeres in most vertebrate embryos are determined by information they receive from their environment and their neighboring cells. This mechanism of determination is called regulated specification, and it gives rise to regulative development. Unlike in mosaic development, the loss of some cells during cleavage in regulative development does not affect the developing embryo, because the remaining cells compensate for the loss. A subset of the remaining cells may add a cell division, or they may even change their fate to compensate for the cells lost. Because development is regulative in humans, a single blastomere can be removed from early embryos without harming the remaining blastomeres or disrupting normal development. Cells removed from embryos produced by in vitro fertilization can be used for preimplantation genetic diagnosis to ensure that healthy embryos are selected for implantation in the mother.

If some blastomeres can change their fate to compensate for the loss of other cells during cleavage and blastula formation, can those cells form an entire embryo? Yes, in species with regulative development, they can. If during cleavage or early blastula formation, the blastomeres are physically separated into two groups, both groups can produce complete embryos. Since the two embryos come from the same zygote, they will be monozygotic twins—genetically identical.

Non-identical twins occur when two separate eggs are fertilized by two separate sperm. Thus, although identical twins are always the same sex, non-identical twins have a 50 percent chance of being the same sex (that is, the same as two non-twin siblings). If genetic or environmental factors cause the blastula or blastocyst to split partially, the result is twins who are conjoined at some point on their bodies and usually share some of their organs and limbs. Conjoined twins occur in about 1 out of 50,000 human pregnancies.