The early cells of the embryo can replace each other and compensate for a missing cell. This was first shown in 1952, when Seidel destroyed one cell of a 2-cell rabbit embryo, and the remaining cell produced an entire embryo. Once the inner cell mass has become separate from the trophoblast, the ICM cells constitute an equivalence group. In other words, each ICM cell has the same potency (in this case, each cell can give rise to all the cell types of the embryo, but not to the trophoblast), and their fates will be determined by interactions among their descendants. Gardiner and Rossant (1976) also showed that if cells of the ICM (but not trophoblast cells) are injected into blastocysts, they contribute to the new embryo. Since its blastomeres can generate any cell type in the body, the cells of the blastocyst are called totipotent (see Chapter 4).

This regulative capacity of the ICM blastomeres is also seen in humans. Human twins are classified into two major groups: monozygotic (one-egg, or identical) twins and dizygotic (two-egg, or fraternal) twins. Fraternal twins are the result of two separate fertilization events, whereas identical twins are formed from a single embryo whose cells somehow dissociated from one another. Identical twins may be produced by the separation of early blastomeres, or even by the separation of the inner cell mass into two regions within the same blastocyst.

Identical twins occur in roughly 0.25% of human births. About 33% of identical twins have two complete and separate chorions, indicating that separation occurred before the formation of the trophoblast tissue at day 5 (Figure 11.32A). The remaining identical twins share a common chorion, suggesting that the split occurred within the inner cell mass after the trophoblast formed. By day 9, the human embryo has completed the construction of another extraembryonic layer, the lining of the amnion. This tissue forms the amnionic sac (or water sac), which surrounds the embryo with amnionic fluid and protects it from desiccation and abrupt movement. If the separation of the embryo were to come after the formation of the chorion on day 5 but before the formation of the amnion on day 9, then the resulting embryos should have one chorion and two amnions (Figure 11.32B). This happens in about two-thirds of human identical twins. A small percentage of identical twins are born within a single chorion and amnion (Figure 11.32C). This means that the division of the embryo came after day 9. Such newborns are at risk of being conjoined (“Siamese”) twins.

The ability to produce an entire embryo from cells that normally would have contributed to only a portion of the embryo is called regulation, and is discussed in Chapter 3. Regulation is also seen in the ability of two or more early embryos to form one chimeric individual rather than twins, triplets, or a multiheaded individual. Chimeric mice can be produced by artificially aggregating two or more early-cleavage (usually 4- or 8-cell) embryos to form a composite embryo. As shown in Figure 11.33A, the zonae pellucidae of two genetically different embryos can be artificially removed and the embryos brought together to form a common blastocyst. These blastocysts are then implanted into the uterus of a foster mother. When they are born, the chimeric offspring have some cells from each embryo. This is readily seen when the aggregated blastomeres come from mouse strains that differ in their coat colors. When blastomeres from white and black strains are aggregated, the result is commonly a mouse with black and white bands. There is even evidence (de la Chappelle et al. 1974; Mayr et al. 1979) that human embryos can form chimeras. Some individuals have two genetically different cell types (XX and XY) within the same body, each with its own set of genetically defined characteristics. The simplest explanation for such a phenomenon is that these individuals resulted from the aggregation of two embryos, one male and one female, that were developing at the same time. If this explanation is correct, then two fraternal twins have fused to create a single composite individual.

Markert and Petters (1978) have shown that three early 8-cell embryos can unite to form a common compacted morula and that the resulting mouse can have the coat colors of the three different strains (Figure 11.33B). Moreover, they showed that each of the three embryos gave rise to precursors of the gametes. When a chimeric (black/ brown/white) female mouse was mated to a white-furred (recessive) male, offspring of each of the three colors were produced.

According to our observations of twin formation and chimeric mice, each blastomere of the inner cell mass should be able to produce any cell of the body. This hypothesis has been confirmed, and it has very important consequences for the study of mammalian development. When ICM cells are isolated and grown under certain conditions, they remain undifferentiated and continue to divide in culture (Evans and Kaufman 1981; Martin 1981). Such cells are called embryonic stem cells (ES cells). As shown in Chapter 4, cloned genes can be inserted into the nuclei of these cells, or the existing genes can be mutated. When these manipulated ES cells are injected into a mouse blastocyst, they can integrate into the host inner cell mass. The resulting embryo has cells coming from both the host and the donor tissue. This technique has become extremely important in determining the function of genes during mammalian development.