The Questions of Developmental Biology

Twins
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 (twoegg,
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.