By Levi Clancy for Student Reader on
In blastomere separation experiments (aka blastomere isolation experiments) the cells of the early embryo are separated and allowed to develop independent of one another.
There is a long history of this type of experiment, with the purpose of ascertaining whether the fate of cells was determined very early during cleavage. Basically, experiments on various species had one of two outcomes. The first outcome was that each portion of the embryo developed as it would have if left together, taking its prospective fate. The second outcome was that the separated portions each developed into a whole embryo, a fate greater than their prospective fate.The former result was taken to demonstrate mosaic development (each part of the embryo is specified very early, the potency of each blastomere is the same as its prospective fate); the latter result was taken to demonstrate regulative development (the blastomer's potency, or developmental potential, is greater than its prospective fate).
Although each of the separated first two and four blastomeres of sea urchins and mammals can give rise to a whole organism (regulative development), for certain other organisms, separated blastomeres only give rise to the part of the embryo they would have become if left in contact with the other blastomeres (thus showing mosaic development).
Are mosaic and regulative development really fundamentally different types of development? We think not. As explained by E.B. Wilson, the apparent difference between regulative and mosaic development is a function of whether localized determinants are divided differentially among the early blastomeres by the early cleavage planes.
|Urchin||If: The blastomeres of a two-cell urchin embryo are shaken apart.|
Then: Each blastomere can give rise to a normal urchin.
|Frog||If: The blastomeres of a two-cell frog embryo are shaken apart.|
Then: Each blastomere can give rise to a normal frog.
|Mammal||If: The blastomeres of a 2-, 4- or even 8-cell mammalian embryo are shaken apart.|
Then: Each blastomere can give rise to a normal mammal.
|Conclusion||Blastomeres are totipotent until after the 2-8 cell stage. Thus they must contain the same genetic information. The nuclear transplantation experiment reveals whether genomic equivalence continues.|