Mosaic and Regulative Eggs

Eggs as well as early cleavage embryos are occasionally divided into two categories:

i) Mosaic or determinate embryos and

ii) Regulative or indeterminate embryos.

The terms mosaic and regulative relate to the time for which the restrictions in development are imposed on the blastomeres. For instance, the embryos of tunicates related to mosaic or determinate category. In these organisms the various blastomeres become restricted to make only specific structures as soon as they are formed or during the first few cleavages. In other words, the respective fates of dissimilar blastomeres are finally determined early in mosaic embryos. As a result, the removal or loss of specific blastomeres cannot be compensated by the other blastomeres and the embryos will be defective in structures that are normally derived from the missing blastomeres.

In regulative or indeterminate embryos the restrictions on blastomeres take place later in development. Amphibians and sea-urchins belong to this category. Because the cell fate of blastomeres is lastly determined later, any removal or destruction of blastomeres early in development can be compensated by the other blastomeres that are able to form all the structures. It is also believed that the differences between mosaic and regulative embryos are directly connected to the extent of their usage of extrinsic and intrinsic mechanisms for cell determination. The mosaic embryos depend on ooplasmic determinants, i.e. intrinsic factors for cell fate determination. In contrast, the regulative embryos mostly rely upon extrinsic factors derived from cell interaction for cell determination.