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Modern Genetic Analysis Chapter 16. The Genetic Basis of Development
Solved Problem

3.

In the embryogenesis of mammals, the inner cell mass, or ICM (the prospective fetus), quickly separates from the cells that will serve as enclosing membranes and respiratory, nutritive, and excretory channels between the mother and the fetus.
a. Design experiments using mosaics in mice to determine when the two fates are decided.
b. How would you trace the formation of different fetal membranes?

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Solution

a.We must have markers that enable us to distinguish different cell lineages. This can be done with mice by using strains that differ in chromosomal or biochemical markers. (Other ways would be to use differences in sex chromosomes of XX and XY cells or to induce chromosome loss or aberrations by irradiating embryos.)

When you have decided on the marker difference to be used, one way to answer the question is to inject a single cell from one of two strains into embryos of the other at various developmental stages. Another approach is to fuse embryos of defined cell numbers from the two strains. In either case, you would inspect the embryos when the ICM and membranes are distinct and recognizable. When cell insertion or fusion results in membranes and ICM that are exclusively made up of one cell type and never a mosaic of the two, the two developmental fates have been set.

b.Carry out the same injection or fusion experiment on early embryos. Now look for the pattern of mosaicism. Correlate the presence of cells of similar genotype in different membranes. It should be possible to determine the cell lineage of cells in each set of membranes.

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© 1999 by W. H. Freeman and Company. All rights reserved.