The distinctive start of life in mammals

A University of Bath research group has found that mammalian eggs are not pre-packaged in the way that the eggs of all other animals are.

This work by Dr Tony Perry of the Department of Biology & Biochemistry, was produced in collaboration with mathematical biologists in Germany and published recently in The EMBO Journal

The cells within all two-, three- and four- cell embryos studied so far in non-mammals differ from each other depending on which 'end' (or pole) of the egg they are derived from.  The difference between the cells is partly because they inherit different sets of messenger ribonucleic acid (mRNA) molecule from different egg poles.

This non-uniform mRNA inheritance is critical to ensure normal development, so it was widely assumed that the same would apply to mammals. However, no mammals had actually been studied because their eggs are too small and scarce, making the problem technically difficult.

Dr Perry says: “Our research addressed this by sampling mRNAs from parts of individual mouse eggs and one-cell embryos and from the cells of embryos immediately after they had divided.

“For the technically-minded, this is perhaps the first single sub-cellular transcriptome profiling in any system.”

The research group found that there was indeed a non-uniform distribution of mRNAs within mouse eggs, as predicted from non-mammalian species.

However, and contrary to expectations, the group found no reproducible differences between the mRNA make-up of sister cells within a given mouse embryo.

Dr Perry adds: “This is in marked contrast to the situation in non-mammalian species and has implications for a fundamental question that is still wide open in early 21st century biology: how is a mammalian embryo formed?”

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