The significance of fertilization.

The fertilization of a human egg, often thought of as initiating the life of a person, is in reality but the beginning of a beginning for one or more individuals. While pronuclear fusion establishes a diploid genome, this is at first a structural entity without function. No significant RNA synthesis occurs between germinal vesicle breakdown and early cleavage, and in fact embryonic genes do not begin to find expression until about the 4- to 8- cell stage. Gene expression then progressively spreads throughout the genome, during prenatal development and beyond. The progressive nature is well shown in the early mouse embryo by the widening range of energy sources utilizable, by the rising levels of glycogen storage and by the increasing uptake of nucleic acids and protein precursors. While HCG-B RNA is transcribed in human embryos about 2 days after fertilization, it is not expressed until 16-cell stage is not linked to physical or functional integration, each cell being inherently capable of giving rise to an entire person (together with a complex of placental structures); alternatively, cells from separate embryos on being brought together can jointly lead to the establishment of a chimeric individual. Multiplicity can also originate later, the primitive streak stage being the normal time for monozygotic twinning. Only when that stage has passed does true individuality exist, for (excluding anomalies) just one person can now eventuate. The gene-transfer function of fertilization can be replaced or augmented by intranuclear or intra-blastocyst gene injection, or by the use of teratocarcinoma or embryo-stem cells.