In earlier chapters the fundamental differences between germ line and somatic cells have been stressed. The germ line that provides continuity from generation to generation does not age, except in unusual circumstances. At least a proportion of the cells must be potentially immortal, with this immortal cell lineage stretching back to the progenitors of existing species. Whatever provides the basis for the indefinite proliferation of germ cells is therefore shut off in most, and perhaps all, somatic cells. In other words, regulatory changes controlled by the genetic programme and development come into play. Since different mammalian species have very different lifespans, the nature or extent of these genetically controlled changes must vary from species to species. This is in contrast to the germ line, which must have common species independent features keeping it in a permanently juvenile state.

THE PRESERVATION OF THE GERM LINE

Germ line cells retain their juvenile state irrespective of parental age, with a few notable exceptions that are discussed later. In effect, a steady state must be maintained in the sense that the fertilised egg of one generation has the same potential for development as that of any earlier or later generation. One might expect genetic or other abnormalities to gradually accumulate from generation to generation, and this has given rise to the question, ‘Why are babies born young?’ (Bernstein 1981). In answer to this question, it has been proposed that a major function of meiosis is to provide the opportunity for the repair of DNA damage, and that this is an essential mechanism for avoidance of ageing in the germ line (Bernstein & Bernstein 1991).