1 How Metazoans Reach Their Full Size: The Natural History of Bigness

Abstract

Life forms range enormously in size, but as yet, we have little understanding of the mechanisms that determine the size of a cell or the size of an organism. Not only do we have little understanding of the mechanisms, but there is also a lack of appreciation of what is represented by growth control. Until recently, growth control was equated with the control of proliferation (Raff 1996; Su and O’Farrell 1998). However, the simple observations that cells can grow to different sizes, and that cells can divide without growth to produce larger numbers of smaller cells, suggest that the processes ought to be considered separately. It is recognized that growth should be considered in terms of increase in mass rather than increase in cell number. To many investigators, this appears to be a formalism, because they are so familiar with growth situations in which the two go hand in hand. The formalism becomes much more concrete when examining growth and cell proliferation in metazoans. It turns out that there is an extraordinary and fundamental segregation of organism growth and cell proliferation in the life histories of most metazoans.

There has been a recent surge of interest in growth control and wider recognition of the distinctions between the control of mass increase and the control of cell proliferation. Despite newly emphasized distinctions between growth and cell proliferation, in most of the systems that are currently being investigated, growth is largely exponential and cells are actively proliferating in close parallel. This includes...