Cell size and cell number in tissue growth and development. An old hypothesis reconsidered.

The development of tissues and organs in the post-embryogenic period has commonly been thought to be by two main consecutive processes: the multiplication of cells followed by a growth in their size. The capacity for recovery following growth restriction has been considered to depend on whether restriction occurred during the phase of cell multiplication, in which case catch-up was permanently inhibited; or later, in which case recovery could occur. This model is now no longer valid for a number of reasons, the main one being that the original assumptions about the sequence of developmental processes have been shown to be false. There certainly seems to be an early period of vulnerability during which restriction is not followed by catch-up, but this cannot now be related to the multiplication of cells.