Intrinsic and extrinsic control of growth in developing organs.

The growth rate and final size of developing organs is controlled by organ-intrinsic mechanisms as well as by hormones and growth factors that originate outside the target organ. Recent work on Drosophila imagined discs and other regenerating systems has led to the conclusion that the intrinsic growth-control mechanism that controls regenerative growth depends on position-specific interactions between cells and their neighbors, and that these interactions also control pattern formation. According to this interpretation, local growth by cell proliferation is stimulated when cells with disparate positional information are confronted as a result of grafting or wound healing. This local growth leads to intercalation of cells with intervening positional values until the positional information discontinuity is eliminated. When all discontinuities have been eliminated from a positional field, growth stops. In this article we consider the possibility that organ growth during normal development may be controlled by an intercalation mechanism similar to that proposed for regenerative growth. Studies of imaginal disc growth are consistent with this suggestion, and in addition they show that the cell interactions thought to control growth are independent of cell lineage. Developing organs of vertebrates also show intrinsic growth-control mechanisms, as demonstrated by the execution of normal growth programs by immature organs that are transplanted to fully grown hosts or to hosts with genetically different growth parameters. Furthermore, these organ-intrinsic mechanisms also appear to be based on position-specific cell interactions, as suggested by the growth stimulation seen after partial extirpation or rearrangement by grafting. In organs of most adult vertebrates, the organ-intrinsic growth-control system seems to be suppressed as shown by the loss of regenerative ability, although it is clearly retained in the limbs, tails and other organs of salamanders. The clearest example of an extrinsic growth regulator is growth hormone, which plays a dominant role along with insulin-like growth factors, thyroid hormone and sex hormones in supporting the growth of bones and other organs in postnatal mammals. These hormones do not appear to regulate prenatal growth, but other hormones and insulin-like growth factors may be important prenatally. The importance of other growth factors in regulating organ growth in vivo remains to be established. It is argued that both intrinsic and extrinsic factors control organ growth, and that there may be important interactions between the two types of control during development.