The long term objective for this project is to understand the cellular processes involved in growth control and pattern formation in developing mammalian limbs. Experiments of the type proposed have recently become possible due to the development of techniques for in situ surgery on early stage mouse embryos. Experiments are proposed to begin a developmental analysis of wildtype limbs and, in addition, limbs of selected mutants in an effort to characterize alterations in the normal course of growth which correlates with defective patterns. Growth will be analyzed by utilizing a number of different techniques including, BrdUrd incorporation to establish normal growth parameters and cell type-specific birthdays; the introduction of a marker gene to trace the fate of small populations of cells in the limb bud; the use of growth specific antibodies to define the pattern of growth termination during limb development. Finally, mutant limbs will be screened for autonomy of the mutational effect by microsurgical grafting onto wildtype host embryos. The information gained from these studies will ultimately be of significance to our understanding of congenital birth defects, and possibly to their correction. In addition, understanding how growth is controlled in the limb during development may provide insight into how regenerative growth may be enhanced in more mature mammalian limbs.