The growth of nonlymphoid thymic components in vitro: age-related differences during development.

An explant culture procedure has been developed that makes it possible to measure the relative growth capacity of the epithelial and mesenchymal cells of the canine thymus gland. Standardized growth conditions were obtained by size-grading thymic fragments and counting to allow uniform fragment density during culture. After 6 d in culture, outgrowth from the fragments formed colonies that could be classified into epithelial, mixed, or spindle cell type. Uniform fragment size and number in each flask allowed calculation of the total plating efficiency, relative distribution of colony types, and mean colony diameters for thymic fragments collected from fetuses (50 d of gestation), neonates (0 d postpartum), and juveniles (70 d postpartum). Data show age-related changes in the proliferative capacity of the cells in all three colony types. The most significant difference was seen in the epithelium, which showed a 30% reduction in mean colony diameter over the 2 wk between fetal and neonatal ages and a 23% reduction over the postnatal period of 70 d. Significant reductions were seen in the other colony types as well. Because the severity of the effect of many injurious agents is proportional to the rate of growth of the target cells, these data suggest that the thymus gland of the fetus may be more sensitive to physical or chemical injury than is the neonate or adult.