Differences in the decrease in regenerative capacity of various brain regions of Xenopus laevis are related to differences in the undifferentiated cell populations.

The extent of the undifferentiated cell population in normal and regenerating brains of larvae and metamorphosed individuals of Xenopus laevis has been analyzed by means of an immunocytochemical method and mitotic index determinations. Results show that the decrease in regenerative capacity of the brain during larval development and after metamorphosis is in relation with the gradual reduction of the population of undifferentiated cells and that the different regenerative capacities of the various brain districts are related to quantitative and qualitative differences in this cell population. While in the early larval stages the cell population formed of actively cycling cells is very large and widespread, in late larval stages and after metamorphosis these cells localize in some encephalic areas (matrix zones). This localization occurs later in the telencephalon than in the rhombencephalon and in mesencephalon. The less conspicuous decrement in the regenerative capacity of the telencephalon than of other encephalic districts of froglets, particularly the mesencephalon, is related to the presence of a larger number of actively cycling cells together with a rather large number of undifferentiated cells which are in a temporary quiescent state from which they may re-enter the actively cycling state in response to proliferation promoting factors.