The Effects of Early Hypo- And Hyperthyroidism on the Development of Rat Cerebellar Cortex. III. Kinetics of Cell Proliferation in the External Granular Layer

Abstract The effects of early hypo- and hyperthyroidism on the rates of cell acquisition and proliferation have been studied in the external granular layer (EGL) of the developing rat cerebellar cortex at 10 days of age using quantitative autoradiographic methods. Both altered thyroid reduce the rate of cell acquisition in the EGL, but appear to do so for different reasons. Hyperthyroidism shortens the average length of the cell cycle by decreasing the duration of the pre-DNA synthetic phase (G1), indicating that excess thyroxine may exert a direct effect on the EGL. This action involves the early onset of neuronal differentiation (cessation of proliferation)46 which presumably leads to the observed decrease in the rate of cell acquisition (increased doubling time). Such differentiating cells do not, however, leave the proliferative zone or the EGL prematurely, resulting in a reduced labeling index, mitotic index, and growth fraction as non-dividing cells dilute the proliferating cell population. Hypothyroidism, on the other hand, leads to no significant change in the length of the cell cycle or in the mitotic index, but causes a decreased labeling index and growth fraction, as well as a reduced rate of cell acquisition (increased doubling time). No significant change in the amount of cell death in the EGL could be found to explain this apparent discrepancy between the rate of cell proliferation (cell cycle length) and cell acquisition. The answer to this puzzle appears to lie in the mitotic index, which is not affected to the same extent as the labeling index, although it is also slighly reduced. If cells were to remain longer in mitosis, this could result in a decreased labeling index and growth fraction but nearly normal mitotic index and cell cycle length (as measured using the % labeled mitoses method), since those cells dropping out of the cycling population would be counted as mitoses. This hypothesis is supported by the build-up of labeled mitoses with time after [3H]thymidine injection as more and more labeled cells reach the border between mitosis and interphase (M/G1) and fail to pass it, as well as the increased time required for the labeled cell population to double. Such mitotic cells appear to move from the proliferative zone of the EGL (decreased mitotic index) to the subproliferative zone where they accumulate, primarily in anaphase-telophase (increased index of anaphase-telophase mitotic figures), but eventually resume proliferation, as evidenced by a significantly increased labeling index in this zone. These results indicate that the rate of cell proliferation may actually be reduced in hypothyroidism corresponding to the decreased rate of cell acquisition, but this effect is masked by the prolongation of mitosis when analyzed by the method of % labeled mitoses. Possible mechanisms underlying the effects of hypo- and hyperthyroidism on cell acquisition and proliferation in the EGL are discussed. These results indicate the importance of proper levels of thyroid hormone during postnatal development for normal cell acquisition and proliferation in the rat cerebellar cortex.