The purpose of the present study is to determine whether or not the differences in cellular proliferation in rat nasal septal cartilage and tibial cartilage are reflected by differences in the pattern of age-related changes in thymidylate synthetase (TS) and thymidine kinase (TK) activities. TS and TK are responsible for thymidine monophosphate formation via, respectively, the de novo and the salvage pathways. The present study also examines the effects of glucocorticoid s, which are known to affect the proliferation of chondrocytes, on TS and TK activities in these cartilaginous tissues. Both enzyme activities declined with age, and the decline in TK activity was more rapid than that of TS. The TK/TS ratio decreased with age more rapidly in nasal septal cartilage. These findings suggest that nasal septal cartilage might mature earlier than tibial cartilage and, furthermore, that in rapidly growing tissues the salvage pathway functions predominantly. Prednisone injection (2.5 mg/ 100 g, at 4 days after birth) clearly disturbed the gain of body weight. TK activity significantly decreased concomitantly with this arrest in weight gain, while in contrast TS activity was not depressed. This marked difference between TS and TK activities induced by prednisone suggests that these two enzymes may be regulated independently.

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The morphogenesis of the muscle spindles of the masseter muscle was investigated by electron microscopy. The mice used in this study were those from the 16-day-old fetuses to the four-day old young, and the three-month-old adult ICR mice. At the time of observation the formation of the masseter muscle spindle had already started in the 16-day-old fetus. In the 17-day-old fetus, the sensory nerve terminated not only in the myotube but also in the myoblast in which no myofilaments were yet formed. From the observations on the 16-day-old and 17-day-old fetuses, it is suggested that the myotubes of the intrafusal muscle fibers are formed by the fusion of the myocytes already containing the myofilaments. The gamma motor end-plate appeared two days later than the sensory nerve terminal formation in the muscle spindle. The initial inner capsule of the spindle was observed in the four-day-old young. The cell without the myofilaments appeared amidst the cluster of the intrafusal muscle fibers in the three-day-old young. It is considered that this cell may be the origin of the inner capsule cell. The periaxial space formation and the myelination of the masseteric nerve fiber occurred in the four-day-old young. The formation of the postsynaptic folds was observed in the extrafusal muscle fiber at the same stage. Thus, the muscle spindles, nerve fibers and the extrafusal muscle fibers showed a remarkable development at the same time in this early postnatal stage.

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