The variation in thin filament length was investigated in slow and fast muscle from adult and neonatal rats. Soleus (slow) muscle from adult, 3-, 7-, and 9-d-old rats, and extensor digitorum longus (EDL; fast) muscle from adult rats were serially cross-sectioned. The number of thin filaments per 0.06 microns2 (TF#) was counted for individual myofibrils followed from the H zone of one sarcomere, through the I-Z-I region, to the H zone of an adjacent sarcomere TF# was pooled by distance from the Z band or AI junction. In both adult muscles, thin filament length varied from 0.18 to 1.20 microns, with approximately 25% of the thin filaments less than 0.7 microns in length. In 7- and 9-d soleus, thin filament length ranged from 0.18 to 1.08 microns; except for the longest (0.18 to 1.20 microns) filaments, the distribution of thin filament lengths was similar to that in adult muscle. In 3-d soleus, thin filament length was more uniform, with less than 5% of the filaments shorter than 0.7 microns. In all neonatal muscles, there were approximately 15% fewer thin filaments per unit area as compared to adult muscles. We conclude: (a) In rat skeletal muscle, thin filaments are not of uniform length, ranging in length from 0.18 to 1.20 microns. (b) There may be two stages of thin filament assembly in neonatal muscle: between 3 and 7 d when short thin filaments may be preferentially or synthesized or inserted near the Z-band, and between 9 d and adult when thin filaments of all lengths may be synthesized or inserted into the myofibril.

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