This study investigates the slow component of axonal transport in mammalian neurons, focusing on identifying structural polypeptides transported into the axon from the cell body. Using radioactively labeled axonal polypeptides from rat ventral motor neurons and cat spinal ganglion sensory neurons, the researchers found that the slow component consists of five major polypeptides accounting for over 75% of the total radioactivity. Two of these polypeptides were tentatively identified as tubulin, the microtubule protein. The remaining three polypeptides, with molecular weights of 212,000, 160,000, and 68,000 daltons, are associated with the 10-nm neurofilament. The 212,000-dalton polypeptide was found to migrate with the heavy chain of chick muscle myosin. These findings suggest that the slow component contains fundamental structural elements of vertebrate neurons. The slow component is a mechanism for transporting newly synthesized proteins from the cell body to the axon and its terminals. Radioactive tracer studies show that the slow component moves at a rate of 1–2 mm per day and constitutes more than half of the total proteins entering the axon. The slow component is thought to contain microtubules and 10-nm neurofilaments, which are prominent structures in the axon. The presence of tubulin in the slow component supports the idea that it plays a structural role. The study also compared the slow component polypeptides from rat ventral motor neurons and cat spinal ganglion sensory neurons, finding them to be identical. Electrophoretic analysis revealed that the slow component consists of a small number of polypeptides with identical molecular weights in different mammalian species, suggesting they are fundamental structures of vertebrate neurons. The results indicate that the slow component is composed of five major polypeptides, with the 212,000-dalton polypeptide being identical to the heavy chain of chick muscle myosin. The 160,000-dalton and 68,000-dalton polypeptides are associated with the 10-nm neurofilament. These findings support the hypothesis that the slow component is involved in the structural maintenance of the axon. The study also suggests that the slow component may play a role in the transport of other axonal structures, such as mitochondria and vesicles, through its linear organization. The results are consistent with the view that the slow component represents a mechanism for the continuous renewal of major structural elements of the axoplasm, possibly microtubules and neurofilaments.This study investigates the slow component of axonal transport in mammalian neurons, focusing on identifying structural polypeptides transported into the axon from the cell body. Using radioactively labeled axonal polypeptides from rat ventral motor neurons and cat spinal ganglion sensory neurons, the researchers found that the slow component consists of five major polypeptides accounting for over 75% of the total radioactivity. Two of these polypeptides were tentatively identified as tubulin, the microtubule protein. The remaining three polypeptides, with molecular weights of 212,000, 160,000, and 68,000 daltons, are associated with the 10-nm neurofilament. The 212,000-dalton polypeptide was found to migrate with the heavy chain of chick muscle myosin. These findings suggest that the slow component contains fundamental structural elements of vertebrate neurons. The slow component is a mechanism for transporting newly synthesized proteins from the cell body to the axon and its terminals. Radioactive tracer studies show that the slow component moves at a rate of 1–2 mm per day and constitutes more than half of the total proteins entering the axon. The slow component is thought to contain microtubules and 10-nm neurofilaments, which are prominent structures in the axon. The presence of tubulin in the slow component supports the idea that it plays a structural role. The study also compared the slow component polypeptides from rat ventral motor neurons and cat spinal ganglion sensory neurons, finding them to be identical. Electrophoretic analysis revealed that the slow component consists of a small number of polypeptides with identical molecular weights in different mammalian species, suggesting they are fundamental structures of vertebrate neurons. The results indicate that the slow component is composed of five major polypeptides, with the 212,000-dalton polypeptide being identical to the heavy chain of chick muscle myosin. The 160,000-dalton and 68,000-dalton polypeptides are associated with the 10-nm neurofilament. These findings support the hypothesis that the slow component is involved in the structural maintenance of the axon. The study also suggests that the slow component may play a role in the transport of other axonal structures, such as mitochondria and vesicles, through its linear organization. The results are consistent with the view that the slow component represents a mechanism for the continuous renewal of major structural elements of the axoplasm, possibly microtubules and neurofilaments.