[Stages in the postnatal maturation of the optic nerve: morphometric changes during the critical period]

The postnatal differentiation in rat optic nerve fibers was examined by electron microscopy from birth until the age of 44 days. We have estimated the evolution of the following parameters: nerve, axons and fibers caliber, number of axons, number and density of microtubules and neurofilaments, and number of myelin lamellae. In this study, we have determined four phases in the postnatal maturation of the optic nerve fibers, and we have described the ultrastructural changes during the critical period. The first week is a period of slow growth in axonal size and there is a lack of myelinated fibers. Microtubules and neurofilaments are relatively stable at this time, being the microtubules more numerous. The second phase corresponds with the second and third postnatal weeks, and shows a rapid increase in axonal size, microtubule and neurofilament number. The number of neurofilaments was more numerous than the number of microtubules. The third phase corresponds with the fifth postnatal week, and there are no changes in axonal caliber and in microtubule and neurofilament number. The fourth and last phase occurred from the sixth postnatal week to adult age. In this phase the axonal size, the fiber caliber, the microtubule and the neurofilament number increased. Our findings support the idea that neurofilaments are the most relevant morphological structure in the determination of axonal maturation in rat optic nerve.