Vimentin-positive rosary-bead-like formations in the rabbit superior colliculus during postnatal development

Strings of vimentin-positive rosary-bead-like formations in the rabbit superior colliculus were studied. The size and distance between beads varied from one string to another, and within a given string. The formations were similar in number and distribution in the two superior colliculi. Successions of beads were found mainly in the medial and deep layers, and represented not more than 1% of the radial glial fibers. The strings were in continuity at deep levels with segments of varicose fibers of radial glia, which in turn were continuous with fibers of normal appearance. These observations suggest a temporal sequence of evolution, starting with the normal radial glial fiber, which acquires swellings that in turn give rise to discrete beads; in subsequent stages, the beads become smaller and more widely separated. In the newborn rabbit, these formations are seen mainly in laterobasal regions of the superior colliculus, and spread throughout the colliculus by the end of week one of postnatal life, becoming more numerous in medial regions. We suggest that these formations represent a mechanism of removal of at least part of the radial glial fibers, and discuss the possible relation between these formations and the transformation of radial glia into astrocytes.