Postnatal development of vascularity in the inferior colliculus of the young rat

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Abstract
The inferior colliculus in the rat midbrain is an auditory relay center whose functional maturation occurs postnatally. We examined by morphometry the vascularity and the nuclear profile density of the inferior colliculus in normal young rats at different ages (before and after the onset of auditory input). We also compared this region with a frontal region of the cerebral cortex in 24‐day‐old rats. The inferior colliculus from aldehyde‐perfused Sprague‐Dawley rats aged 5, 9, 14, and 24 days was analyzed by light microscopy of semithin plastic sections. The central region (mostly the central nucleus) was sampled at 5 levels representing its entire rostrocaudal extent. Patent‐blood‐vessel profiles were counted and classified according to their size and profile orientation. Counts of nuclear profiles in the same sections were also made. In the inferior colliculus of rats between 5 and 24 days of age, the small (>10‐μm diameter) cross‐sectioned vessel profiles increased over 6‐fold in number per unit area. Correspondingly the vascular volume density, estimated by differential point counting, increased between these ages. However, there was a decrease in the number of neuronal and glial nuclear profiles per unit area, probably because of growth in the volume of the neuronal perikarya and processes, along with cell emigration reported to occur at early postnatal ages. This study has shown that an increase in vascularity in the central region of the rat inferior colliculus continues for up to 2 weeks after the onset of hearing. The numerical density of small vessels in the rat inferior colliculus had reached the adult level of vascularity by 24 days of age. Also, by 24 days and possibly earlier, regional differences in capillary density in the brain were evident; the frontal cortex was found to have fewer small vessel profiles per unit area and a lesser vascular volume density than the inferior colliculus. Thus, such micro‐regional differences in vascularity must have arisen earlier during brain and capillary growth.