Death of the subcallosal glial sling is correlated with formation of the cavum septi pellucidi

In this study we have examined the developmental fate of a population of cells that is located beneath the rostral corpus callosum during the perinatal period. These cells form a distinct slinglike structure along the geographically defined corticoseptal boundary (CSB) and may play a role in guiding callosal axons across the midline. The sling is a transient structure present in fetal and neonatal animals but not in adults. Here we show that the CSB cells die and that this debris is removed by macrophages. The sequence of cell degeneration in the CSB is highly stereotyped and follows a spatiotemporal pattern that is correlated with fusion of the cerebral hemispheres and subsequent growth across the midline of the callosal axons. The subcallosal location of the resorbing CSB is found in the exact place in which a fluid‐filled cavity (the cavum septi pellucidi) is transiently found during the perinatal period. The tight temporal and spatial correlation between callosal axon decussation, degeneration of the CSB, and cavum septi formation suggests that these three phenomena may be causally related.