The glial sling — a transient bridge-like structure that spans the septum of the developing mammalian forebrain — is something of an enigma. Since it was discovered in the early 1980s, it has been assumed to consist of glioblasts, which were thought to provide a guidance substrate for the axons of the corpus callosum. However, because of a lack of suitable molecular markers, this idea was never corroborated. Now, as reported in Development, Shu and colleagues have revisited the glial sling in the mouse, using the array of markers that has since become available, and they present compelling evidence that it actually consists of migratory neurons.

Shu et al. found that the sling did not stain positive for commonly used glial markers such as GFAP, RC2 or GLAST. Markers for mature neurons, such as neurofilament, were also absent. However, the sling did express the early neuronal markers NeuN and TUJ1, indicating that the cells were most likely to be immature neurons. The sling cells also showed electrophysiological activity that was consistent with an early neuronal identity, including spikes that resembled Na+- and Ca2+-mediated action potentials. By filling the cells with the tracer biocytin, the authors showed that they had a neuronal morphology, with a long leading process and shorter processes that resembled dendrites.

Most of the neurons in the sling seem to originate from the cortical subventricular zone (SVZ), although Shu et al. also found evidence of cell proliferation within the sling itself. Interestingly, in spite of this proliferation, and the presumed continuous replenishment from the SVZ, the sling does not increase significantly in size during development. On the contrary, it begins to shrink after birth, and it is undetectable by postnatal day 10.

The ultimate fate of the sling cells remains a mystery. It was previously thought that they underwent programmed cell death. However, although Shu et al. identified a few apoptotic cells in the sling just before birth, the extent of cell death was insufficient to account for its disappearance. The authors therefore make the tantalizing suggestion that the sling neurons might survive and migrate to other regions of the brain. The question of where the cells go, and what contribution, if any, they make to the adult brain, should provide ample scope for future study.