Glial cells in the optic chiasm arise from the suboptic necrotic centers of the diencephalon floor: morphological evidence in the chick embryo.

During early development of the optic chiasm in the chick embryo, before the arrival of the first optic axons (Stage 20), a characteristic spongy tissue appears medially between the suboptic necrotic centers (SONCs). By the time the first optic axonal fascicles arrive in the contralateral tract (Stage 23), cell death has ceased to occur in the SONCs, which become dense clusters of primitive glial cells. The clusters are steadily displaced medially during development, invading the spongy tissue and converging on the midline at Stage 27. These cell clusters appear to defect rostrocaudally the course of optic axons arriving at the chiasm.