Developmental microvascular architecture of the rat cerebellar cortex.

External and internal microvascular architectures of the developing rat cerebellar cortex, from embryonic day 18 to postnatal day 14 and in adults, were studied using a cerebrovascular casting method for scanning electron-microscopic observation. The external vascularization of the developing cerebellum showed the most significant alteration in vascular morphology at the stage of intensive proliferation of matrix cells in the external granular layer (EGL) from birth to postnatal day 4. It consisted of multiple luminal protrusion of the vessels, septum formation in the lumina, and small, ring-like anastomoses. Moreover, at the end of this stage, these structures of the vessels disappeared and the subarachnoid space was filled with newly-formed microvascular networks. Thereafter, architectural change of the developing pial vessels was mainly accomplished by elongation of each contorted vessel of the network. Concerning internal vascularization, a few vessels connected with the pial vessels were observed in the cerebellar plate forming a loose, simple network in the deeper neural parenchyma before the stage of foliation began. During the period of thickening of the EGL, however, there was no alteration in vascularity of the parenchyma other than the architectural changes proportionate to the newly-formed folia. It was during the synaptogenetic stage in the internal granular layer that the earliest intraneural vascular plexuses were formed. The vascular network in the molecular layer was formed after disappearance of the EGL. These findings suggest that vascular proliferation correlated with EGL-formation pertains to the pial vessels, and not the intraneural ones, which develop after neuronal cell migration in the developing cerebellum has taken place.