Ultrastructural aspects of tumour invasion in the central nervous system.

Tumour invasion in the central nervous system (CNS) was studied using a mouse melanoma cell model. Attention was concentrated on the third ventricle as this site is distant from any implantation artefact. Transmission electron microscopy (TEM) revealed the formation of tumour cell aggregates within the ventricle. Underlying ependymal cells showed loss of cilia and microvilli. Progressive thinning of the ependymal cells took place until continuity of the ependyma was lost through ependymal retraction. Tumour cells were then in direct contact with sub-ependymal neuroglial processes. Further invasion was then accomplished by penetration between neuropil processes, followed by tumour cell infiltration along the intact basement membrane of cerebral capillaries with displacement of astrocyte foot processes from the capillary basement membrane.