Response of intraventricular macrophages to crotoxin-coated microcarrier beads injected into the lateral ventricle of postnatal rats.

Crotoxin-coated microcarrier beads were injected into the lateral ventricles of 5 days old postnatal rats. The morphology of the cells attached to the beads at various time intervals was studied by scanning and transmission electron microscopy. Scanning electron microscopy showed that very few cells were associated with the surface of the beads 18 hours after the injection. After 2 days a large number of spherical cells showing blebs and filopodia were attached to the surface of the beads. One week after the injection, these cells became oval and, in longer survival periods between 2 weeks and 30 days after the injection, the cells developed a flattened or angular cell body bearing a number of radiating slender processes. Transmission electron microscopy of the re-embedded materials from animals killed 2 days after the injection showed many cells with an eccentric nucleus containing dense chromatin masses. Their abundant cytoplasm was endowed with a variable number of lysosome-like dense granules and vacuoles. In longer surviving animals, the cells became elongated with scanty cytoplasm showing relatively fewer dense granules and cytoplasmic vacuoles. It is postulated from this study that the cells attached to the crotoxin-coated beads are derived from the intraventricular macrophages. These are functionally active initially in response to the beads injected. With time, however, they undergo morphological alteration and regress into quiescent cells which are microglia-like.