Oligodendrocytes differentiate in organotypic cultures of rat visual cortex and myelinate efferent axons

We have investigated the presence and function of glia cells, especially of oligodendrocytes (OL) in organotypic cultures of rat visual cortex grown for 1-6 weeks in vitro. OL identified by strong Galactocerebroside-immunoreactivity (GalC-ir) displayed rather small somata and elaborately ramified processes. They were most concentrated in layers VIa and VIb and the remnant of the white matter. Silver staining revealed long descending or oblique processes in layers V and VI, which were often arranged in patches, and horizontal processes in the white matter. Proximal processes of OL cell bodies were connected to these long processes. DiI-labeling revealed very similar patches of processes, termed OL domains. They were identified as membraneous sheaths formed by processes of single OL around axons passing the OL domain. Confocal microscopy revealed single axons running through the membrane sheaths. We compared the molecular differentiation of glial cells in cultures to the in vivo situation with protein blots and immunohistochemistry for glial cell marker molecules. In homogenates of visual cortex in vivo, protein blots revealed the increase in expression by OL of myelin basic protein (MBP) during the fourth postnatal week. The astrocytic marker glial fibrillary acidic protein (GFAP), blotted as a control, increased over time in vivo, beginning at P14, indicating the differentiation of astrocytes. In homogenates of organotypic cortex cultures, the times course of expression of GFAP was very similar: it increased dramatically during the first 10 DIV, and remained fairly constant in older cultures.(ABSTRACT TRUNCATED AT 250 WORDS)