Maturation of neurons in neocortical slice cultures: A light and electron microscopic study on in situ and in vitro material.

Using light and electron microscopic methods, we investigated the development and morphology of neurons in neocortical slice cultures. Slices taken from the visual cortex of 6-day-old rats and cultivated for 14 or 20 days were compared with in situ material of corresponding age (P 20 and P 26). Maturation and differentiation of pyramidal and non-pyramidal cells kept in vitro were found to have progressed considerably. In the light microscope the neurons exhibited a morphological appearance strikingly similar to that of the neurons of the neocortex in situ at the same age. The fine structure of the tissue in vitro also had a mature appearance, corresponding in most respects to the material in situ. Synapses and dendritic spines were well-developed. Sometimes a spine apparatus was contained in the sections and occasionally a myelinated fiber could be seen. GABA-immunoreactive cells making symmetric synaptic contacts were also present. Despite these similarities, some quantitative differences could be observed. In slice cultures, only 52% of the synapses were located on spines (78% in situ). In vitro, a larger proportion of synapses (30%) showed a postsynaptically concave curvature than was the case in situ (12%). The areal density of synapses in vitro reached only about 70% of that in situ. This was probably a side-effect of the larger size of dendritic and axonal profiles on electron micrographs of in vitro-material. The most striking difference was that large synapses and synapses containing a large amount of synaptic vesicles were considerably more frequent in vitro than in situ.