Dynamic morphological responses of mouse astrocytes in primary cultures following medium changes

Within 5 minutes of changing the medium of 3–4 week old mouse astrocyte cultures, dramatic morphological alterations were seen in the cultures at the phase microscope level. These alterations included the appearance of membrane ruffles, followed by the formation of phase light vacuolar regions. Ultrastructurally, these ruffles could be seen as phagocytic arms, and the vacuoles appeared as blisters of the superficial cell layer. These morphological responses were due to the introduction of new serum proteins. Colloidal gold‐labelled serum proteins were used to visualize the dynamics of the macromolecular uptake following this medium change. Some proteins were taken up by phagocytosis, whereas others were associated with coated vesicles and endosomal vesicles. The majority of these gold‐labelled proteins were concentrated in vacuoles (presumably lysosomes) and were localized in the superficial cellular sheets. Inner cellular sheets contained little colloidal gold‐labelled serum proteins, but displayed prominent pinocytotic profiles. Glucose consumption in these cultures remained constant at 1.6 μmoles/mg protein/hr. Glycogen content varied among individual cells, but it remained constant in the cultures at 60 nmoles bound glucose/mg protein throughout the 48 hour examination period. These results suggest that dense cultures of primary astrocytes may act similarly to the glia limitans.