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Process formation in astroglia: scanning electron microscopy study

Abstract Astroglia acquire processes during normal differentiation both in vitro and in vivo. During process formation, astrocytes form contacts with neighboring blood vessels, synapses and pia. These morphological changes, which may correlate with functional changes, suggest the importance of studying astrocyte process formation. Astroglia colony cultures were prepared from mouse neopallia. Using scanning electron microscopy the appearance of astroglia and their processes, during the differentiation in colony cultures and after treatment of astroglia with dibutyryl cyclic AMP (dBcAMP) and cytochalasin D (CD), were described. The small cells in the center of the astroglia colonies had many microvilli near the intercellular junctions. More peripheral, medium-sized cells and large cells (LC) had many thin processes. The surface of the cell body in the region adjacent to the processes had folds and blebs. The LC at the periphery of the colony had similar folds and blebs proximal to lamellipodia which extended from their free surfaces. The mature, small stellate astrocytes (SSA) had fewer, thicker, and longer processes, some of which ended in structures resembling growth cones. When the astroglia were treated with dBcAMP, the cells acquired many thin processes. When both the untreated and the dBcAMP-treated astroglia were exposed to CD, the cells acquired long, thick processes instead. The formation of the thin processes in astroglia, during their differentiation in colony cultures and after treatment with dBcAMP, may be due to polymerization of actin filament bundles at the cell periphery followed by retraction of the cytoplasm away from these bundles. However, the formation of the much longer and thicker processes in SSA and CD-treated cells after disassembly of actin filaments may instead be due to the retraction of cytoplasm away from the bundles of the more stable intermediate filaments.