FIGURE 22-21 Astrocytes and the tripartite synapse. (a) Many synapses are ensheathed by astroglial processes, as shown in this electron micrograph of a synapse in the rodent hippocampus. The postsynaptic compartment (dendrite and dendritic spine) is highlighted in green, the presynaptic terminal in orange, and astroglial process in blue. (b) Astrocytes not only ensheath the synapse, but they also secrete a number of factors that promote correct synapse formation. These include thrombospondin, hevin, glypicans, and cholesterol. Indicative of the importance of astrocytes in neuronal synapse formation, when neurons are grown in cell culture, they require astrocytes for proper synapse formation and development.

[Part (a) republished with permission of Elsevier, from Bourne, J. and Harris, K. M., “Do thin spines learn to be mushroom spines that remember?,” Curr. Opin. Neurobiol., 2007, 17(3):381–6.]