Spinules on mushroom dendritic spines. a, Micrograph through the center of a spinule (turquoise) emerging from a perforation (arrowhead) in the postsynaptic density into the presynaptic axon. b, Reconstruction of the spine illustrated in a with the spinule in turquoise and the PSD surface area in red. c, d, Serial sections through the presynaptic axon (pre; green) and a spinule (arrowhead; turquoise) emerging from the edge of a mushroom spine head into the presynaptic axon and another spinule also emerging from the edge of the spine head (arrowhead; lavender) but invaginating a neighboring axon (n). e, High magnification of serial section beyond d showing a coating along the cytoplasmic surface of the spinule on the side of the invaginated presynaptic axon (arrowhead). f, Later sections of the mushroom spine head showing where the presynaptic axon deeply invaginated the spine head in a vesicle-free zone adjacent to a cell-adhesion (arrow) that is adjacent to the postsynaptic density on subsequent serial sections. g, Three-dimensional reconstruction of the mushroom spine (beige) with perforated synapse (red) and several small spinules into the presynaptic axon (turquoise spinules) or neighboring axon (lavender spinules). Reproduced from ref. 76 with the permission of the Society for Neuroscience (copyright 2004) and Dr. Harris.