Fig. 8.

Presynaptic terminal/axon-derived invaginating projections. a In the butterfly, Agraulis vanillae, invaginating projections form between axons in the compound eye units or ommatidia; each ommatidium has nine photoreceptor (retinula) cell axons including two close triads of axons, each from two diagonal (d) and one horizontal (h) retinula cells (Gordon 1985). The diagram shows a diagonal and horizontal axon from one ommatidium on the left, and a horizontal axon from an adjacent ommatidium on the right. The invaginating projections link diagonal and horizontal cell axons within a triad and also form between the horizontal axons from triads in two adjacent ommatidia. Note how the invaginated membrane is lined with a regular array of microtubules (indicated by black dots in all three drawings); only the microtubules associated with the invaginations are illustrated, although the entire axon cross sections are filled with microtubules and other filamentous structures. b Pseudopodial interdigitations (PSIs) between axon terminals in the globus pallidus. Only the sides of the terminals where they interdigitate are illustrated. PSIs can be simple and short (b1 example from the squirrel monkey; Fox et al. 1974) or long and compound (b2 example from the rat; Boyne and Tarrant 1982). Note that some synaptic vesicles are found within the PSIs. c In the cerebellum of chick embryos (Palacios-Prü et al. 1981) and early postnatal rats (Altman 1971, 1993; Altman and Bayer 1997), first contacts between parallel fiber axons (ax) and Purkinje cell dendrites (Pj de) appear to induce formation of thin spinules from the axons and these invaginate into coated pits on the dendrite. d In some synapses of the molecular layer of the dentate gyrus of adult rats, a central presynaptic terminal can be enwrapped completely by a concave spine to form a synapse (Desmond and Levy 1983) (dendritic processes in blue and glial processes in red) (Color figure online)