The organization of dense projections in synapses of the brain of a marsupial, Setonix brachyurus

Synaptic junctions from the lateral geniculate nucleus of the quokka brain have been examined by electron microscopy after glutaraldehyde fixation and PTA staining. Junctions can be divided into two groups: discontinuous-continuous and continuous-continuous, depending on the continuity or lack of it of the presynaptic membrane. The postsynaptic membrane always has a continuous appearance. The dense projections have the following characteristics: 1. an irregular outline due to the presence of spikes; 2. in some cases, an external electron-opaque “skin” and an internal, relatively electron-translucent core; 3. adjacent projections may have one or more stands of the presynaptic network between them and extending to the internal coat of the presynaptic membrane; 4. the gap between the bases of dense projections varies from 5 to 69 nm, with a mean value of 23 nm. Seventy-six percent of these measurements fall within the range 5-25 nm, while many of the larger gaps separating dense projections are subdivided into smaller components by the presence of one or more strands of the presynap-tic network. Features 2 and 3 have not been previously described in exactly the form seen here. It is not known whether they are confined to marsupial synapses or whether they highlight important general features of synaptic organization.