Fig. 7. Ultrastructural comparison of synapses within the outer plexiform layer of wild-type and laminin β2 chain-deficient retinas at postnatal day 21. Sections of wild-type (A) and laminin β2 chain-deficient (B–H) retinas were viewed with transmission electron microscopy. In the normal retina, rod photoreceptors make synapses, called triads (T), with three postsynaptic elements (*); the ribbon is a presynaptic specialization that marks the active site for release; the three postsynaptic elements invaginate into the base of the photoreceptor such that two horizontal cell dendrites lie laterally, and one bipolar cell dendrite lies centrally. The photoreceptor wraps around the bipolar cell (arrows). In the β2 chain-deficient animal (B–H), several different types of synapses are present. Triads (T) like those in the wild type animal are rare; dyads, with only one or two horizontal cell processes apposed to the ribbon, are more common (D); dyads are seen in wild-type retinas as well (not illustrated). Also common in the β2 chain-deficient retinas are floating synapses (F), wherein a fully formed ribbon, often with vesicles associated, is seen without any postsynaptic element apposed. This type of synapse is seen extremely rarely in the wild-type retina (see Table 1). Finally, occasionally, two ribbons from the same photoreceptor will be apposed to a single postsynaptic element (2); this was also rarely observed in the wild-type animal. Scale bar: A, D, E, 850 nm; B, C, F–H, 630 nm.