An ultrastructural analysis of the photoreceptors of the squid and their synaptic connections. II. Intraretinal synapses and plexus

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Abstract
The plexus level of the retina of the common squid, Loligo pealei was studied by electron microscopy and was found to be layered in the sclerad direction by imprecise zones respectively characterized by receptor collaterals, synaptic profiles, horizontal fibers, and receptor axons and other neurites ensheathed in glia. Receptor axons characteristically contained microtubules and dense mitochondria, passed vertically through the layer of horizontal fibers and then turned horizontally as they became collectively ensheathed. Receptor collaterals emerged at the level of the most proximal receptor nuclei and at intermittent regions across the retina entered into gap junctions with similar appearing processes. Horizontal fibers may represent efferents which originate in similar fibers sheathed in retinal glia and also seen in the optic nerve and lobe. They occasionally formed en passant synapses on receptors which were characterized by clusters of small clear vesicles and increased membrane density. They also gave rise to branches whose terminals have not been directly traced. Some of these branches showed both clear and dense‐cored vesicles. Synapses on receptors had a cluster of small clear vesicles and one to three large dense‐cored vesicles which were not involved in the cluster. Increased density of the synaptic membrane was best seen after primary osmium fixation. Aldehydeosmium and direct osmium fixed material are presented for comparison. Fibers somewhat similar in appearance to horizontal fibers but which were smaller and more dense had regions filled with smaller dense‐cored vesicles which never showed clustering on a membrane or a consistent relationship with a putative post‐synaptic cell. The possible origins of the neurites of the retinal plexus are discussed.