An ultrastructural analysis of the photoreceptors of the squid and their synaptic connections. I. Photoreceptive and non‐synaptic regions of the retina

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Abstract
As part of a larger ultrastructural study on the photoreceptors of the squid,Loligo pealei, this paper reports findings on non‐synaptic regions. The cross‐sectional profiles of individual distal segments coincided in horizontal sections as far as 75 μ apart, but progressively less with greater separations. Areas of cross‐section showed a unimodal distribution skewed towards 6 μ2, the smallest areas. The distribution of ratios of villous to smooth border was unimodal and skewed with 1.0 being the most numerous and minimal ratio. This ratio tended to increase linearly with area. Cells of small cross‐section were completely surrounded by villous border of other cells and about 60° of the perimeter of any cell had microvilli on both sides of the border. Cells participated in odd or even numbers of rhabdomes, up to eight. Pentalaminar membrane junctions between microvilli of the same or neighboring cells were observed. Cytoplasmic microtubules were concentrated near villous borders. In light adaptation some ommin granules entered the photoreceptor tips, and the rhabdomes which formerly began near the surface, now began somewhat below this level. A terminal bar zone existed proximal to the rhabdomes. Here glial cytoplasm separated photoreceptors and both cells contained ommin granules. Photoreceptor somas lacked glial separation and were highly interplicated. Mitochondria were longitudinally arranged along the somal membrane. The planes of the lamellate membrane bodies, seen in the somal cytoplasm of the cells, did not exhibit a rectilinear pattern such as that displayed by the basically rectangular distal segment cross‐sections and microvillar orientations.

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