Ontogeny of the visual system in the cuttlefish, sepiella japonica. II. Intramembrane particles, histofluorescence, and electrical responses in the developing retina

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Abstract
To study molecular and functional differentiation of photoreceptive membranes, the development of the retina in embryos of the cuttlefish, Sepiella japonica, was examined by the freeze‐fracture technique, by histofluorescence of retinal‐related protein, and by electrical responses to photic stimulation. P‐face particles in the microvillar plasma membrane of the receptor cell increased in density from about 2,000/μm2 to about 6,000/μm2 during the early stages of rhabdomere formation (from stage 31 to stage 36). The size distribution of P‐face particles in the microvillar and the cell body plasma membranes showed a similar monomodal pattern with a peak at 8 nm in diameter from stage 31 to 33. In the cell body plasma membrane, the size distribution pattern hardly changed throughout later stages, but in the microvillar membrane, the peak shifted progressively to 9 nm (stages 35 and 36) and then to 10 nm (stage 37). The histofluorescence of reduced rhodopsin was first visible at stage 36 in the layer of growing apical processes. The fluorescence of reduced retinochrome first appeared faintly at stage 40 on both sides of the basal lamina. Electroretinograms (ERGs) were first obtained at stage 34 when many microvilli appeared from the apical processes, increasing rapidly in amplitude with increase in regularity of the microvillar arrangement from stage 35 to 36. Early receptor potentials (ERPs) elicited by a bright light flash were first recorded at the earliest stage of the apical process formation (stage 30), increasing gradually in amplitude during development. The increase was correlated with an increase in the total number of P‐face particles in the microvillar membrane within the retina. These findings suggest that the P‐face particles in the microvillar membrane are associated with rhodopsin, that the receptor cells have a small amount of rhodopsin before rhabdomere formation, and that differentiation of the apical plasma membrane of the receptor cells into the functional photoreceptive membrane occurs in conjunction with its morphological differentiation into rhabdomeres.