Membrane particles and gap junctions in the retinas of two species of cephalopods, Octopus ocellatus and Sepiella japonica

Ultrastructure of electrical synapses: review

This article reviews studies providing information on the ultrastructure of electrical synapses. Although the review focuses on electron-microscopic investigations, its aim is to examine how the structure of an electrical synapse relates to its function. It begins by presenting a historical overview of the early studies which were responsible for the recognition of electrical synapses. The structure of gap junctions which are the morphological correlates of electrical synapses is illustrated and the ultrastructure and function of the two types of electrical synapse, rectifying and non-rectifying, described. Recent papers investigating the ultrastructure of electrical and mixed electrical-chemical synapses in invertebrates and vertebrates are reviewed. For earlier references, the reader is directed to previous reviews on the subject. Much new information, however, on the structure and formation of electrical synapses has been obtained from work on cultured neurons and from electron-microscopic, immunocytochemical, conformational and molecular studies. This article reviews those studies and in light of their findings, re-examines the relationships of the structure of electrical synapses with their function.

Daily changes of structure, function and rhodopsin content in the compound eye of the crab Hemigrapsus sanguineus

The compound eye of the crab hemigrapsus sanguineus undergoes daily changes in morphology as determined by light and electron microscopy, both in the quantity of chromophore substances studied by HPLC and in visual sensitivity as shown by electrophysiological techniques. 1. At a temperature of 20 degrees C, the rhabdom occupation ratio (ROR) of an ommatidial retinula was 11.6% (maximum) at midnight, 8.0 times larger than the minimum value at midday (1.4%). 2. Observations by freeze-fracture revealed that the densities of intra-membranous particles (9-11 nm in diameter) of rhabdomeric membrane were ca. 2000/microns 2 and ca. 3000/microns 2 for night and daytime compound eyes, respectively. 3. Screening pigment granules migrated longitudinally and aggregated at night, but dispersed during the day. Reflecting pigment granules migrate transversally in the proximal half of the reticula layer i.e. cytoplasmic extensions containing reflecting pigment granules squeeze between neighbouring retinula cells causing optical isolation (Fig. 4). Thus the screening pigment granules within the retinula cells show longitudinal migration and radial movement so that the daytime rhabdoms are closely surrounded by the pigment granules. 4. At 20 degrees C, the total amount of chromophore of the visual pigment (11-cis and all-trans-retinal) was 1.4 times larger at night than during the day i.e. 46.6 pmol/eye at midnight and 33.2 pmol/eye at midday. Calculations of the total surface area of rhabdomeric membrane, total number of intra-membranous particles in rhabdomeric membrane and the total number of chromophore molecules in a compound eye, indicate that a considerable amount of chromophore-protein complex exists outside the rhabdom during the day. 5. The change in rhabdom size and quantity of chromophore were highly dependent on temperature. At 10 degrees C both rhabdom size and amount of chromophore stayed close to daytime levels throughout the 24 hours. 6. The intracellularly determined relative sensitivity of the dark adapted night eye to a point source of light was about twice as high as the dark-adapted day eye. Most of the increase in the sensitivity is attributed primarily to the effect of reflecting pigment migration around the basement membrane and, secondarily, to the changes in the amount and properties of the photoreceptive membrane. The results form the basis of a detailed discussion as to how an apposition eye can function possibly as a night-eye.

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

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 histo-fluorescence 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/micron2 to about 6,000/micron2 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.

Ontogeny of the visual system in the cuttlefish, Sepiella japonica. I. Morphological differentiation of the visual cell

To provide morphological bases for functional studies, the differentiation of photoreceptors in the retina of a cuttlefish, Sepiella japonica, was examined by electron microscopy. Development of the retina occurs in four phases. In phase 1 (stages 21-23), the retinal epithelium consists of uniform columnar cells. In phase 2 (stages 24-29), the epithelial cells are classified into light and dark cells on the basis of the electron-density of their cytoplasm. The perikarya of the light cells are located in the distal half of the epithelium, each extending proximally (i.e., sclerad) a thin foot, which protruded out of the epithelium through a narrow break in the basal lamina. The nuclei of the dark cells are distributed in the proximal half of the epithelium. Both cell types have a short cilium in the distal end. In phase 3 (stages 30-35), the light and the dark cells morphologically differentiate into receptor and supporting cells, respectively. The distal end of the light (receptor) cell begins to form an apical process, which gives off short irregular microvilli from its lateral surface. Beneath the basal lamina the feet of the receptor cells swell and the nuclei migrate into them through narrow breaks in the basal lamina. Thus, at the end of phase 3, each receptor cell becomes divided into an outer and inner segment. In phase 4 (stages 36-40), the receptor cells complete the specific structure as photoreceptive cells. The microvilli arising from the apical processes increase in length, density, and regularity to be organized into rhabdomes. The lamellated membranes of the myeloid bodies appear first in the outer segment and later increase in number in the inner segment. Morphometric data on the rhabdomeric microvilli in the adult and embryonic retinas are presented.