Visualizing Biological Membrane Organization and Dynamics

Biological membranes are fascinating. Santiago Ramón y Cajal, who received the Nobel prize in 1906 together with Camillo Golgi for their work on the nervous system, wrote “[..]in the study of this membrane[..] I felt more profoundly than in any other subject of study the shuddering sensation of the unfathomable mystery of life”[1]. The visualization and conceptualization of these biological objects have profoundly shaped many aspects of modern biology, drawing inspiration from experiments, computer simulations, as well as from the imagination of scientists and artists. The aim of this review is to provide a fresh look on current ideas of biological membrane organization and dynamics by discussing selected examples across fields [1] The full quotation is “I must not conceal the fact that in the study of this membrane I for the first time felt my faith in Darwinism (hypothesis of natural selection) weakened, being amazed and confounded by the supreme constructive ingenuity revealed not only in the retina and in the dioptric apparatus of the vertebrates but even in the meanest insect eye. There, in fine, I felt more profoundly than in any other subject of study the shuddering sensation of the unfathomable mystery of life.” from the autobiography Recollections of My Life.

Morphology of the Eye of the Hydrothermal Vent Shrimp, Alvinocaris Markensis

The bresiliid shrimp Alvinocaris markensis is a predator that inhabits the base of sulphide mounds built by the black smoker chimneys of active hydrothermal vents at the Snake Pit site on the Mid-Atlantic Ridge. Casual examination of animals collected with theDSV ‘Alvin’ suggests that, like other biesiliid shrimp from hydrothermal vents, the eyes of this species are adapted for vision in very dim light. However, examination of the structure and ultrastructure of eyes of animals collected and immediately fixed shows that the expected massive array of photoreceptors is partially or completely missing. The eye is enlarged, its dioptric apparatus has disappeared, its screening pigment is essentially gone, and its reflecting pigment cells have formed an enlarged mass of white diffusing cells behind the expected layer of photoreceptors. In half of the animals examined, there were no recognizable photoreceptors in the retina, and in the remaining animals there were only scattered photoreceptors with poorly organized microvillar arrays of photosensitive membrane. We conclude that this species is blind despite some retinal adaptations for vision in very dim light. Apparently, the ambient light of this animal's environment is below the quit point (the minimum level that can be exploited) so that the retina has begun to degenerate by losing its photoreceptors.

Possible role of the central retinular cells in the ommatidia of male black flies (Diptera: Simuliidae)

In Cnephia dacotensis, a species that mates on rocks and plants without swarm formation, the eyes of the males are separate and undivided. Each ommatidium consists of two general regions: a distal dioptric apparatus and a sensory receptor layer with eight retinular cells. Six of these cells (R1–6) are located peripherally and two centrally; R7 occurs distally and R8 basally. In males of previously studied species in which females are detected as they fly above a male swarm, the compound eyes are holoptic and divided into distinct dorsal and ventral regions. Ommatidia in the dorsal region lack the R7 cell. If in black flies R7 is a blue receptor and R8 a uv receptor, then the absence of R7 means that swarm-forming males see the females against a background that provides a sharper contrast than a background of a uv to blue range. This would sharpen the visibility of the dark female against the background skylight, enabling the male to perceive her more swiftly.

Retinal mosaics of the principal eyes of two primitive jumping spiders, Yaginumanis and Lyssomanes : clues to the evolution of Salticid vision

Retinae of the principal eyes of jumping spiders (Salticidae) have four tiers of photoreceptors. A previous paper described the receptor mosaics in all four layers at the retinal fovea for two forms, Lagnus and Portia , shown by behavioural experiments to possess high visual acuities. Only Layer I farthest from the dioptric apparatus has a mosaic quality that can sustain the demonstrated visual discriminations, leaving the roles of Layers II—III open to question. We now describe the retinal mosaics of two presumptively primitive species Yaginumanis is placed with Portia in a newly erected subfamily of Salticidae, the Spartaeinae, whose members possess functional posterior median eyes; those of Lyssomanes are vestigial as in advanced Salticids, but the genus is usually considered primitive and its ethology indicates restricted visual capabilities. Layer I receptors of Yaginumanis each bear two rhabdomeres on opposite faces of the cell which are contiguous with those of adjacent receptors, so that optical pooling substantially limits acuity. The same arrangement is present in receptors in the peripheral retina of Lyssomanes but at the fovea each cell bears only a single rhabdomere, as in Salticids with high visual acuities. In both species, Layer II receptors have two rhabdomeres throughout the retina and centre-to-centre spacings that approximately match those of the Layer I receptors that they overlie. Layers III and IV are essentially the same in all Salticids: Layer III amounts to a virtually continuous sheet of rhabdomeres, and Layer IV is divided into three regions with a complex organisation. Neither is suitable to sustain fine visual discriminations. The organization of the salticid principal retina emerges as anatomically conservative, and contrasts with that of the secondary eyes which varies greatly between primitive and advanced species. A working hypothesis is proposed to explain its evolution.

Circulation and dioptric apparatus in the photophores of Euphausia pacifica: some ultrastructural observations

The dioptric or light-directing apparatus of the abdominal photophores of Euphausia pacifica is briefly described. Electron micrographs of the lens, reflector, and lamellar ring are presented, with accompanying explanation concerning their probable optical role. Elements of the circulatory system are also discussed, and it is proposed that the 'rod mass' of classical cytologists is a highly ordered capillary bed.