Fine structure of ocelli in sphinx moths

The internal ocelli of sphinx moths have receptor cells with a rhabdomere structure that is unique for insects. The rhabdomere consists of an invagination of a single receptor cell membrane to produce a cavity lined with microvilli and containing small lumen.     

Fine structure of the cerebral ocelli of a sipunculid,Phascolosoma agassizii

Summary The brain ofPhascolosoma agassizii, a sipunculid worm, contains a pair of ocelli. Each ocellus lies at the inner end of the ocular tube, an invagination that connects the concavity of the ocellus with the anterior surface of the head. The cuticle which covers the epidermis of the worm extends into and lines the ocular tube. The cells of the neck of the tube are columnar and contain longitudinally oriented tonofilaments extending into microvilli that project into the cuticle in the tube. Cilia also project from the apices of these cells. Toward the base of the ocular tube are two kinds of columnar cells: supportive and photoreceptive. The supportive cells contain varying concentrations of melanin-like granules forming the pigmented component of the ocellus. Numerous longitudinally oriented tonofilaments in these cells extend into microvilli projecting into the cuticular layer. The photoreceptor cells, containing many microtubules, lie between the supportive cells and spread out at their tips giving off an irregular array of miorovilli, the presumed photoreceptors, and cilia. These photoreceptors are regarded as belonging to the rhabdomeric type, albeit cilia are present.This investigation was financed by grant number GM 10292 from the National Institute of General Medical Science.     

Fine structure of the segmental ocelli of Armandia brevis (Polychaeta: Opheliidae)

Summary Armandia brevis responds negatively to light during the benthic phase and positively to light during the epitokous phase of its life history. In addition to the prostomial photoreceptors this slender translucent marine worm possesses eleven pairs of ocelli arranged serially from the 7th to the 17th segments.Each ocellus is located at the inner edge of the epidermis slightly in front of the parapodium and contains a single photoreceptor cell which gives off approximately 15 sensory processes. These processes are composed of a central core of neurofibrils surrounded by a mitochondrial layer and a compact array of microvilli. The sensory processes project into and nearly fill the ocellar cavity which is lined by squamous glial cells.The pigment cup enclosing the photoreceptor is composed of about 30 cuboidal cells packed with brown granules. The pigment cells form a mesothelium, being in direct continuity with the coelom. The cup is separated from the glial cells by a basal lamina which separates the epidermal tissues from the mesodermal derivatives of the body wall. Slender muscle fibers traverse the coelom and pass between the cells of the pigment cup.The prostomial photoreceptors were re-examined and found in this material to be composed of microvilli rather than of folds containing labyrinthine tubular infoldings of the cell surface as previously reported.The author thanks Dr. Richard M. Eakin for support and criticism. This investigation was financed by a postdoctoral fellowship, number 1-F2-GM-20, and grant number GM 10292 from the National Institute of General Medical Science.     

Evolutionary significance of fine structure of archiannelid eyes

Summary The structure of the ocelli of representatives of four of the five families of archiannelids (Polychaeta: Annelida) was studied by light and electron microscopy. The apparent photoreceptoral organelle in each species is an array of microvilli (rhabdomere). Cilia were observed in the eyes of only a couple of specimens in one species of archiannelid (Nerilla antennata). They were unassociated with the rhabdomeres; we regard them as adventitious. Support is given by this study to the theory that the photoreceptoral organelle of the ancestral annelid was a rhabdomere. Other features of the ocelli are described and illustrated.We acknowledge the support of a grant-in-aid of research (GM 10292) from the United States Public Health Service; the skillful transformation of our sketches into finished drawings by Phyllis Thompson; the assistance of Jean Brandenburger in the electron microscopy of villar particles; and the critical reading of the paper by Mrs. Brandenburger and Drs. Colin O. Hermans and Ralph I. Smith     

Immunohistochemical and ultrastructural analysis of the muscular and nervous systems in the interstitial polychaete Polygordius appendiculatus (Annelida)

The systematic position of Polygordiidae is still under debate. They have been assigned to various positions among the polychaetes. Recent molecular analyses indicate that they might well be part of a basal radiation in Annelida, suggesting that certain morphological characters could represent primitive character traits adopted from the annelid stem species. To test this hypothesis, an investigation of the muscular and nervous systems by means of immunological staining and confocal laser scanning microscopy and transmission electron microscopy was conducted. With the exception of the brain, the nervous system is entirely basiepidermal and consists of the brain, the esophageal connectives, the subesophageal region, the ventral nerve cord and several smaller longitudinal nerves. These are connected by a considerable number of ring nerves in each segment. The ventral nerve cord is made up of closely apposed longitudinal neurite bundles, a median and two larger lateral ones. Since distinct ganglia are lacking, it represents a medullary cord. The muscular system mainly consists of longitudinal fibers, regularly distributed oblique muscles and strong septa. The longitudinal fibers form a right and a left unit separated along the dorsal midline, each divided into a dorsal and ventral part by the oblique muscles. Anteriorly, the longitudinal musculature passes the brain and terminates in the prostomium. There is no musculature in the palps. In contrast to earlier observations, regularly arranged minute circular muscle fibers are present. Very likely, a basiepithelial and non-ganglionic organization of the ventral nerve cord as well as an orthogonal nervous system represent plesiomorphic characters. The same applies for the predominance of longitudinal muscle fibers.     

Fine structure studies of the ocelli of Polyorchis penicillatus (Hydrozoa,Anthomedusae) and their connection with the nerve ring

Summary The fine structure of the small compact ocelli (50–100 m in diameter) of Polyorchis penicillatus is described. The ocellar cup is formed of pigment cells and receptor cells. The pigment cells occur in approximately a 2:1 ratio to the receptor cells. Each pigment cell has a process that may pass through the presumed photosensory region. Pigment cells are connected to adjacent receptor cell processes by septate junctions. The sensory cells are bipolar with the apical part forming the receptor process and the basal part forming an axon 8–15 m long and 1–2 m in diameter. Each receptor cell axon forms a synapse with a single second order neuron but the sensory cells are also connected to the second order neurons postsynaptically. There are also synapses between adjacent second order neurons. The second order neurons lie outside the ocellar cup, next to the tentacular mesogloea. Each second order neuron forms an axon of about 1 m thickness. The axons on each side group together to form an optic nerve having 30–40 axons that travel around the tentacle base on either side and enter the outer nerve ring independently.     

Fine structure and function of the abdominal chloride epithelia in caddisfly larvae

Summary The larvae of the caddisfly Anabolia nervosa Curt. (Limnephilidae) possess 10 fields on the dorsal and ventral sides of the 3rd to 7th abdominal segments, which were formerly regarded as specialized sites of respiration. The epithelial fine structure and histochemical localization of chloride unequivocally show that the main function of these sites is the transport of electrolytes. They probably participate in osmoregulation by the absorption of salt. Therefore, these specialized areas of the hypodermis are termed chloride epithelia.     

Fine structure and function of the gut epithelium of pike eel larvae

The development of the alimentary canal of pike eel, Muraenesox cinereus , that were hatched and reared in the laboratory, was examined. The larvae did not feed on zooplankton or phytoplankton and survived for 10 days. Histological and histochemical investigations showed that the alimentary canal was morphologically and functionally differentiated in 3 days after hatching and before exhaustion of the yolk. The foregut was not functional in absorption. The midgut absorptive cell was characterized by a well-developed lamellar membranous structure in the basal portion of the cytoplasm. Na¹-K¹ ATPase activity was demonstrated on the lamellar membrane of the structure. It suggests that water and solute transport is quite active in the midgut epithelium. Orally administered HRP (horseradish peroxidase) was ingested and intracellularly digested in the hindgut absorptive cells. Some of the HRP was transported, without complete digestion, into tissue spaces under the epithelium. Evidence suggests that pike eel larvae possibly utilize dissolved organic matter, including soluble macromolecules, as their nutrition sources.     

Fine structure and function of the rectal chloride epithelia of damselfly larvae

Zygopterous larvae posses three epithelial pads in the rectum which show the fine structural organization of ion transporting epithelia. They are directly exposed of the external medium, which is ventilated through the anus. The ventilation frequency is related to the external salinity. Adaptation to highly hypotonic salt solution (10 μM NaCl) leads to a high ventilation activity. Histochemical precipitation of chloride results in dense silver chloride deposits specifically lining the lumen side of the epithelia. These results suggest that the rectal chloride epithelia are involved in osmotic hyperregulation by the absorption of electrolytes.     

The fine structure of the ocelli of Schistocerca gregaria

Summary A study of the organisation of the locust dorsal ocellus shows that the structure is designed to provide the maximum possible effective aperture. The condenser-like cuticular lens and the dispersal of the rhabdome over a large proportion of the circumferential area of the retinula cells increases the light gathering power of the eye. The synaptic plexus of the ocellus has two major features: (i) the retinula cells are repeatedly and reciprocally connected by synapses and junctions, and (ii) there is an extensive lateral and feedback network between the receptors and interneurons. A unified structure is described for a synapse that presents differing profiles dependent upon the angle of section. A distinct morphological class of junction is described between retinula cells. The synaptic arrangements of morphologically identical retinula cells vary from cell to cell and the synaptic plexus is not organised with a high degree of spatial precision. The overall synaptic configurations are discussed in terms of the varied response characteristics of units in the ocellar nerve.     

Fine structural organization of the lateral ocelli in two species of Scolopendra (Chilopoda: Pleurostigmophora): an evolutionary evaluation

The lateral ocelli of Scolopendra cingulata and Scolopendra oraniensis were examined by electron microscopy. A pigmented ocellar field with four eyes arranged in a rhomboid configuration is present frontolaterally on both sides of the head. Each lateral ocellus is cup-shaped and consists of a deeply set biconvex corneal lens, which is formed by 230–2,240 cornea-secreting epithelial cells. A crystalline cone is not developed. Two kinds of photoreceptive cells are present in the retinula. 561–1,026 cylindrical retinula cells with circumapically developed microvilli form a large distal rhabdom. Arranged in 13–18 horizontal rings, the distal retinula cells display a multilayered appearance. Each cell layer forms an axial ring of maximally 75 rhabdomeres. In addition, 71–127 club-shaped proximal retinula cells make up uni- or bidirectional rhabdomeres, whose microvilli interdigitate. 150–250 sheath cells are located at the periphery of the eye. Radial sheath cell processes encompass the soma of all retinula cells. Outside the eye cup there are several thin layers of external pigment cells, which not only ensheath the ocelli but also underlie the entire ocellar field, causing its darkly pigmented. The cornea-secreting epithelial cells, sheath cells and external pigment cells form a part of the basal matrix extending around the entire eye cup. Scolopendromorph lateral ocelli differ remarkably with respect to the eyes of other chilopods. The dual type retinula in scolopendromorph eyes supports the hypothesis of its homology with scutigeromorph ommatidia. Other features (e.g. cup-shaped profile of the eye, horizontally multilayered distal retinula cells, interdigitating proximal rhabdomeres, lack of a crystalline cone, presence of external pigment and sheath cells enveloping the entire retinula) do not have any equivalents in scutigeromorph ommatidia and would, therefore, not directly support homology. In fact, most of them (except the external pigment cells) might be interpreted as autapomorphies defining the Pleurostigmophora. Certain structures (e.g. sheath cells, interdigitating proximal rhabdomeres, discontinuous layer of cornea-secreting epithelial cells) are similar to those found in some lithobiid ocelli (e.g. Lithobius). The external pigment cells in Scolopendra species, however, must presently be regarded as an autapomorphy of the Scolopendromorpha.     

Fine structure of wall bodies in large cells of an Anabaena species.

A new type of inclusion in blue-green algae termed a wall body is described. These are elongate bodies of medium electron density, from 6 to 70 nm in thickness and at least 0.2 micrometer in depth found in large cells of Anabaena sp. B387. The bodies are up to 12 micrometer long, extending the complete length of the cell in some cases. Some of the bodies are bifurcated and the ends appeared to come in contact with the plasma membrane. The wall bodies lie at various angles in the cells. Up to six have been observed in a single thin section of a cell. The large cells are found in greater numbers in cultures grown in a medium solidified with agar. Comparison of the bodies to the mucopolymer layer (layer 2) of the cell wall is made.     

The fine structure of the dorsal ocelli in the male bibionid fly

The dorsal ocelli of bibionid flies, details of which have not previously been described, were examined in males of Dilophus febrilis. The three ocelli are combined within an elevated chitin capsule, in a medial position between the enlarged dorsal compound eyes. The biconvex lenses show a multiple layering of up to 150 regularly spaced, clear and dense cuticle zones (100 nm spacing) which probably provide some spectral filtering, suggested by in vivo observations with an epifluorescence microscope. The corneagenous cells and the retina with 100-200 photoreceptor cells are adjoined proximally. A distal retina zone comprises the rhabdoms, which are laterally connected in an hexagonal network. The rhabdoms are between 4 and 15 mum in length; they decrease gradually from the dorsal to the ventral retina region. A middle retina zone comprises the receptor somata, a proximal zone, their axons. Synaptic contacts between axons and interneuron dendrites, feedback synapses to axons, and axo-axonic synapses are found, showing varying pre-synaptic structures. A possible functional role of the ocelli is discussed.     

Fine structure of Methanospirillum hungatii.

The fine structure of Methanospirillum hungatii was studied by electron microscopy. The topography of the cell wall and the mechanism of cell division are not typical of gram-positive or gram-negative bacteria. A novel architectural arrangement of cells in continuous spiral filaments is described. Filamentous cells are connected by spacers and enclosed within a rigid outer envelope. The unique ultrastructural features of cells and cell spacers were examined.