Egg envelope synthesis and chorion modification after oviposition in the dragonfly Libellula depressa (Odonata, Libellulidae).

Libellula depressa (Odonata, Libellulidae) is an exophytic dragonfly ovidepositing eggs in clutches on the surface of floating plants and algae. The present work investigates, at ultrastructural level, the gradual differentiation of the egg envelopes and the chorionic changes after egg deposition in water. The ovary of the mature female of L. depressa is composed of numerous strings of panoistic ovarioles, where the eggshell formation takes place gradually throughout the activity of the follicle cells. The present data show that the egg envelopes are constituted of a very thick electrondense vitelline envelope, a thin endochorion and an extremely thick exochorion composed of a fibrillar matrix resting on a thin electrondense layer. After deposition in water, L. depressa eggs, initially white and almost transparent, gradually become brown spots in a semitransparent jelly coat, rich of incorporated debris. The jelly coat enveloping the eggs of L. depressa derives exclusively from the exochorion, constituted of a fibrillar matrix, which swell at contact with water. The jelly-like coat performs an adhesive function and presumably a protective role during egg segmentation and ensuing larval hatching.

Post-larval development of the commercial sponge Spongia officinalis L. (Porifera, Demospongiae).

This study investigated the development of the larvae of Spongia officinalis in experimental conditions, after settlement on plastic substrates, using electron and light microscopy. The released larvae show a dark pigmented ring distinguishes the posterior larval pole. The youngest larvae, covered with a flagellate epithelium, move onwards by rotating on their longitudinal axis. Over time a creeping-like motion prevails, probably linked to the need for settlement. After a free-swimming period of 24-48 h, larvae settle on the artificial substrate by the anterior pole. At settlement, the flagellate epithelium is substituted by flattened cells, which delimit the outermost surface. Post-larvae were reared to about three months. The early phase of post-larval differentiation shows a solid interior mainly consisting of granular cells varying in shape and size. They are included in a dense collagen matrix that contains a conspicuous amount of bacteria. Lacunae are already evident in the initial phase of metamorphosis. In several of them, cell debris and nucleate cells are visible. This feature is consistent with a progressive reduction of the cell mass (autolysis). Neither choanocyte chambers nor canals differentiate. The morphogenetic process leads to a metamorph only consisting of vacuolated cells and collagen fibrils included in a thin fibrous coat.

Dissociated cells of the calcareous sponge clathrina: a model for investigating cell adhesion and cell motility in vitro.

The study of cell-cell and cell-substratum adhesion in vitro is useful for understanding cell behavior in a three-dimensional pattern. We have used dissociated cells (choanocytes represent the main fraction) from the calcareous sponge Clathrina, namely C. cerebrum and C. clathrus, to illustrate our present understanding on three main aspects of cell-cell and cell-substratum adhesion in vitro: (1) cytoskeletal protrusions; (2) cell behaviours on organic substrata; and (3) paths of locomotory sponge cell. Cell locomotion occurs by the extensions of scleropodial and lamellipodial protrusions, by way of actin polymerization. The extent to which cells produce these cytoplasmic processes varies according to the substratum (e.g., collagen, fibronectin, laminin, polylysine). It was found that more cell extensions were produced on collagen substrata, and this led to greater cell movement. Advancing choanocytes are not polarized. Their paths are particularly complicated, showing linear segments, which produce a more efficent cellular translocation, and winding tracts with frequent turns or loops. Small amoeboid cells describe more linear paths with a wide range of speed variation than larger cells. The presence of cell-derived substratum reduces the progressive dispersion of cells and allows cells to encounter one another in such a way that the initial random walking later turns into non-random displacement. Even though cAMP-treated cells exhibit different aggregative tactics, cAMP 10(-8) M remarkably enhances cell encounters and supports the existing information that this cyclic nucleotide represents a signal that affects cell morphology and locomotion. The bulk of data on sponge cell-cell and cell-substratum adhesion has been evaluated by mentioning the significant advances and references concerning studies of other cell systems.

Larval antennal sensilla in water-living insects.

An overview of larval antennal sensilla in hemimetabolous and holometabolous water-living insects is given by updating current knowledge on the fine structure of these sensory systems. In the absence of successful electrophysiological studies, the possible function of sensilla is deduced from their architecture. Various kinds of sensilla are described in hemimetabolous insects, such as Ephemeroptera, Odonata, and Plecoptera, and holometabolous insects, such as Diptera Nematocera (Culicidae, Simuliidae, Psychodidae, Chaoboridae) and Trichoptera. Their possible function in responding to stimuli from the freshwater environment is illustrated and discussed. The importance of sensilla as taxonomic and phylogenetic traits is reported in Baetidae (Ephemeroptera) and in Diptera Nematocera. Some homologies outlined in Diptera Nematocera give evidence of a link between species, such as Chaoboridae and Psychodidae, whose antennae are highly modified, prehensile in the former and very reduced in the latter. Particular features, such as cuticle without pores in chemosensory sensilla and naked perikarya, are so far found exclusively in some water-living arthropods, thus reflecting a possible adaptation to the aquatic habitat. The structure of sensilla and chloride cells, which have a similar external morphology, is presented and discussed in various insect groups, considering the possible derivation of the chloride cells from sensilla.

Peculiar digestion patterns of sponge-associated zoochlorellae in the caddisfly Ceraclea fulva.

The caddisfly Ceraclea fulva feeds exclusively on the freshwater sponge Ephydatia fluviatilis. Sponge spicules are accumulated in the insect midgut and arranged perpendicularly to the longitudinal axis of its gut. The peritrophic membrane of the midgut is so thick that it prevents spicules from damaging the epithelium during their transit. The digestion process of the endocellular zoochlorellae, which are vehiculated by the sponge cells, was examined by transmission electron microscopy (TEM). Zoochlorellae were seen in the midgut lumen, close to the peritrophic membrane and in the underlying space. Discrete algal cells became evident in tight apposition to the brush border of the midgut cells and were enveloped by the microvilli. Digestion progressed to the final transformation of the organism into membrane-delimited vacuoles.

Morphological responses of dissociated sponge cells to different organic substrata.

To study interactions between sponge cells and components of the extracellular matrix (ECM), cells of the calcareous sponge Clathrina cerebrum were investigated in vitro by scanning electron microscopy. Cells were settled on glass coverslips, used as controls, and on coverslips coated with various ECM components (laminin, collagens and fibronectin), and with an adhesive substance (polylysine). Cells tended to conserve a rounded shape, producing thin, stiff processes (scleropodia) and lamellipodia, whose shape and extension varied according to the substrata. Spreading was observed only on polylysine, inducing cells to assume a fibroblast-like aspect. On laminin, cell adhesion was assured only by scleropodia. On fibronectin, scleropodia and lamellipodia were present, but reduced in size and length. On collagens, laminar processes occurred among prevailing scleropodia. Measurements of cell area and perimeter allowed statistical comparison of substrata, on the basis of their induction of cell flattening and protuberance formation. In summary, sponge cells were found to modulate their morphology in response to the external environment, expressing features for dynamic activities most fully in the presence of substances close to their natural ECM constituents. These results are discussed in the context of tissue rearrangement as a basic adaptation occurring throughout the life span of these organisms.

Parasitic diatoms inside antarctic sponges.

Antarctic sponges may host large populations of planktonic and benthic diatoms. After settling on the sponge, these diatoms enter its body through pinacocytes (1) and form, there, large mono- or pauci-specific assemblages. Yet the total amount of carbohydrates in the invaded sponge tissue is inversely correlated with that of chlorophyll-a. We suggest, therefore, that endobiont diatoms utilize the products of the metabolism of their host as an energy source. This is the first evidence indicating that an endobiotic autotrophic organism may parasitize its animal host. Moreover, this unusual symbiotic behavior could be a successful strategy that allows the diatom to survive in darkness.

Sperm ultrastructure of a member of the black coral family Aphanipathidae: Rhipidipathes reticulata (Anthozoa, Antipatharia).

Fertile male polyps of three colonies of the black coral Rhipidipathes reticulata (Aphanipathidae) from Togian Islands (Indonesia) have been the source of the sperm investigated at ultrastructural level, in order to compare their organization with that of other members belonging to the family Antipathidae and Myriopathidae. The extension of the studies to a representative of the family Aphanipathidae stresses once more the structural similarity of the male gametes in antipatharians. A sketch of the sperm model reports the similarity and differences in the examined taxa. Among the micro-characters, the cup-like body linked to the pericentriolar apparatus, is exclusive of the antipatharians. Other inclusions concern the more common pro-acrosomal vesicles or the acrosomelike structure observed only in Antipathella subpinnata and Myriopathes japonica. Lipid vesicles are occasionally present. A typical inclusion, the electron-dense content of which has a C-shaped configuration, is restricted to Rhipidipathes reticulata and is associated to the cup-like body or to the mitochondrion.

Oogenesis of the mayfly Habrophlebia eldae: synthesis of vitelline and chorionic envelopes.

The ultrastructure of developing ovarian follicles inside the panoistic ovarioles of Habrophlebia eldae were examined to observe the events occurring during egg maturation up to the full formation of the chorionic envelopes. The early vitellogenic follicles are coupled by gap junctions and are extensively interlocked with the oocyte plasma membrane via microvilli. With the onset of vitellogenesis, coated pits and coated vesicles are precursors to yolk deposition and are visible at the follicle cell-oocyte interface. Postvitellogenic development entails the deposition of the egg envelopes. The vitelline envelope arises from the coalescence of rectangular plaques whose precursors are visible in Golgi complexes as heterogeneous electron-opaque granules. A chorionic pattern of ridges on the egg surface characterizes the shell of H. eldae. The fully developed chorion shows three distinct regions with differently organized patterns. A fine layer of fibrous material (a secretion of the follicle cells, Ephemeroptera devoid of accessory glands) adheres to the egg chorion and is probably involved in attachment to the substrate.

Structural and ultrastructural aspects of growth in Oscarella lobularis (Porifera, Demospongiae).

Light and electron microscopy were used to study projections emerging from areas of the sponge Oscarella lobularis. These projections seem to arise from foldings of the exopinacoderm, a process also involving mesohyl cells. Besides folding, exopinacocytes invaginate giving rise to small pockets. These may open directly to the outside and represent the precursors of flagellated chambers. Exopinacocytes lining pockets and spaces between projections transform into endopinacocytelike cells. As pockets become isolated from the exterior, cells further differentiate into choanocytes. Remodeling processes, involving fusion of adjacent protrusions and transformation of the intervening spaces into canals, eventually realize a functional aquiferous system. Allowing the sponge to increase its size, these events represent the steps of a growth mechanism.

Ultrastructural study of the mature egg of Tethya citrina Sarà and Melone (Porifera, Demospongiae).

Tethya citrina is an oviparous demosponge in which eggs are distributed in clumps within the choanosome. The cytoplasm of the mature egg presents a peripheral cortex consisting of a slightly granular layer sandwiched between two densely granular, vesiculated ones. The cortex probably has a specialized, trophic function. Mesohyl bacteria are phagocyted at the egg surface, included in vacuoles, and transferred across the cortical sheath toward the inner cytoplasm. The region of the egg extending between the cortex and the nucleus shows a lacunary system mostly developed beneath the cortical envelope. The noncortical cytoplasm also contains lipid droplets, dense rodlike bodies, and phagosomelike granules. Most of the latter are probably autophagosomes, forming lacunae and supporting autosynthetic vitellogenesis. Rodlike inclusions are probably proteinaceous; they likely originate within the phagosomes and represent the actual yolk material.

Biochemical and ultrastructural effects of nitrite in rainbow trout: liver hypoxia as the root of the acute toxicity mechanism.

In Salmo gairdneri specimens exposed for 12, 24, 48, and 72 hr to nitrite (450 micrograms NO2-N) the main biochemical parameters of cerebral and hepatic hypoxia were studied. Observations on liver ultrastructure were also made to follow nitrite hepatotoxicity. Furthermore, some physiological alterations in liver mitochondria incubated in a medium containing a nitrite concentration similar to that found in the liver of in vivo exposed trout were studied. Results suggest that tissue hypoxia, due to the nitrite-induced high methemoglobinemia, is too low to be directly responsible for animal death. Nevertheless liver hypoxia is thought to be at the root of nitrite acute toxicity mechanism by producing suitable conditions for toxic potentialities. Thus irreversible and deadly damages arise in liver biochemistry and ultrastructure, particularly at the mitochondrial level.