Ultrastructural and cytochemical study of spermatogenesis in Scrobicularia plana (Mollusca, Bivalvia)

The ultrastructure of the mature spermatozoa and spermatogenesis of the bivalve Scrobicularia plana are described. Support cells extend from the basal lamina to the lumen of the testis and are laterally connected to the germinal epithelium. Germ cells present intercellular bridges and flagella since the spermatogonial stage. While spermatogonia and spermatocytes appear connected to support cells by desmosome-like junctions, elongated spermatids are held at the acrosomal region by support cell finger-like processes. During spermiogenesis, the acrosomal vesicle differentiates from a golgian saccule and then migrates to the nuclear apex. A microtubular manchette arising from centrioles surrounds the acrosomal vesicle, the nucleus, and the mitochondria at the time these three organelles start their elongation, disappearing after that. The mature spermatozoon of S. plana lacks a distinct midpiece because the mitochondria extend from the region of the pericentriolar complex along the nucleus anteriorly for approximately 1.4 microns. The features of this bivalve type of modified spermatozoon are compared with those of other animal groups having similar modifications.

[Analysis of Ostrea gigas Thunberg from Xinhua, Jiangsu]

This paper compares the ultramicrostructure, infrared spectrum and Meigen reaction of the ancient Ostrea gigas shell from Xinhua County, Jiangsu Province, with those of fresh Ostrea gigas shell from Fujian Province. It was shown that the histomorphology of the ancient Ostrea gigas shell is the same as that of the fresh one and both shells are composed of calcite.

Electron microscopy of negatively stained scallop myosin molecules. Effect of regulatory light chain removal on head structure

The heads of myosin molecules from the striated adductor muscle of scallop have been studied by electron microscopy after negative staining. In common with vertebrate skeletal muscle myosin visualized by this method, the scallop myosin heads were pear-shaped and often showed pronounced curvature. Staining suggestive of two or, more frequently, three domains could often be observed. Removal of regulatory light chains (R-LCs) resulted in a reduction in the length of the heads of about 2.6 nm, with no significant change in maximum width. In desensitized preparations a majority of heads displayed anticlockwise curvature, whereas intact heads were usually seen curved clockwise. Analysis of the head curvature in both intact and desensitized molecules was consistent with an ability of each head to rotate about its long axis. Desensitization resulted in an increased incidence of heads showing two domains. It seems likely that the reduction in length upon removal of the R-LC is due to the two small domains located in the neck region of the head collapsing into one.

Effect of Ca2+ on the dimeric structure of scallop sarcoplasmic reticulum

Scallop sarcoplasmic reticulum (SR), visualized in situ by freeze-fracture and deep-etching, is characterized by long tubes displaying crystalline arrays of Ca2+-ATPase dimer ribbons, resembling those observed in isolated SR vesicles. The orderly arrangement of the Ca2+-ATPase molecules is well preserved in muscle bundles permeabilized with saponin. Treatment with saponin, however, is not needed to isolate SR vesicles displaying a crystalline surface structure. Omission of ATP from the isolation procedure of SR vesicles does not alter the dimeric organization of the Ca2+-ATPase, although the overall appearance of the tubes seems to be affected: the edges of the vesicles are scalloped and the individual Ca2+-ATPase molecules are not clearly defined. The effect of Ca2+ on isolated scallop SR vesicles was investigated by correlating the enzymatic activity and calcium-binding properties of the Ca2+-ATPase with the surface structure of the vesicles, as revealed by electron microscopy. The dimeric organization of the membrane is preserved at Ca2+ concentrations where the Ca2+ binds to the high affinity sites (half-maximum saturation at pCa approximately 7.0 with a Hill coefficient of 2.1) and the Ca2+-ATPase is activated (half-maximum activation at pCa approximately 6.8 with a Hill coefficient of 1.84). Higher Ca2+ concentrations disrupt the crystalline surface array of the SR tubes, both in the presence and absence of ATP. We discuss here whether the Ca2+-ATPase dimer identified as a structural unit of the SR membrane represents the Ca2+ pump in the membrane.

Sperm nuclear transformations consist of enlargement and condensation coordinate with stages of meiotic maturation in fertilized Spisula solidissima oocytes

Rates of sperm nuclear expansion were measured and correlated with processing of the maternal chromatin in synchronous populations of fertilized surf clam (Spisula solidissima) oocytes fixed at regular intervals following insemination and stained with the DNA fluorochrome Hoechst 33342. Sperm nuclei expanded in four distinct phases each temporally coordinate with events of meiotic maturation: germinal vesicle stage (phase A), germinal vesicle breakdown (phase B), polar body formation (phase C), and female pronuclear development (phase D). Sperm nuclei were essentially unchanged during phase A (rate = 0.1 micron2/min, enlarged during phases B (rate = 8.2 microns2/min) and D (rate = 6.2 microns2/min), and condensed during phase C (rate = -1.9 micron2/min). Sperm nuclear enlargement during phase D was significantly less in polyspermic and polygynic zygotes. The effects of various treatments (temperature, microtubule disruption, pH alterations, and metabolic and protein synthesis inhibitions) which perturbed sperm nuclear enlargement and meiotic processing of the maternal chromatin indicated that the two processes are coupled and may be linked by common regulatory agents.

Polarized site of sperm entrance in the egg of a freshwater bivalve, Unio elongatulus

We studied the organization of the egg of a freshwater bivalve, Unio elongatulus. This egg is markedly polarized. At the vegetal pole there is a crater which constitutes the point of attachment of the growing oocyte to the ovarian wall. This has previously been interpreted as a micropyle. We show that the sperm does not enter the egg through the crater but in a differentiated region around it, mostly at its base. This region is characterized by a wrinkled surface and is the only site of the vitelline coat which specifically binds the lectin from Lotus tetragonolobus. The egg reacts explosively upon fertilization, ejecting vacuolar material from the crater. The role of this "egg reaction" in relation to the prevention of polyspermy is discussed.

Processes in formation of mucus by the body wall of Ariolimax columbianus

The mucus secreted by the body surface of the terrestrial pulmonate slug, Ariolimax columbianus, is a variable mix of products of the giant mucous and channel cells of the skin. An in vitro sac preparation allows the study of control and products of these two cell types. Mucins in membrane-bound granules are released by the mucous cells on mechanical or electrical stimulation of the skin. The product of the channel cells is a blood ultrafiltrate, modified by reabsorption of Na+ and Cl- ions and transfer into the fluid of K+ and HCO3- ions (processes of secretion inhibited by ouabain, amiloride and furosemide; and by other epithelial blocking agents). Rates of secretion can be increased or decreased by gastropod neurotransmitters (acetylcholine, serotonin, and others), and by prostaglandin E2 and indomethacin.

A method for cytogenetic and cytological examination of small shelled larvae of bivalves and other zooplankton

Studies on chromosomes and nuclei of very small bivalve larvae have been impeded by the veliger shell. It has been determined that the alcohol:acetic acid fixative commonly used in cytogenetic techniques can be made to act as a decalcifying agent upon repeated heating. In addition, transfer of formalin fixed shelled specimens, routinely used as marine bioassay organisms, into ethyl alcohol:acetic acid (3:1) fixative also yields clear cells for cytological examination of decalcified but otherwise intact oyster larvae and other zooplankton. Identification of cell type, such as germ-line primordia, in, for example, reproductive and ploidy level studies, and observations on the presence of bacteria can be made from the preparations. Material can be examined up to at least a year after preservation. The method is evaluated and its modifications are discussed.

Evidence for neurosecretory control of the optic gland in terrestrial pulmonates

The optic gland in terrestrial pulmonates secretes gonadotropic hormone. The present study investigates the fine structure of the optic gland to clarify the control of secretion. Animals used were the slug, Limax marginatus, and the snail, Euhadra peliomphala. The optic gland cell has a process and the cytoplasm is filled with large granules about 750-1300 nm in diameter with a thin cortex in L. marginatus and 800-1700 nm delimited by a thick peripheral layer in E. peliomphala. These gland cells are characterized by well-developed granular endoplasmic reticulum. Granule formation was seen in the region of the Golgi apparatus. During the breeding season, the medial neurosecretory cells of the brain are active in the production and release of secretory materials. Although no neurosecretory cell bodies occur in the tentacular ganglion, neurosecretory axons penetrate into the optic gland cells. Cobalt filling reveals that axons of the medial neurosecretory cells project to the tentacular ganglion, near the optic gland. These results suggest that the optic gland is controlled by a neurohormone originating from the medial neurosecretory cells of the brain.

[Ultrastructure of the osphradium of Viviparus sp. (Mollusca, Prosobranchia)]

By means of scanning and transmissive electron microscopy, osphradial structure has been studied in the freshwater herbivorous mollusc Viviparus sp. Ciliary supporting and microvillous cells are described; they do not form any specific for the osphradium zones on its surface. The number of the villi, belonging to one supporting cell, is within 20-40. Peripheral processes of the receptory subepithelial cells have microvilli and 1-2 kinocilia on their apical surface. Central processes of the primary sensitive receptor cells go into the neuropil of the osphradial ganglion, that is situated in the center of the osphradial torus. Among uni- and multipolar ganglionic cells typical neurosecretory cells with electron opaque granules 200 nm in diameter are revealed. Release of neurosecretion takes place in the neuropil and in the connective tissue of the osphradial ganglion. A suggestion is made on modulating role of neuropeptides in the osphradial ganglion, that enables to change sensitivity of this receptor organ depending on physiological state of the animal and on the environment.

Dimer ribbons in the three-dimensional structure of sarcoplasmic reticulum

The three-dimensional structure of scallop sarcoplasmic reticulum membranes has been determined from electron micrographs of two classes of stain-filled tubules by helical reconstruction methods. These structures are characterized by dimer ribbons of Ca2+-ATPase molecules running diagonally around the tube wall. Deep right-handed grooves separate the ribbons. The elongated, curved units of the dimer (approximately 95 A long in the radial direction; 60 to 70 A axially, and about 30 A wide) are displaced axially by approximately 34 A and are connected at their outer ends by a bridge running nearly parallel to the tube axis. The monomers make a second contact at their inner ends. Adjacent units with the same orientation form a strong contact that is responsible for the ribbon appearance. Comparison of tubules of different diameter shows that one set of connections between the dimer ribbons is conserved: the inner ends of axially displaced dimers appear to make contact along a left-handed path almost perpendicular to the major grooves. The lipid bilayer cannot be clearly identified. The two-dimensional map obtained from flattened tubules is consistent with the three-dimensional reconstruction in showing dimer ribbons connected by a weak contact across the grooves, strongly resembling the inter-dimer bond observed in three dimensions. The two-dimensional map shows a 2-fold axis relating units of the dimer, but the three-dimensional tubes show a slight axial polarity that may arise from the presence of proteins other than the Ca2+-ATPase.

[Ultrastructure of the osphradium of Siphonaria grisea (Mollusca, Pulmonata)]

By means of scanning and transmissive electron microscopic methods osphradium of Siphonaria grisea has been studied. The osphradium of the animal is presented as a small torulus formed by supporting ciliated cells. Among them bodies of receptory cells are situated; they are of smaller size and decorated with a bundle of microvilli. Central processes of the receptory cells, penetrating through a thick layer of the connective tissue, reach the subepithelial neural trunk. Concentration of the neural cells in the periphery of the trunk is small, and in the central area no chemical synapses are revealed. The data presented demonstrate a primitive structure of the osphradial chemoreceptory organ in Siphonaria.

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

To study correlation between membrane structure and photoreceptor function, we compared the size and density of intramembrane particles (IMPs) in various membrane compartments of freeze-fractured retinas in a cuttle-fish, Sepiella japonica, and an octopus, Octopus ocellatus. Distribution of gap junctions in the retinas was also examined. Similar results were obtained in the two species. P-faces of both rhabdomeric microvillar membrane and non-rhabdomeric plasma membrane of the apical process were characterized by a random distribution of dense IMPs (ca. 5500-6500/microns2), which showed a unimodal size distribution with a mean diameter of ca. 10 nm. Unlike other invertebrate ocelli, the plasma membrane of the cell body in both the outer and inner segments had significantly denser P-face particles (ca. 7500-8000/microns2) than the rhabdomeric microvillar membrane. The size distribution of IMPs in each part of the membrane was also unimodal, but with a mean diameter of ca. 8 nm. In tangential fractures, each lamella of the myeloid body showed a patchwork of P-faces with irregularly arranged, dense particles and E-faces with orderly patterened granulation. Density and size distribution of the P-face particles in the myeloid membrane resembled those in the rhabdomeric microvillar membrane. The plasma membranes of the supporting cell and the gial cell had relatively sparse P-face particles (ca. 1500-3000/microns2). In addition to the previously reported gap junctions, which connected visual cell inner segments with each other, directly or via collaterals, small gap junctions were found between the visual cell axons and presumed efferent nerve fibres in the plexiform layer. Large-sized gap junctions provided mutual connections for both supporting cells and glial cells. In conclusion, IMPs of 10 nm in mean diameter in the microvillar and non-microvillar parts of the apical process plasma membrane and in the myeloid membrane represent the molecules or their clusters of two photopigments in the cephalopod visual cell, rhodopsin and retinochrome, respectively, and electrical transmission plays a role in visual cell-efferent nerve interactions.

Analysis of contact-rehydration in terrestrial gastropods: absorption of 14C-inulin through the epithelium of the foot

Contact-rehydration in terrestrial slugs involves a specific drinking behaviour during which water is rapidly absorbed through the integument of the foot. When dehydrated slugs were placed on wet filter paper containing 14C-inulin, they displayed the characteristic drinking posture and absorbed both water and 14C-inulin. Samples of haemolymph from dehydrated slugs after 12 min of contact-rehydration contained about 6 micrograms of 14C-inulin 100 mg-1 of haemolymph (0.24 mmol l-1 14C-inulin in the substrate). The haemolymph of hydrated slugs however contained no detectable radioactivity after 12 min on the filter paper. Electron microscopy revealed that the intercellular spaces between the epithelial cells of the foot were reduced in dehydrated slugs, but were rapidly enlarged during contact-rehydration. It is concluded that contact-rehydration in terrestrial slugs is mediated by bulk flow of water through an epithelial paracellular pathway in the integument of the foot.

Galactogen synthesis-stimulating factor in the slug, Limax maximus: cellular localization and partial purification

Galactogen synthesis in the albumen gland of the slug Limax maximus is under direct endocrine control [J. van Minnen, J. Wijdenes, and P. G. Sokolove (1983) Gen. Comp. Endocrinol. 49, 307-314]. The present results show that dorsal body cells (DBC) present throughout the connective tissue around the circumesophageal brain are the major cellular source of a galactogen synthesis-stimulating factor (GAL-SF). The amount of GAL-SF varies during sexual maturation of L. maximus (a protandrous hermaphrodite): it is low when animals are immature and reaches a peak in the late female phase when the animals prepare to lay eggs. The morphology of DBC also changes during maturation. DBC are small and release little secretory product in immature and early male-phase animals; release is higher and DBC are larger in late female-phase animals. Gel filtration of homogenates of cerebral ganglia on Sephadex G75 yields one peak of GAL-SF activity with an estimated Mr of 4000-7000 Da. Subsequent anion exchange chromatography showed that GAL-SF could be eluted with a linear NaCl gradient; GAL-SF was found in fractions with a NaCl concentration between 0.04 and 0.07 M.

The ultrastructure of the lens of the cephalopod Sepiola: a scanning electron microscopic study

The ultrastructure of the eye lens of Sepiola atlantica was investigated using scanning electron microscopy. The main lens elements in both the anterior and posterior half of the Sepiola lens are plate-like configurations with fiber-like extensions at their margins. Anteriorly the plates are plano-convex, posteriorly subspherical. The central, primordial, posterior plates are spherical with no marginal extensions. The plates are mutually anchored by protrusions and invaginations and by push-button attachments. The posterior and anterior halves are separated by a septum which consists of concentric zones of radially orientated elongated cells. The marginal extensions of the plates and the septal elements are closely associated. The unique structure of the septum makes it a good candidate for the high resistance barrier between the posterior and anterior halves of the Sepiola lens (Jacob and Duncan, 1981).

The utilization of cnidarian nematocysts by aeolid nudibranchs: nematocyst maintenance and release in Spurilla

Some nudibranchs that feed on cnidarians are known to store nematocysts within cnidophage cells and use them for their own defense. Most of the nematocysts are in direct contact with the cytoplasm of the cnidophage. Nematocysts are not subjected to lysosomal enzymes because any phagocytic membrane that surrounded the nematocyst after engulfment does not persist. Cnidophage organelles are restricted to regions surrounding the nematocysts and may aid in the maintenance and development of the nematocysts. The release of cnidophages is initiated by a contraction of a dense muscle complex surrounding the cnidosac. Nematocysts do not discharge if the cnidophage membrane does not rupture upon release. A comparison of nematocysts maintenance in Spurilla neapolitana and nematocyst retention in other organisms is presented.

Crystalline structure of sarcoplasmic reticulum from scallop

Negatively stained sarcoplasmic reticulum from the scallop Placopecten magellanicus presented a variety of crystalline forms, the most common being tubular structures. These were characterized by paired rows of morphological units, spaced at approximately 120 A, running diagonally across the tubules. The orthogonal unit cell (120 X 55 A) contained two units, related by a twofold axis, which probably represented the part of the Ca2+-ATPase molecule projecting from the outer surface of the membrane.

The fine structure of epidermal lines on arms and head of postembryonic Sepia officinalis and Loligo vulgaris (Mollusca, Cephalopoda)

The structure and cell types of the epidermal lines that are found on the surface of the head and arms of Sepia officinalis and Loligo vulgaris, were investigated by light- and transmission electron microscopy. The lines consist of ciliated cells and non-ciliated, accessory cells. The axon of the ciliated cell and the ultrastructure of the latter indicate that this element is sensory; therefore, the epidermal lines of cephalopods may be regarded as a type of sensory organ.

An ultrastructural analysis of Dentalium vulgare (Mollusca, Scaphopoda) gametes with special reference to early events at fertilization

The ultrastructure of Dentalium gametes, and their fate at fertilization, were investigated. Unexpectedly, fixation carried out in 2% osmium tetroxide, at 4 degrees C, although destroying most cytoplasmic organelles, reveals the presence of bundles of "stress fibers" in the microvilli and the fertilization cone of the eggs. Anti-actin labeling supports the view that the bundles are made of actin. Fertilization, as in other molluscs, does not cause any cortical granule exocytosis, at least during the first 15 min following insemination. Ultrastructure of the unreacted and reacted spermatozoon is presented and a tentative model for the morphological interpretation of the acrosomal reaction is proposed. A reevaluation of cytoplasmic heterogeneity, e.g., the so-called cytoplasmic prelocalization, as observed after the germinal vesicle breakdown is also provided as well as an analysis of concomitant surface changes.

The distribution of troponin-like proteins on thin filaments of the bay scallop, aequipecten irradians

Antibodies reacting either with bay scallop troponin-C-like protein or both bay scallop troponin-I-like protein and a possible scallop troponin-T were used to determine the distribution of the troponin complex on bay scallop striated muscle thin filaments. Both antibodies cause thin filaments to associate laterally and electron microscopy of such aggregates indicates a periodicity of approximately 38 nm, a distribution characteristic of proteins comprising the troponin complex in vertebrate and arthropod striated muscle. These studies therefore provide additional evidence for the presence of thin filament-linked regulation in bay scallops.

Restriction of developmental potential and trochoblast ciliation in Patella embryos

At the 64-cell-stage embryos of Patella develop a prototroch consisting of four groups of four cilia-bearing cells. Ciliogenesis of isolated blastomeres and trochoblasts was studied, as well as the effect on it of cleavage arrest caused by cytochalasin B treatment. Isolation of blastomeres or trochoblast cells has no influence on ciliogenesis; neither has arrest of cleavage in whole embryos after the third cleavage. However, cleavage arrest before third cleavage completely prevents ciliogenesis. Thus, third cleavage is decisive for the expression of the developmental potential of the primary trochoblasts. Impairment of DNA synthesis by aphidicolin in the S-phase preceding third cleavage also prevents ciliogenesis. It is concluded that a determinant for ciliogenesis as well as certain nuclear factors must be segregated into the micromeres at third cleavage for ciliogenesis to occur in the prototroch cells.

Reconstitution of ciliary membranes containing tubulin

Membranes from the gill cilia of the mollusc Aequipecten irradians may be solubilized readily with Nonidet P-40. When the detergent is removed from the solution by adsorption to polystyrene beads, the proteins of the extract remain soluble. However, when the solution is frozen and thawed, nearly all of the proteins reassociate to form membrane vesicles, recruiting lipids from the medium. The membranes equilibrate as a narrow band (d = 1.167 g/cm3) upon sucrose density gradient centrifugation. The lipid composition of reconstituted membranes (1:2 cholesterol:phospholipids) closely resembles that of the original extract, as does the protein content (45%). Ciliary calmodulin is the major extract protein that does not associate with the reconstituted membrane, even in the presence of 1 mM calcium ions, suggesting that it is a soluble matrix component. The major protein of reconstituted vesicles is membrane tubulin, shown previously to differ hydrophobically from axonemal tubulin. The tubulin is tightly associated with the membrane since extraction with 1 mM iodide or thiocyanate leaves a vesicle fraction whose protein composition and bouyant density are unchanged. Subjecting the detergent-free membrane extract to a freeze-thaw cycle in the presence of elasmobranch brain tubulin or forming membranes by warming the extract in the presence of polymerization-competent tubulin yields a membrane fraction with little incorporated brain tubulin. This suggests that ciliary membrane tubulin specifically associates with lipids, whereas brain tubulin preferentially forms microtubules.