Ultrastructure, histology, and innervation of the mantle edge of the freshwater pulmonate snails Lymnaea stagnalis and Biomphalaria pfeifferi

The temptin gene of the clade Lophotrochozoa is involved in formation of the prismatic layer during biomineralization in molluscs

The biomineralization mechanism of mollusc shell has been studied for a long time, but there is a lack of understanding about the relationship between the shell formation in vitro and the signaling system in vivo. In this study, we cloned a novel shell matrix protein gene (hc-temptin), which only be characterized as a water-borne protein pheromone of molluscs in previous studies, from the freshwater mussel Hyriopsis cumingii. By bioinformatics analysis we found that temptin was a gene unique to the clade Lophotrochozoa, and it exists in all mollusc taxa except Cephalopoda. The current data supported the premise that temptin was generated in the early emergence of molluscs and that it maintained a high mutation rate to evolve relative independently. The specificity of hc-temptin expression in the mantle tissue suggests its potential to participate in biomineralization. Its sequence contained typical Ca²⁺ binding sites. Our experiments involving the pearl formation process, damaged shell repair process, and RNAi experiment showed that hc-temptin was a shell matrix protein that plays an important role in formation of the prismatic layer. The results of this study provided new insights about the origin of the temptin gene and its role in molluscs.

Localization of keyhole limpet hemocyanin-like immunoreactivity in the nervous system of Biomphalaria alexandrina

Recent years have led to increased effort to describe and understand the peripheral nervous system and its influence on central mechanisms and behavior in gastropod molluscs. This study revealed that an antibody raised against keyhole limpet hemocyanin (KLH) cross-reacts with an antigen(s) found extensively in both the central and the peripheral nervous systems of Biomphalaria alexandrina. The results revealed KLH-like immunoreactive (LIR) neurons in the cerebral, pedal, buccal, left pleural, right parietal, and visceral ganglion within the CNS with fibers projecting throughout all the peripheral nerves. Numerous KLH-LIR peripheral sensory neurons located in the foot, lips, tentacles, mantle, esophagus, and penis exhibited a bipolar morphology with long tortuous dendrites. KLH-LIR cells were also present in the eye and statocyst, thus suggesting the labeling of multiple sensory modalities/cell types. KLH-LIR cells did not co-localize with tyrosine hydroxylase (TH)-LIR cells, which have previously been described in this and other gastropods. The results thus provide descriptions of thousands of peripheral sensory neurons, not previously described in detail. Future research should seek to pair sensory modalities with peripheral cell type and attempt to further elucidate the nature of KLH-like reactivity. These findings also emphasize the need for caution when analyzing results obtained through use of antibodies raised against haptens conjugated to carrier proteins, suggesting the need for stringent controls to help limit potential confounds caused by cross-reactivity. In addition, this study is the first to describe neuronal cross-reactivity with KLH in Biomphalaria, which could provide a substrate for host-parasite interactions with a parasitic trematode, Schistosoma.

Mollusk shell structures and their formation mechanism

In nature, mollusk shells have a role in protecting the soft body of the mollusk from predators and from the external environment, and the shells consist mainly of calcium carbonate and small amounts of organic matrices. Organic matrices in mollusk shells are thought to play key roles in shell formation. However, enough information has not been accumulated so far. High toughness and stiffness have been focused on as being adaptable to the development of organic–inorganic hybrid materials. Because mollusks can produce elaborate microstructures containing organic matrices under ambient conditions, the investigation of shell formation is expected to lead to the development of new inorganic–organic hybrid materials for various applications. In this review paper, we summarize the structures of mollusk shells and their process of formation, together with the analysis of various organic matrices related to shell calcification.

High net calcium uptake explains the hypersensitivity of the freshwater pulmonate snail, Lymnaea stagnalis, to chronic lead exposure

Previous studies have shown that freshwater pulmonate snails of the genus Lymnaea are exceedingly sensitive to chronic Pb exposure. An EC20 of <4microgl(-1)Pb for juvenile snail growth has recently been determined for Lymnaea stagnalis, which is at or below the current USEPA water quality criterion for Pb. We characterized ionoregulation and acid-base balance in Pb-exposed L. stagnalis (young adults approximately 1g) to investigate the mechanisms underlying this hypersensitivity. After 21-day exposure to 18.9microgl(-1)Pb, Ca(2+) influx was significantly inhibited (39%) and corresponding net Ca(2+) flux was significantly reduced from 224 to -23nmolg(-1)h(-1). An 85% increase in Cl(-) influx was also observed, while Na(+) ion transport appeared unaffected. Finally, a marked alkalosis of extracellular fluid was observed with pH increasing from 8.35 in the control to 8.65 in the 18.9microgl(-1) Pb-exposed group. Results based on direct measurement of Ca(2+) influx in 1g snails gave an influx nearly an order of magnitude higher (750nmolg(-1)h(-1)) than in comparably sized fish in similar water chemistry. Under control conditions, specific growth rate in newly hatched snails was estimated at 16.7% per day over the first 38-day post-hatch and whole body Ca(2+) concentrations were relatively constant at approximately 1100nmolg(-1) over this period. Based on these data, it is estimated that newly hatched snails have net Ca(2+) uptake rates on the order of 7600nmolg(-1)h(-1). A model was developed integrating these data and measured inhibition of Ca(2+) influx rates of 13.4% and 38.7% in snails exposed to 2.7 and 18.9microgl(-1)Pb, respectively. The model estimates 45% and 83% reductions in newly hatched snail growth after 30-day exposure in these two Pb-exposed groups. These results compare well with previous direct measurements of 47% and 90% reductions in growth at similar Pb concentrations, indicating the high net Ca(2+) uptake is the controlling factor in observed Pb hypersensitivity.

Functional morphology of the mantle ofNautilus pompilius (Mollusca, Cephalopoda)

This study presents histological and cytological findings on the structural differentiation of the mantle of Nautilus pompilius in order to characterize the cells that are responsible for shell formation. The lateral and front mantle edges split distally into three folds: an outer, middle, and inner fold. Within the upper part of the mantle the mantle edge is divided into two folds only; the inner fold disappears where the hood is attached to the mantle. At the base of the outer fold of the lateral and front mantle edge an endo-epithelial gland, the mantle edge gland, is localized. The gland cells are distinguished by a distinct rough endoplasmic reticulum and by numerous secretory vesicles. Furthermore, they show a strong accumulation of calcium compounds, indicating that the formation of the shell takes place in this region of the mantle. Numerous synaptic contacts between the gland cells and the axons of the nerve fibers reveal that the secretion in the area of the mantle edge gland is under nervous control. The whole mantle tissue is covered with a columnar epithelium having a microvillar border. The analyses of the outer epithelium show ultrastructural characteristics of a transport active epithelium, indicating that this region of the mantle is involved in the sclerotization of the shell. Ultrastructural findings concerning the epithelium between the outer and middle fold suggest that the periostracum is formed in this area of the mantle, as it is in other conchiferan molluscs.

Localization of putative neurotransmitters in the mantle and siphuncle of the mollusc Nautilus pompilius L. (Cephalopoda)

The neurotransmitter supply in the nerve endings of the mantle and the siphuncle, i.e. in organs that are responsible for the shell formation in the ectocholeate Nautilus pompilius, were investigated with electron microscopical, fluorescence-, immuno- and enzyme histochemical methods as well as with high pressure liquid chromatography (HPLC). Using antibodies against serotonin and the tetrapeptide FMRF-amide, positive reactions were demonstrated immunohistochemically within the terminal nerve fibres of the mantle and the vessels of the siphuncle. Enzyme histochemical proof of the presence of specific acetylcholinesterase yielded positive results in the muscle fibres of the mantle and siphuncle. Additionally, in the mantle, glyoxylic acid-induced fluorescence was shown within the nerve endings indicating catecholamines as neurotransmitters, whereas in the siphuncle such fluorescence did not appear. However, the HPLC-analyses showed that in the mantle and also in the siphuncle the content of dopamine is higher than that of noradrenaline whereas only traces of adrenaline occur in both organs suggesting dopamine as a putative neurotransmitter. Transmission electron microscopical examination of the nerve endings of both organs revealed that different types of vesicles were distinguished that could be considered as cholinergic, aminergic and peptidergic structures.

Calcium localization in the shell-forming tissue of the freshwater snail, Biomphalaria glabrata: a comparative study of various methods for localizing calcium

The routes calcium might take across the mantle to the shell have been investigated with various electron-microscopical techniques in the freshwater snail Biomphalaria glabrata (Planorbidae, Basommatophora). In chemically-fixed tissue, calcium was precipitated with a tannic acid-antimonate technique in predominantly the intercellular spaces of the outer mantle epithelium and the interstitium below it. Some vacuoles of the outer mantle epithelium and one type of mucus cell in the inner mantle epithelium also contained precipitate. The presence of calcium in the precipitates was proved by electron energy loss spectroscopy combined with electron spectroscopic imaging. Incubation with lead acetate and uranyl acetate revealed binding-sites for calcium in the intercellular spaces of the epithelia interstitium and the mucus cells of the inner mantle epithelium. Precipitates were also seen after all incubations in the calcium spherites of the connective tissue. The concentrations of calcium and other elements were analysed in freeze-dried ultrathin sections of cryofixed mantle tissue by means of energy-dispersive X-ray microanalysis. Only in mitochondria of the musculature could high amounts of calcium and phosphorous be detected.

Ultrastructural localization of carbonic anhydrase in tissues involved in shell formation and ionic regulation in the pond snail Lymnaea stagnalis

The ultrastructural localization of carbonic anhydrase (CA) was investigated with the cobalt-bicarbonate method in three epithelia of the pond snail Lymnaea stagnalis. In the epidermis a selective population of "positive cells" was observed. In these cells, CA is confined to the apical and to small parts of the lateral plasma membrane. In cells of the outer mantle epithelium, CA is localized in the lateral and basal parts of the plasma membrane. In cells of the ureter, CA was found apically as well as basally. The localization of CA is discussed in relation to the different functions of the epidermis (electrolyte uptake), mantle (HCO3- secretion, calcification) and ureter (electrolyte uptake, acid-base regulation).