A new model for periostracum and shell formation in Unionidae (Bivalvia, Mollusca)

The periostracum in Unionidae consists of two layers. The outer one is secreted within the periostracal groove, while the inner layer is secreted by the epithelium of the outer mantle fold. The periostracum reaches its maximum thickness at the shell edge, where it reflects onto the shell surface. Biomineralization begins within the inner periostracum as fibrous spheruliths, which grow towards the shell interior, coalesce and compete mutually, originating the aragonitic outer prismatic shell layer. Prisms are fibrous polycrystalline aggregates. Internal growth lines indicate that their growth front is limited by the mantle surface. Transition to nacre is gradual. The first nacreous tablets grow by epitaxy onto the distal ends of prism fibres. Later growth proceeds onto previously deposited tablets. Our model involves two alternative stages. During active shell secretion, the mantle edge extends to fill the extrapallial space and the periostracal conveyor belt switches on, with the consequential secretion of periostracum and shell. During periods of inactivity, only the outer periostracum is secreted; this forms folds at the exit of the periostracal groove, leaving high-rank growth lines. Layers of inner periostracum are added occasionally to the shell interior during prolonged periods of inactivity in which the mantle is retracted.     

The ontogeny of shell secretion in Terebratalia transversa (Brachiopoda, Articulata). II. Formation of the protegulum and juvenile shell

The fine structure of the shell and underlying mantle in young juveniles of the articulate brachiopod Terebratalia transversa has been examined by electron microscopy. The first shell produced by the mantle consists of a nonhinged protegulum that lacks concentric growth lines. The protegulum is secreted within a day after larval metamorphosis and typically measures 140-150 micron long. A thin organic periostracum constitutes the outer layer of the protegulum, and finely granular shell material occurs beneath the periostracum. Protegula resist digestion in sodium hypochlorite and are refractory to sectioning, suggesting that the subperiostracal portion of the primordial shell is mineralized. The juvenile shell at 4 days postmetamorphosis possesses incomplete sockets and rudimentary teeth that consist of nonfibrous material. The secondary layer occuring in the inner part of the juvenile shell contains imbricated fibers, whereas the outer portion of the shell comprises a bipartite periostracum and an underlying primary layer of nonfibrous shell. Deposition of the periostracum takes place within a slot that is situated between the so-called lobate and vesicular cells of the outer mantle lobe. Vesicular cells deposit the basal layer of the periostracum, while lobate cells contribute materials to the overlying periostracal superstructure. Cells with numerous tonofibrils and hemidesmosomes differentiate in the outer mantle epithelium at sites of muscle attachments, and unbranched punctae that surround mantle caeca develop throughout the subperiostracal portion of the shell. Three weeks after metamorphosis, the juvenile shell averages about 320 micron in length and is similar in ultrastructure to the shells secreted by adult articulates.     

Microboring in a Freshwater Fluvial Unionid Bivalve Substrate

Samples of the unionid bivalve Elliptio complanata were collected from the channel of the freshwater Saint John River, from Fredericton, New Brunswick, Canada. Scanning electron microscopy imaging of prepared shell samples revealed an assemblage of microborings. No borings are noted on the periostracum or prismatic shell layers. Boring structures are instead confined to the underlying nacreous aragonitic shell material, together with its associated organic conchiolin layers. Three main styles of boring are encountered, encompassing both predominantly surficial structures and penetrative tubular borings. Surficial structures are represented by a polygonal network on an exposed conchiolin shell layer. The penetrative borings take two forms, one being simple unbranched tubes, steeply aligned (perpendicular to the shell surface) and traversing the full thickness of the nacreous shell layer. The other penetrative boring style, again occurring within the nacreous layer, comprises a complex irregular network of randomly oriented rarely branching tubular borings. Borings generally display diameters of micron scale. Biofilm and extracellular polymeric substances, with bacterial, diatomaceous and filamentous components are also observed, often displaying a close association with both the borings and the conchiolin layers within the shell. The formation of the borings may be attributed to cyanobacteria, cyanophyte or fungal progenitors.     

A hairy business—Periostracal hair formation in two species of helicoid snails (Gastropoda,Stylommatophora, Helicoidea)

In molluscs, the calcareous shell is covered externally by a thin organic layer, the periostracum. The periostracum of some pulmonate species is of special taxonomic interest because it bears distinct microscale architectures. Where and how these structures are formed is as yet unknown. Using histological sections through their shells, gelatin cuts, and live observations I studied the pattern by which the periostracal hair‐like projections in two helicoid land snail species are secreted and evenly arranged on the shell. The results indicate a complex mechanism: a hair is formed in the periostracal groove independently of the periostracum, after which it is attached to the edge of the shell, drawn out of the tissue, and finally swivelled to the upper side of the periostracum. Upon further growth of the periostracum, the hairs are finally fixed upright on the shell. J. Morphol. 2011. © 2011 Wiley‐Liss, Inc.     

Aragonitic dendritic prismatic shell microstructure in Thracia (Bivalvia,Anomalodesmata)

The shells of most anomalodesmatan bivalves are composed of an outer aragonitic layer of either granular or columnar prismatic microstructure, and an inner layer of nacre. The Thraciidae is one of the few anomalodesmatan families whose members lack nacreous layers. In particular, shells of members of the genus Thracia are exceptional in their possession of a very distinctive but previously unreported microstructure, which we term herein “dendritic prisms.” Dendritic prisms consist of slender fibers of aragonite which radiate perpendicular to, and which stack along, the axis of the prism. Here we used scanning and transmission electron microscopical investigation of the periostracum, mantle, and shells of three species of Thracia to reconstruct the mode of shell calcification and to unravel the crystallography of the dendritic units. The periostracum is composed of an outer dark layer and an inner translucent layer. During the free periostracum phase the dark layer grows at the expense of the translucent layer, but at the position of the shell edge, the translucent layer mineralizes with the units typical of the dendritic prismatic layer. Within each unit, the c‐axis is oriented along the prismatic axis, whereas the a‐axis of aragonite runs parallel to the long axis of the fibers. The six‐rayed alignment of the latter implies that prisms are formed by {110} polycyclically twinned crystals. We conclude that, despite its distinctive appearance, the dendritic prismatic layer of the shell of Thracia spp. is homologous to the outer granular prismatic or prismatic layer of other anomalodesmatans, while the nacreous layer present in most anomalodesmatans has been suppressed.     

Function of the flexible periostracal hairs in Trichotropis cancellata (Mollusca, Gastropoda)

Abstract. The marine snail Trichotropis cancellata possesses hairy projections of periostracum (outer shell layer) whose function is unknown. Although rigid shell projections in molluscs have been studied extensively, the selective advantage of flexible extensions of periostracum is less clear. None of the functions proposed previously for periostracum (e.g., protection from erosion and boring) are promoted when it is drawn into hair-like projections. We investigated hypothetical functions that may be served by flexible periostracal hairs, including predator deterrence, alteration of flow vectors to promote feeding or affect turbulence dynamics during freefall, and providing a differential substratum for epibionts. Our laboratory results indicate that crabs, Cancer oregonensis , and sea stars, Pycnopodia helianthoides , consumed snails with the periostracum removed more often than snails with an intact hairy periostracum. However, in both predatory species, some individuals showed no significant preference, and another crab species ( Cancer productus ) did not strongly discriminate based on the shell periostracum. Field studies showed no difference in the rate of predation on hairy- versus smooth-shelled snails. The hairs did not alter flow around the shells consistently in laboratory flume experiments. Additionally, hairy- and smooth-shelled kleptoparasitic snails grew at rates that were statistically indistinguishable, while hairy, suspension-feeding snails grew more slowly. The hairs did not impact the orientation of a snail after a falling event or the time to righting after a fall. The presence of the hairs did deter settlement by barnacles. We conclude that the hairy periostracum acts as a slight deterrent to crab and sea star predators and as a stronger deterrent to the settlement of large calcareous epibionts, such as barnacles, that would increase the weight the snail must bear and potentially increase drag.     

Studies on shell formation. VIII. Electron microscopy of crystal growth of the nacreous layer of the oyster Crassostrea virginica

Electron microscope observations have been made by means of the replica method on growth processes of calcite crystals of the nacreous layer of the shell of the oyster, Crassostrea virginica. Layer formation is initiated by the secretion of a conchiolin matrix and the deposition of rounded crystal seeds on or in this material. In some areas crystal seeds are elongate and within a given area show a similar orientation, probably due to slower deposition. The seeds appear to increase in size by dendritic growth, and smaller seeds become incorporated into larger ones which come into contact to form a single layer. With further growth, crystals overlap, forming a step-like arrangement. The direction of growth is frequently different in neighboring regions. Crystal seeds deposited on crystal surfaces are usually elongate and oriented. Well developed crystals have a tabular idiomorphic form and are parallel in their growth. Rounded and irregular crystals were also observed. The crystals show reticular structure with units of the order of 100 A and striations corresponding with the rhombohedral axes of the crystals. The role of the mantle is discussed in relation to the growth patterns of crystals and shell structure.     

THE MANTLE AND SHELL OF SOLEMYA PARKINSONI (PROTOBRANCHIA: BIVALVIA)

The shell of Solemya exhibits considerable flexibility which is further enhanced by the marked extension of the periostracum beyond the calcareous portions of the valves. This fcature, more than any other, has made possible the habit, unique among bivalves, of burrowing deep within the substrate without direct contact with the water above. The inner calcareous layer of tho valves is restricted to a small area near the umbones while the outer calcareous layer is thin and contains a high proportion of organic material. The shell conchiolin consists mainly of protein, varying in composition, but much of it strengthcned by quinone-tanning, and in ccrtain regions probably by the presence of appreciable quantities of chitin. The ligament, although superficially resembling an amphidetic structure, is opisthodetic, the extcnsion anterior to the umbones consisting of anterior outer layer only.
The mantle is characterized by an extension of the outer fold of the mantle margin which has effected equally both the inner and outer surfaces of this fold. The secretory epithelium and the modified pallial musculature, contraction of which results in the intucking and plaiting of the periostracum, is dcscribed. Simple tubular oil glands open at the mantlo margin and are responsible for the water-repellent nature of the periostracum.
The form of the mantlelshell and that of the enclosed body are discussed and compared with those of other bivalves in which elongation of the mantle/shell is achieved in a different way. It is concluded that the mantlelshell of Solemya is of little value in determining its relationships, and that the greatly elongatod ligament, the edentulous hinge and the flexible shell are all adaptations to a specialized mode of life.     

Neuroendocrine Control Mechanisms in Shell Formation

The brain of Helisoma duryi contains several neurodendocrinecentres. Factors) present in the cerebral ganglia are thoughtto be involved in normal shell growth while neurosecretory substancespresent in the visceral ganglion are involved in the repairof damaged shell. In Lymnaea stagnalis a growth hormone is producedby the cerebral ganglion which stimulates periostracum formationand the calcification of the inner shell layer. The second effectis thought to occur through the action of a mantle edge calciumbinding protein. In Helisoma, mantle collar is able to produce the periostracumin vitro. The presence of brain from a fast growing donor increasesthe amount of periostracum produced by a mantle collar froma slow growing animal. This effect is further enhanced by theremoval of the lateral lobes. The periostracum produced by fastgrowing animals has a higher glycine content than that producedby slow growing snails. The presence of dorsal epithelial tissueenhances the incorporation of calcium into periostracum formedin vitro. These findings suggest that a single factor is present in thebrain of fast growing Helisoma which modulates shell formationrates in vivo and periostracum formation in vitro.     

Topography of retinal axons in the diencephalon of goldfish

Summary The projections of horseradish peroxidase-filled axons from each quadrant of the retina were studied to determine whether retinal projections of goldfish are topographically organized in diencephalic target nuclei. A distinct topography of the dorsal, nasal, ventral and temporal retina exists in the lateral geniculate nucleus and in the dorsolateral optic nucleus of the thalamus. The projections of retinal quadrants show minimal spatial overlap in each of these nuclei. The suprachiasmatic nucleus of the hypothalamus is extensively innervated by ventral retinal fibers, whereas the nucleus is sparsely innervated by fibers from the other three retinal quadrants. A rudimentary topography also exists in the pretectum where the dorsal pretectal area receives projections primarily from the ventral retina and the ventral pretectal area receives projections mostly from the dorsal retina. These data show that retinal projections to some diencephalic nuclei are topographically organized.This work was supported by Research Grant EY-01426 to S.C.S.     

Afferent projections of the trigeminal nerve in the goldfish,Carassius auratus

The horseradish peroxidase (HRP) histochemical technique was used to examine the peripheral distribution and afferent projections of the trigeminal nerve in the goldfish, Carassius auratus. Sensory fibers of the trigeminal nerve distribute over the head via four branches. The ophthalmic branch distributes fibers to the region above the eye and naris. The maxillary and mandibular branches innervate the regions of the upper and lower lip, respectively. A fourth branch of the trigeminal nerve was demonstrated to be present in the hyomandibular trunk. Upon entering the medulla the trigeminal afferent fibers divide into a rostromedially directed bundle and a caudally directed bundle. The rostromedially directed bundle terminates in the sensory trigeminal nucleus (STN) located within the rostral medulla. The majority of fibers turn caudally, forming the descending trigeminal tract. Fibers of the descending trigeminal tract terminate within three medullary nuclei: the nucleus of the descending trigeminal tract (NDTV), the spinal trigeminal nucleus (Spv), and the medial funicular nucleus (MFn). All projections, except for those to the MFn, are ipsilateral. Contralateral projections were observed at the level of the MFn following the labeling of the ophthalmic and maxillomandibular branches. All branches of the trigeminal nerve project to all four of the trigeminal medullary nuclei. Projections to the STN and MFn were found to be topographically organized such that the afferents of the ophthalmic branch project onto the ventral portion of these nuclei, while the afferents of the maxillo- and hyomandibular branches project to the dorsal portion of these nuclei. Cells of the mesencephalic trigeminal nucleus were retrogradely labeled following HRP application to the ophthalmic, maxillary, and mandibular branches of the trigeminal nerve. In addition to demonstrating the ascending mesencephalic trigeminal root fibers, HRP application to the above-mentioned branches also revealed descending mesencephalic trigeminal fibers. The descending mesencephalic trigeminal fibers course caudally medial to the branchiomeric motor column and terminate in the ventromedial portion of the MFn.     

INÄQUIVALVER SCHALENBAU BEI CRANIA ANOMALA

As a result of attachment over its entire surface, the ventral valve of the brachiopod Crania anomala (Müiller) differs from the dorsal valve not only in growth form, time sequence of calcification, and distribution of endopuncta, but also (withoutap-parent functional reason) in ultrastructure. The secondary layer of the dorsal valve grows together from flat crystals (up to 5μ. diameter) showing screw-like dislocations. The same layer of the ventral valve is formed later and has an irregular structure. Spiral crystals are not secreted in the ventral valve until the adult growth-stage is attained. The periostracum and primary layer are developed in both valves in a normal fashion. Fossil representatives, in which the ventral valve is partly or completely free, show the same ultrastructure in both valves.
Infolge ganzflächiger Anheftung weicht die Ventralklappe von Crania anomala (Müller) nicht nur in Wuchsform, zeitlichem Ablauf der Kalzifikation und Verteilung der Endopuncta von der Dorsalklappe ab, sondern ohne erkennbare funktionelle Gründe auch in der Ultrastruktur. Während die Sekundärschicht in der Dorsalklappe aus spezifischen, bis 5μ. Großen Kristallen mit Schrauben-versetzungen zusammenwächst, wird sie in der Ventralklappe verzögert und mit regelloser Struktur angelegt. Erst in adulten Wachstumsstadien können auch in der Ventralklappe spiralige Kristalle abgeschieden werden. Periostrakum und Primärschicht sind bei C. anomala normal entwickelt. Fossile Anpassungstypen der Craniacea, deren Ventralklappen nur punktförmig oder überhaupt nicht zementiert sind, zeigen in beiden Klappen dieselbe Ultrastruktur.     

Formation of the prismatic layer in the freshwater bivalve Hyriopsis cumingii: the feedback of crystal growth on organic matrix

The squeezing hypothesis and the organic frameworks preformation hypothesis propose two different mechanisms to explain the interaction between organic frameworks and crystals during biomineralization of the prismatic layer of the mollusk shell. In this study, we began to study Hyriopsis cumingii shell formation and discover that this species seemed to follow the squeezing hypothesis. During the formation of the aragonite prismatic layer in the freshwater bivalve H. cumingii, we found that crystal growth was involved in controlling initiation of formation of the interprismatic organic membranes. First, newly formed crystals were embedded in the periostracum. Next, the interprismatic organic membranes of the prismatic layer were produced via squeezing between neighboring crystals. The organic matrix secreted by the mantle continuously self‐assembled into the interprismatic organic membranes as the crystals grew. In the mature stage, the interprismatic organic membranes were shaped by crystal growth. These findings provide evidence to support the squeezing hypothesis and add to the existing knowledge about interactions that occur at the organic–inorganic interfaces during mollusk shell biomineralization.     

Ultrastructural studies of the mantle and the periostracal lamina in the manila clam, Ruditapes philippinarum

The four folds of the mantle and the periostracal lamina of R. philippinarum were studied using light, transmission and scanning electron microscopy to determine the histochemical and ultrastructural relationship existing between the mantle and the shell edge. The different cells lining the four folds, and in particular those of the periostracal groove, are described in relation to their secretions. The initial pellicle of the periostracum arises in the intercellular space between the basal cell and the first intermediate cell. In front of the third cell of the inner surface of the outer fold, the periostracal lamina is composed of two major layers; an outer electron-dense layer or periostracum and an inner electron-lucent fibrous layer or fibrous matrix. The role and the fate of these two layers differ; the outer layer will recover the external surface of the shell and the inner layer will contribute to shell growth.     

STRUCTURE OF THE CONCHIOLIN CASES OF THE PRISMS IN MYTILUS EDULIS LINNE

The prisms in the shell of Mytilus edulis Linné are calcite needles. Their small size and their thin conchiolin cases distinguish them from the prisms of many other species of mollusks. These Mytilus prisms have been studied with the electron microscope. The material consisted of positive replicas of surfaces of the prismatic layer, etched with chelating agents, and of preparations of tubular cases from decalcified prisms which were compared with the conchiolin from decalcified mother-of-pearl of the same species. In the replicas, the cases appear as thin pellicles in the intervals between the prism crystals. Both the prism cases and the nacreous conchiolin, disintegrated by exposure to ultrasonic waves and sedimented on supporting films, appear in the form of tightly meshed, reticulated sheets, described as "tight pelecypod pattern" in former studies on nacreous conchiolin of Mytilus. The results show that in the shell of this species the same conchiolin structure is associated with aragonite in mother-of-pearl and with calcite in the prismatic layer.     

Studies on Shell Formation : VIII. Electron Microscopy of Crystal Growth of the Nacreous Layer of the Oyster Crassostrea virginica

Electron microscope observations have been made by means of the replica method on growth processes of calcite crystals of the nacreous layer of the shell of the oyster, Crassostrea virginica. Layer formation is initiated by the secretion of a conchiolin matrix and the deposition of rounded crystal seeds on or in this material. In some areas crystal seeds are elongate and within a given area show a similar orientation, probably due to slower deposition. The seeds appear to increase in size by dendritic growth, and smaller seeds become incorporated into larger ones which come into contact to form a single layer. With further growth, crystals overlap, forming a step-like arrangement. The direction of growth is frequently different in neighboring regions. Crystal seeds deposited on crystal surfaces are usually elongate and oriented. Well developed crystals have a tabular idiomorphic form and are parallel in their growth. Rounded and irregular crystals were also observed. The crystals show reticular structure with units of the order of 100 A and striations corresponding with the rhombohedral axes of the crystals. The role of the mantle is discussed in relation to the growth patterns of crystals and shell structure.     

Periostracal mineralization in the gastrochaenid bivalve Spengleria

We investigated the spikes on the outer shell surface of the endolithic gastrochaenid bivalve genus Spengleria with a view to understand the mechanism by which they form and evaluate their homology with spikes in other heterodont and palaeoheterodont bivalves. We discovered that spike formation varied in mechanism between different parts of the valve. In the posterior region, spikes form within the translucent layer of the periostracum but separated from the calcareous part of the shell. By contrast those spikes in the anterior and ventral region, despite also forming within the translucent periostracal layer, become incorporated into the outer shell layer. Spikes in the posterior area of Spengleria mytiloides form only on the outer surface of the periostracum and are therefore, not encased in periostracal material. Despite differences in construction between these gastrochaenid spikes and those of other heterodont and palaeoheterodont bivalves, all involve calcification of the inner translucent periostracal layer which may indicate a deeper homology.     

Morphological studies on the calcification process in the fresh-water mussel Amblema

Light microscopy, transmission electron microscopy, scanning electron microscopy, various histochemical procedures for the localization of mineral ions, and analytical electron microscopy have been used to investigate the mechanisms inherent at the mantle edge for shell formation and growth in Amblema plicata perplicata, Conrad. The multilayered periostracum, its component laminae formed from the epithelia lining either the periostracal groove or the outer mantle epithelium (of the periostracal cul de sac), appears to play the major regulatory and organizational role in the formation of the component mineralized layers of the shell. Thus, the inner layer of the periostracum traps and binds calcium and subsequently gives rise to matricial proteinaceous fibrils or lamellar extensions which serve as nucleation templates for the formation and orientation of the crystalline subunits (rhombs) in the forming nacreous layer. Simultaneously, the middle periostracal layer furnishes or provides the total ionic calcium pool and the matricial organization necessary for the production of the spherical subunits which pack the matricial ‘bags’ of the developing prismatic layer. The outer periostracal layer appears to be a supportive structure, possibly responsible for the mechanical deformations which occur in the other laminae of the periostracum. The functional differences in the various layers of the periostracum are related to peculiar morphological variables (foliations, vacuolizations, columns) inherent in the structure and course of this heterogeneous (morphologically and biochemically) unit. From this study, using the dynamic mantle edge as a morphological model system, we have been able to identify at least six interrelated events which culminate in the production of the mature mineralized shell layers (nacre, prisms) at the growing edge of this fresh-water mussel.     

A horseradish peroxidase study on the mammillothalamic tract in the rat

The mammillary projections to the anterior thalamic nuclei were investigated in the rat, using the horseradish peroxidase (HRP) method. Pars centralis of the medial mammillary nucleus projects to the medial portion of the ateromedial nucleus (AM). Pars medialis (Mm) of the medial mammillary nucleus sends fibers to the ipsilateral AM and sparsely to the medial portion of the contralateral side. The ventral and dorsal portions of Mm project to the anterior and posterior portions of AM, respectively. The pars latralis (Ml) and pars posterior (Mp) of the medial mammillary nucleus send fibers predominantly to the ipsilateral anteroventral nucleus and sparsely to the contralateral side. A slight difference between Ml and Mp projections was observed. The lateral mammillary nucleus projects bilaterally to the anterodorsal nucleus.     

网湖水域中绢丝丽蚌贝壳形态的研究

对网湖1368枚绢丝丽蚌贝壳形态研究表明：前排小棘或棘痕数介于3－5之间,4个者居多;所排小棘或棘痕数介于1－4之间,2个者居多。壳长与壳厚的直线回归方程为：L＝7.2406T＋2．4392,贝壳的角质层最薄,呈棕褐色或者黑色;核柱层稍厚,呈黄褐色;珍珠层最厚,皎白闪亮。生长轮在棱柱层上和珍珠层外表面清晰可见。贝壳外表面背部肋嵴细弱,只在近壳顶处较明显;其棱柱层背部和后部呈黄褐色者为雌蚌。贝壳外表面背部肋嵴粗壮,且整个背部都十分显著;其棱柱层背部和后部呈红色或红褐色者为雄蚌。