Title Formation of the Nacreous and the Innermost Prismatic Layer in Rusticus (Gmelin) ()

Author(s) Uozumi, Satoru; Togo, Yoshihiro

Citation Journal of the Faculty of Science, Hokkaido University. Series 4, Geology and mineralogy, 17(1), 153-172

Issue Date 1975-03

Doc URL http://hdl.handle.net/2115/36052

Type bulletin (article)

File Information 17(1)_153-172.pdf

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Hokkaido University Collection of Scholarly and Academic Papers : HUSCAP Jour. Fac. Sci., Hokkaido Univ., Ser. IV, voL 17, no. 1, March, 1975, pp. 153-l72.

FORMATION OF THE NACREOUS AND TH[E INNERMOST PRISMATIC LAYER IN OMPHALIUS RUSTICUS (GMELIN) (GASTROPODA) by Satoru Uozumi and Yosliihiro Togo (with 2 text-figures and 6 plates)

(Contyibution from the Departinent of Geology and Mineralogy, Faculty of Science, Hokkaido University, No. 1397)

The initiation and the growth of aragonite crystals which make up the nacreous layer have been studied from inorphological, histologicai and biochemical view points. Recent studies carried out with the aid of an electron microscope have shown details of the sequential processes which lead to tlie foi7mation of aragonite crystals. Bevelander and Nakahara (l959) have reported that the initiation and the growth of aragonite occur within the domain of compartments composed of the inter-lamellar conchiolin membranes. This conclusion is weli-known as the "compartment hypothesis". Mutvei (l972) confirmed this hypothesis from scanning electron microscopic observations on the cleveloping surfaces of the aragonite nacreous and the calcite prismatic layers of Mytilus edulis. Most of the works on the shell formatioA have been concentrated in the nacre (sheet nacre) of lamellibranchs, and only a few studies liave been reported on that of marine gastropods (ErbeR et al., l972a, 1972b; Wise, 1970). The purpose of the research presented in this paper was to examkie with a scanniRg electron microscope the formation of the aragonite crystals on the developing nacre surface of Omphalius rusticus (Gastropoda), and also to describe the formation of aragonite crystals in the innermost prismatic layer, which develops directly on the imier surface of the nacre.

Materials and Methods

Speciinens of Omphalius rusticus were obtained from the coast of Shukuzu, Otaru-City, Southwest Hokkaido, and kept in a glass acluaria, filled with artificial sea water (l9-200C, pH 8.S). Pieses of shell approximately 1-2 cm2 in size were removed from the body of the living specimens, using an electric dental machine. These pieces 154 S. Uozu mi and Y. Togo were thoroughly washed in sea water and repeatedly in distilled water to remove the extrapallial fluid and then divided into three groups. One of them was kept in the desiccator with no treatment, another was treated with a 5% sodium hypochlorite solution at several different time intervals to remove the proteinaceous materials in different degrees, and the last one was fixed in a 2.5% glutaraldehyde-sea water solution. Then, the pieces of the latter two groups were washed in distilled water and air dried. A few pieces of the former group were randomly irradiated by an electron beam to break the last-form.ed membrane after coating with carbon. All of them were directly mounted on the specimen-holders using silver paint, follow.ed by a double coating with carbon and gold, and studied with a scanning electron microscope JEM-U3 (Japan Electron Optics Lab. Ltd.) at the Department of Hokkaido University.

Observations and Results

Aiacreous layer The soJcalled nacreous layer of marine shells has roughly been divided into two types based on the difference in construction of the nacre tablets: one is

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Text-Fig. 1. Scanning electron micrographs showing the nacreous layer in vertical polished section (Ieft) and growing stacks of tablets (right). Note the tablets with parallel arrangement, and the inter-lamellar conchiolin membranes (arrows) Iying between the tablets. X 2,OOO (left); X 6,OOO (insert); X 6,OOO (right). FORMATIONOFNACREOUSANDINNERMOSTPRISMATICLAYERINOMPHALIUs 155 - Text-Fig.2. Electron micro- p."'"ig graph showing inter-lamellar top･.ec--eeee.ee.-.-.ifliikll/ee11x,/i,11Gll//$ma･,'ec･ conchiolin membrane; demineralized m EDTA. X l5,OOO. .ee-

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called the "sheet nacre" (Taylor, 1969) or "Backsteinbau" (Schrnidt, 1923) of the lamellibranchs; the other is the "columnar Aacre" (Taylor, .1969) or "Treppen" (Schmidt, 1923) of the gastropods. The nacre of Omphalius in not an exception to these rules: it consists of aragonite tablets arraAged into columns. The growing surface of the nacre was covered by tall conical stacks of aragonite tablets: the early crystals forrn at the tops of the stacks and.then, as growing proceeds, they expand laterally as more crystals form on top. Their lateral growths generate fiat tablets that ultimately get in contact with the crystals of the adjacent stacks. Then, they coalsce to form thin mineral lamellae characterizing the nacre of molluscs. In our preparations of surface of nacre also, conical stacks are covered by the last-formed inter-lamellar conchiolin membrane, lying parallel to the' shell inner surface. This membrane is very thin and fragile, whereas the conical termination of stacks which contact with this membrane are very sharp. Accordingly, these terminal parts of the stacks are occasionally seen as if protruding on the inter-lamellar membrane. This aspect is caused by the artificial distortion or coRstrictioR of the membrane produced in the course of the drying of the preparations. The preparatioRs in which the last-formed membrane was broken by a random irradiation of an electron beam clearly show that this membrane covers the terminations of conical stacks (Pl. 2, Figs. 4, 6). In the preparations in which the membrane was constricted and remove from the foregrouRd, the conical terminal parts were pulled down in the same direction (Pl. 1, Figs. 3-6). 156 S. Uozumi and Y. Togo Also, froin the obsei'vations under the low acceierating voltage 10 kv, tlie conical terminal parts are photographed as the soft-focus picture, and the last-formed membrane is draped rot}ghly on these terminations (Pl. 2, Figs. 1, 2). From the foregoing observations, it is clear that all growing terminations of stacks are always covered by the last-formed membrane. Incidentaliy, most of the membranes examinecl have an smooth surface bt}t it is noticed that a few of thein have a granular surface with many scattered "openings" (Pl. 2, Fig. 5). The granular ones appear to be more dense and thicker than the smooth ones, and seem to be continuous with them (Pl. 2, Fig. 2). It seems, if this observation is correct, that either of the two aspects of membrane shows an artificial pattern. The present writers will re-examine more precisely the natural appearance of the membrane by freeze-drying and critical drying methods recently developed for the preparation of these samples for the electron microscope.

Innernzost Prismatic Layer The junior author (1974) has described the way of growing and the shell structure of the innermost layer of gastropods based on the ontogenetic study of the slitell structure. This layer is composed of aragonite crystals and represents a rather disordered construction of crystals which is clearly distingushed from other structural layers, nacreous, crossed-lamellar and so on.. Moreover, this layer decreases rather smoothly in thickness towards the of the shell, although the inner and the outer layers increase inversely in thickness. The innermost layer of 0mphalitts rustictts consists of aragonite prisms: in the vertical section of this layer various sized prisms, 3 to 20pt in width, are standing close together and inclining with certain angies to the mineral lamellae of nacre underlying this layer. When the developing surface of this layer is observed, the fan-shaped aragonite tablets appear arranged in roughly parallel overlapping rows (Pl. 3, Figs. I, 3). In some places, their marginal portions increase in thickness and rise as a ridge, 4pt to 5pt in width and IOpt to 20pt in length (Pl. 4, Fig. 5). In the areas where the innermost layer is just being formed many spherical crystallites, O.1-O.4u in diameter, can be observed on the nacre crystals or on the iimnature fan-shaped tablets (Pl. 3, Figs. 2, 5). In some parts, maRy acicular crystals, approximately 5pt in length aAd O.3" in diaiineter, are visible on or around the fan-shaped tables (Pl. 3, Figs. 3-5, Pl. 5, Fig. 3). Three shapes of aragonite crystals, spherical, acicular and tabular, are observable in one portion of the preparation, although the proportion of these crystals is variable in places. When spherical crystallites are predominant, the FORMATIONOFNACREOUSANDINNERMOSTPRISMATICLAYERINOMPHALIUs 157 other types have an ill-development. Same relationship is valid between acict}lar aiid tabular crystals respectively. More detailed observations show that tlie fofmation of new tabular crystals is initiated from spherical crystallites: these experiment a dendritic growth producecl by aggregation and ultimately generate the acicular crystals (Pl. 3, Figs. S, 6; Pl. 5, Figs. 2,3). With the increase in size and the nuiinber of tliLe acicular crystals they make contact with the neighboL}ring ones, and form a fan-shaped tablet (Pl. 3, Fig. 6; P}. 4, Figs. 2, 3). Sequentially, the fan-shaped tablets increase their length and widtl/i by further accumulation of acicular crystals. Moreover, they generate larger crystals through pseudohexagonal intergrowth and so on (PI. 4, Fig. 1;Pl. 5, Rg. 5). When the fan-shaped tablets are being almost coinpleted, new minL}te crystallites occur at the riins or on the central portions of their surfaces. These new crystallites are rounded, measuring O.2" to O.7ll in diameter at an early stage (Pl. 4, Fig. I;Pl. S, Fig. 6) and grow rapidly in a vertical direction from the surface of the fan--shaped tablets. Through growth these crystallites become fine prisms (Pl. 6, Figs. 1-3). Some of them grow independently and some coalesce with adjacent ones to form a laiger prism. They are approximately O.2" to O.7u in width and arrange with their long axes nearly normal to the surface of the fan-shaped tablets. They finally overgrow all over the surface of the tablets (Pl. 6, Fig. 6). Vertical growth and coalescence of these forms give rise to an increase of the tlaickness of the fan--shaped tab}ets and to the generation of various sized prisms.

Summary The present study has revealed that the initiation and the growth of nacre crystals in Omphalius take place witliin the interspaces between inter-lamellar conchiolin menabranes, which have been already polymerizecl prior to crystal growth, just as in the case of the lamellibranch nacre. All, or apart, of the extrapallial fEuid that passed throt}gh the several sheets of the inter-lamellar membranes is thought to influence the further increase in size of the tablets of stacks until the growing tablets make contact with tablets of adjacent stacks. In this case, it is still unknown exactly what site of the membrane influences the transportation of tlie extrapallial fiuid. However, it may be possible to some extent that the passage of the fluid takes place through tl'ie mintite "openings", scattered on tlie concliiolin mernbrane (Pl. 2, Fig. 5). The stucly of the developing surface of the innermost layer has revealed the following two phases in the process by which this innermost layer is formed: 1) initiation of spherical crystallites, their growth to form acicular crystal, tlie accumulation and coalescence of the acicular crystals to form fan-shaped 158 s. uozumi and y. Togo tablets and 2) the verticai growth of tablets and the resultant formation of prlsms. The crystal growth in the latter phase is strikingly different from that in the former one, and rather simllar to that in lamellibranch nacre which will be described in other paper in the near future. Meanwhile, it is difficult to explain what is the sjgnificance suggested by the fonnation of the two different shell iayers, nacreous and prismatic, built up from the same mantle tissue. It may be assumed that the difference between them is not due to basic but to slight differences in the secretory activity of the mantle tissue at the respective portion. Namely, the differences may be dependent on the amounts, the degree of supersaturation and the viscosity of the extrapallial fluid.

Acknowledgeinents

The authors wish to express their deepest gratitude to Prof. Masao Minalo and Prof. Seiji Hashimoto, of our University, for their continuous encourage- ment and supervision during the course of this study. Thanks are also given to Dr. Makoto Kato, Dr. Masahiko Akiyama, Dr. KQji Nakamura, and Dr. Keiji' Iwata for their kind discussions of the subject. Also thanks are due to Dr. Chikara Akiba of Hokkaido Educational University for his kind discussion on the crystallographical problems. To Mr. Shigeshi Ohta for electron micrographs and technical assistance and to Mr. Ietaka Watanabe for his help in the laboratory works.

R eferences Beveiander, G. and Nakahara, H. (l969), An electron microscope study of the formation of the nacreous layer in the shell of certain bivalve mollt}scs. ailc. Tiss. Res. Vol. 3 p.84-92. - s) Erben, H.K. (1972), Uber die Bildt}ng und Wachstum von Perlmutt. Biomineralisation, Bd.4, p.15-4O. Mutvei, H. (l970), Ultrastructure of the mineral and organic components of molluscan nacreous layers. Biominetulisation, Bd.2, p.48-68. Mutvei, H. (l972), Formation of nacreous and prismatic iayers in Mlytilus edulis L. (Lamellibranciata). Biomineralisation, Bd.6, p.96-1OO. Schmidt, W.J. (1923), Bau und Bilding der Perlmuttermasse. Zool. Ib. Anat. Abt. Jbna, 45, p.l-l48. Togo, Y. (l974), Shell structure and growth of protoconch and teleoconch in ?Veptunea (Gastropoda). Jour. Geol. Soc. .ldpan, Vol.80, no.8, p.369-380. Uozumi, S. and Iwata, K. (1969), Studies on calcified tissues. Part l. Ultrastructure of the conchiolin in M)ytilus coruscus Gould. Earth Science, Vol.23, no. I, p. I-6. Uozumi, S., Iwata, K. and Togo, Y. (1972), The ultrastructure of the mineral in and the construction of the crossed-lamellar iayer in molluscan shell. Jour. Eac. Sci. Hokkaido FORMATION OF NACREOUS AND INNERMOST PRISMATIC LAYER IN OMPHALIUS 159

Univ., Ser.IV, Vol.XV, nos.3-4, p.447-478. Wada, K. (1972), Nucleation and growth of aragonite crystals in the nacre of some bivalve molluscs. Biomineralisation, Bd,6, p.l41-159. Wise, S.W. (l970), Microarchitecture and mode of formation of nacre (mother-of-pearl) in pelecypods, gastropods and cephalopods. Eclogae GeoL Helv., 63, p.775-797.

Received on Nov. 6, 1974 l60

Explanation of PIates

AII figures are scanning electron micrographs of Omphalius rusticus (Gmelin). Bar scale in mlcrons.

Plate 1 Outcrop pattern of the growing stack ends of tabiets on the inner surface of the nacreous layer.

Figs. I, 2. The growing stacks are covered by the last-formed inter-lamellar conchiolin membrane which somewhat hangs down artificially along the surface of stacks, Fig. 1 : X 3,ooo; Fig. 2: x s,ooo.

Figs. 3-6. The growing stacks are under the horizontal inter-lamellar membrane, The 3-5 tablets of the stack ends are pulled down artificially in the same direction by the constriction ef the inter-lamellar membrane. The outline of the stacks is obscurely photographed for the cover of this membrane. Fig. 3: X 2,OOO;Fig. 4: X 3,OOO; Fig. 5: X 2,OOO; Fig. 6: X 5,OOO. 16ユ

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Fig. 1, 2. 0utcrop pattern of the growing stack ends on the inner surface of the nacreous layer. The outline of the stacks is photographed as the soft-focus picture for the cover of inter-lamellar membrane under the low accelerating voltage 10 kv of the electron beam. The membrane is stretched like a thread between some stack ends. Fig. I: X 3,OOO;Fig. 2: X 3,OOO.

Fig. 3, 4. 0blique view of a right-angle fractured surface of the nacreous Iayer showing a two-dimensional aspect of the growing stacks. Fig.3: X l,OOO;Fig. 4: X 1,OOO.

Fig. 5. Inner surface of the nacreous layer showing the inter-lamellar membrane which covers the stacks without being constricted. Note the granular surface and the numerous minute "openings" of the membrane. X 3,OOO.

Fig.6. 0blique view of the cracked inner surface with stacks intersecting conchiolin membranes at an angle. Note the last-formed inter-Iamellar coRchiolin membrane and next underlying one. X 2,OOO. 茎63

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Fig.'l. Outcrop pattern of growing crystals on the inner surface of the innermost layer. Spherical crystallites develop on and among immature fan-shaped tablets. X l,OeO.

Fig. 2. Same preparation as in Fig. 1 with a higher magnification. X 3,OOO.

Fig. 3. 0utcrop pattern of the growing crystals on the inner surface of the innermost iayer. Acicular crystals develop on and among immature fan-sltaped tablets. X 1,OOO.

Fig. 4. Same preparation as in Fig. 3 with a higher magnification. X 3,OOO.

Fig. 5. Same preparation as in Fig. 2; a higher magnificated micrograph sltowing acicular crystals formed from spherical crystallites by dendritic intergrowth. X 3,OOO.

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Plate 4

Fig. 1. 0utcrop pattern of growing crystals on the inner surface of the innermost layer. All growing acicular and fan-shaped crystals are developing in a dense organic matrix. Fan-shaped tablets represent the phenomenon of pseudohexagonal intergrowth which is observable in the natural growth of aragonite crystals. X 6,OOO.

Figs. 2-S. Micrographs showing the aspect of fan-shaped tablets in various stages of their growth on the inner surface of the innermost layer. Note tips of acicular crystals protruding from the side face of the tablets. Tablets shown in Fig. 3 are in a more advanced stage than those in Fig. 2. Fan-shaped tablets in Fig. 4 correspond to an almost mature stage of their growth, although they remain as the tips of acicular crystals in their side face. Figs. 2, 3, 4: X 3,OOO; Fig. 5: X 1,OOO.

Fig.6. Full developed fan-shaped tablets of the inner surface of the mnermost Iayer showing the smooth side face and an increase in thickness. X 1,OOO. 167

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PIate S

Fig. I. Oblique view of a right-angle fractured surface showing a two-dimensional aspect of the innermost layer. Various sized spherical crystallites are seen on the fan-shaped tablets which overlie the nacreous layer (lower right). X 6,OOO.

Fig. 2, 3, S. Outcrop pattern of immature fan-shaped tabiets on the inner surface of the innermost layer. Note the acicular or rod-like crystals which are formed by aggregation of spherical crystallites. Figs. 2, 3, 5: X 3,OOO.

Fig.4. 0utcrop pattern of the inner surface of the inner-most layer. Most of the organic materials have been removed with a 5% sodium hypochiorite solution. Note the coalescent acicL}lar crystals directly lying on the mineral lamellae of nacre (lower Ieft) and overlaid in turn by immattire fan-shaped tablets. X 3,OOO.

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Plate 6

Figs. I-3. 0utcrop pattern of minute prismatic crystals growing on the rims or central parts of the fan-shaped tablets. Nete the pattern of growth and the Iinear arrangement of the minute prisms. Fig. I-3 : X 6,OOO.

Fig. 4. 0utcrop pattern of mature fan-shaped tablets. The pattern of the inner surface of the fan-shaped tablets is made up of many minute prismatic crystals. Organic material has been partially removed with sea water. X 3 OOO. ) Fig. S. Outcrop pattern of mature fan-shaped tablets. Note the interior structure of the fan-shaped tablets. Organic material has been almost perfectly removed with a 5% sodium hypochlorite solution. X 3,OOO.

Fig.6. 0utcrop pattern of irreguiar prisrns at the final stage of the formation of the innermost layer. The surface of the fan-shaped tablets is almost completely occupied by prismatic crystals which have grown vertically from this surface. X 3,OOO 171

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