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The Key Role of the Surface Membrane in Why Gastropod Nacre Grows in Towers

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The Key Role of the Surface Membrane in Why Gastropod Nacre Grows in Towers The key role of the surface membrane in why gastropod nacre grows in towers Antonio G. Checaa,1, Julyan H. E. Cartwrightb, and Marc-Georg Willingerc aDepartamento de Estratigrafía y Paleontología, Facultad de Ciencias, Universidad de Granada, E-18071 Granada, Spain; bInstituto Andaluz de Ciencias de la Tierra, Consejo Superior de Investigaciones Cientificas–Universidad de Granada, Campus Fuentenueva, E-18071 Granada, Spain; and cDepartamento de Química, Centro de Investigac¸a˜ o em Materiais Ceraˆmicos e Compositos, Campus Universitario de Santiago, Universidade de Aveiro, 3810- 193 Aveiro, Portugal Edited by Steven M. Stanley, University of Hawaii, Honolulu, HI, and approved November 25, 2008 (received for review September 4, 2008) The nacre of gastropod molluscs is intriguingly stacked in towers. in a step-like manner (7, 9). The nacre thus produced is said to It is covered by a surface membrane, which protects the growing have a terraced arrangement (Fig. 1A). nacre surface from damage when the animal withdraws into its In gastropods, however, the biomineralization compartment shell. The surface membrane is supplied by vesicles that adhere to of nacre is enclosed by a surface membrane first reported by it on its mantle side and secretes interlamellar membranes from the Nakahara (8) in Monodonta and Haliotis. Since its discovery, its nacre side. Nacre tablets rapidly grow in height and later expand existence went unremarked, until Cartwright and Checa (10) sideways; the part of the tablet formed during this initial growth realized that it is widespread in nacre-secreting gastropods and phase is here called the core. During initial growth, the tips of the that the interlamellar membranes must necessarily detach from cores remain permanently submerged within the surface mem- it. The surface membrane acts as a protective seal, which brane. The interlamellar membranes, which otherwise separate the prevents the organic compounds and minerals involved in nacre nacre tablet lamellae, do not extend across cores, which are aligned growth from being lost to the external environment when the soft in stacked tablets forming the tower axis, and thus towers of nacre body of the gastropod withdraws into its shell, something evi- tablets are continuous along the central axis. We hypothesize that dently not necessary with bivalves. Below the surface membrane, in gastropod nacre growth core formation precedes that of the many parallel interlamellar membranes with tablets growing interlamellar membrane. Once the core is complete, a new inter- between them can be found. These tablets are typically stacked lamellar membrane, which covers the area of the tablet outside the in towers (Fig. 1B), with the smaller, more recently begun tablets core, detaches from the surface membrane. In this way, the found at the top. Although the nacre of gastropods, in particular tower-like growth of gastropod nacre becomes comprehensible. that of the abalone, i.e., the genus Haliotis, has been intensively studied, there are still many pieces to be assembled in the puzzle. biomineralization ͉ molluscs ͉ organic membranes ͉ epitaxy One, perhaps key piece, is the surface membrane, key both because it is intimately related to the other components of nacre acre is by far the most intensively studied non-human and because the mineral ions and organic molecules for nacre Norgano–mineral biocomposite. It has a high proportion, growth are necessarily introduced into the biomineralization Ϸ 5%, of organic matter (proteins and polysaccharides; ref 1), the compartment through it. Its ultrastructure, growth, and secre- mineral fraction being exclusively in the form of aragonite. tional activity have never been elucidated. Jackson et al. (2) estimated that its work of fracture is 3,000 times This work is dedicated to determining the relationship of the higher than that of inorganic aragonite, although later estimates surface membrane to the interlamellar membranes and mineral reduce this figure considerably (see the review in ref. 3). Its tablets. Our conclusions shed light not only on the dynamics of superior biomechanical properties, together with its interest to gastropod nacre growth but also bear on the present debate the pearl industry and its possible biomedical uses (see e.g., ref. about whether superimposed nacre tablets nucleate and grow 4), make nacre the subject of many biomimetic studies. An onto the organic interlamellar matrix or, alternatively, whether ultimate aim of such work is to mimic nacre in the laboratory, there is crystallographic continuity between them across the following the biological principles used by molluscs to produce interlamellar membranes. such a biomaterial (5). It is sine qua non for this objective to have a complete understanding of the mechanisms involved in nacre Results growth. Surface Membrane. The surface membrane extends between the Nacre is secreted only by the molluscan classes Gastropoda, adoral and apical boundaries of the nacreous layer, usually Bivalvia, Cephalopoda, and, to a minor extent, Tryblidiida. It has bounded by the external spherulithic layer and an internal a lamellar structure consisting of alternating tablets of aragonite aragonitic lamellar layer of uncertain microstructure (Fig. 2A). 300–500 nm thick and 5–15 ␮m wide and organic interlamellar In Gibbula and Monodonta, at least, its mantle-side surface is membranes Ϸ30 nm thick, which have a core of ␤-chitin sur- dotted with bodies adhering to it (Fig. 2 A and F). In transmission rounded by acidic proteins (6). It is now clear that the sequence electron microscopy (TEM) sections these structures are seen to of nacre formation involves the secretion of interlamellar mem- be hollow (Figs. 2 B and C and 3C) and thus may be called branes (7) separated by a liquid rich in silk fibroin (5); only subsequently is the liquid replaced with mineral (7–9). This Author contributions: A.G.C. and J.H.E.C. designed research; A.G.C., J.H.E.C., and M.-G.W. pattern is the same for the bivalves and gastropods, and it is likely performed research; A.G.C. and M.-G.W. analyzed data; and A.G.C. wrote the paper. so too for the other nacre-secreting molluscs, although this is yet The authors declare no conflict of interest. to be determined. There are, however, structural differences This article is a PNAS Direct Submission. between bivalve and gastropod nacre. In the former group, the 1To whom correspondence should be addressed. E-mail: [email protected]. interlamellar membranes are secreted with just the liquid-filled This article contains supporting information online at www.pnas.org/cgi/content/full/ extrapallial space between them and the cells of the mantle 0808796106/DCSupplemental. epithelium, and mineralization within the membranes proceeds © 2008 by The National Academy of Sciences of the USA 38–43 ͉ PNAS ͉ January 6, 2009 ͉ vol. 106 ͉ no. 1 www.pnas.org͞cgi͞doi͞10.1073͞pnas.0808796106 Downloaded by guest on September 29, 2021 membrane and the surface membrane and partly encased within A it (Fig. 2 D and E). Where tablets have been torn off upon contraction of the membranes during sample preparation, the scars remaining can also be seen (Fig. 2D). The width of these cores is estimated to be between 100 and 200 nm (Figs. 2 E and F, and 3 A, C, and D). The topographical relationship observed in SEM samples is so recurrent that the possibility that this is artifactual can be excluded. The relationship is further demon- strated by TEM, which reveals that the tip of a growing tablet (i.e., the last 50–70 nm) is directly embedded within the surface membrane (Fig. 3 A, C, and D). In the few instances when towers are fortuitously sectioned exactly through their central axis, the interlamellar membrane possesses a fuzzy appearance or is totally absent. The disappear- ance of the interlamellar membrane at the very axes of the towers is evident in some exceptional TEM views (Fig. 3). The partial dissolution sometimes produced during sample preparation (Fig. 3 D and E) does not affect the tower axis area. TEM views of decalcified towers of Gibbula umbilicalis show too that the interlamellar membranes are missing at the very axes of the B towers, across a maximal width of Ϸ100 nm, or are replaced by a fuzzy band of organic matter with a different orientation (Fig. 4A). SEM observation in back-scattered electron (BSE) mode of polished axial sections of nacre towers of the same species, in which we can safely assume that membranes have not been disturbed during sample preparation, manifests that the same effect may take place across tens of tablets in a tower (Fig. 4B). The fuzzy band, when present, usually curves slightly toward the top of the tower. When the same samples are decalcified with methanolic solution, the axes of the towers are marked by a succession of holes (Ϸ150 nm wide) with coarsened rims, sometimes traversed by organic threads (Fig. 4 C and D). The regularity and persistence of such structures exclude the possi- bility that they are artifacts caused by dissolution. Treatment with 2% EDTA preferentially removes the calcified organic-rich components: the interlamellar organic membranes, the lateral boundaries between tablets and, interestingly, the parts of the Fig. 1. Bivalve and gastropod nacre growth compared. (A) Oblique view of tablets coinciding with the axes of towers [supporting informa- the terraced nacre of the bivalve P. margaritifera.(B) Oblique view of the tion (SI) Fig. S1]. towered nacre of the gastropod Perotrochus caledonicus. High-resolution TEM observations show that the interface between two superimposed tablets is fully crystalline at the axis (Fig. 5A). Observation of lattice fringes and fast Fourier trans- vesicles. They vary in shape from spherical, when they are just form (FFT) analysis of small areas provide additional evidence touching the surface membrane, to strongly compressed, when of this crystalline character.
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