Forming nacreous layer of the shells of the bivalves Atrina rigida and Pinctada margaritifera: An environmental- and cryo-scanning electron microscopy study

Abstract

A key to understanding control over mineral formation in mollusk shells is the microenvironment inside the pre-formed 3-dimensional organic matrix framework where mineral forms. Much of what is known about nacre formation is from observations of the mature tissue. Although these studies have elucidated several important aspects of this process, the structure of the organic matrix and the microenvironment where the crystal nucleates and grows are very difficult to infer from observations of the mature nacre. Here, we use environmental- and cryo-scanning electron microscopy to investigate the organic matrix structure at the onset of mineralization in the nacre of two mollusk species: the bivalves Atrina rigida and Pinctada margaritifera. These two techniques allow the visualization of hydrated biological materials coupled with the preservation of the organic matrix close to physiological conditions. We identified a hydrated gel-like protein phase filling the space between two interlamellar sheets prior to mineral formation. The results are consistent with this phase being the silk-like proteins, and show that mineral formation does not occur in an aqueous solution, but in a hydrated gel-like medium. As the tablets grow, the silk-fibroin is pushed aside and becomes sandwiched between the mineral and the chitin layer.

Aragonite crystals formation on nacre substrate

“…It is known that fresh nacre substrate is usually covered by a thin organic layer [27][28][29], which can be eliminated by 5% NaClO solution. Fig.…”

“…Fresh nacre surface is usually covered by a sheet of organic matrix [27][28][29], which is composed of insoluble matrix and soluble matrix [11]. Previous reports show that ACC exists in nacre and is stabilized by soluble organic matrix [21][22][23][24].…”

Gastropod nacre: Structure, properties and growth — Biological, chemical and physical basics

“…This amphiphilic property could be an important driving force for selforganisation, resulting in the observed layered organic sheets. The modified chitin fraction might form the interface between the chitin core and the interlamellar matrix surface, which is in contact with the proposed hydrophobic silk-like gel [49,[150][151][152][153] between the interlamellar layers. 2 In 2009, Checa et al showed TEM images of extracellular vesicles, fusing with the surface of the interlamellar matrix sheets at the mantle side of gastropods [154].…”

The growth of the screw dislocation of nacreous layer on Pteria penguin

“…The organic matters in the nacreous layer are distributed between the layers of aragonite crystals or wrapped around nanoscale aragonite crystals continuously and in parallel [35][36][37][38]. The development of modern molecular biology encourages people to explore the biomineralization mechanism and nacreous layer's formation mechanism from the perspective of genetic control [39,40].…”