Conchiolin

Abstract: The periostracum is an outer, quinone-tanned protein layer of the bivalve shell. The conchiolin interprismatic walls of the prismatic outer layer of some bivalves are in structural continuity with the periostracum. It is suggested that calcium carbonate deposition begins so close to the margin that all the periostracal material has not polymerised by the time the first crystals of calcium carbonate are precipitated. Consequently, the periostracal protein is pushed to the edge of growing carbonate crystals, preventing their further lateral growth. Growth proceeds in a vertical direction, as columns, until the influence of the periostracum has waned. Confirmatory evidence is provided by natural-regneration periostracum sheets formed on the inner layers of the shell, which are associated with thin prismatic layers.

Abstract: 1, C and D). The dominant crystalline mineral phase is pyrite, whereas greigite, the sulfide analog of magnetite, is present in lower proportions and accounts for the ferrimagnetism. The interior of the sclerites is penetrated by a pulp of pedal tissue that extends almost to the tip. Between the pulp and the outer mineralized layers there is a tough, laminated, organic layer, histologically indistinguishable from molluscan conchiolin. The conchiolin is studded with minute (typically about 1 m in diameter) iron sulfide granules and finely dispersed iron sulfur compounds; these minerals are more common in the outer layers of the conchiolin. Where the sclerite surface is overlain by adjacent sclerites, the sclerites are covered by a coat of bacteria (Fig. 1C; fig. S2G). Iron sulfide as a skeletal material is not known in metazoans (4), although accumulations of metal sulfides occur in animal tissues

Abstract: The variation in histology between the epithelial regions of the mantle in Anodonta cygnea concerned with the secretion of the different layers of the shell has been investigated. The epithelia on the outer surface of the outer mantle fold which secrete the outer layer of the ligament and the corresponding outer calcareous layer of the valves are basically alike in structure. They are also comparable in appearance with the epithelium on the inner surface of the outer mantle fold which secretes the periostracum and with that of the fused outer folds responsible for the formation of the fusion layer of the ligament. All these epithelial regions can readily be distinguished histologically from the epithelia of the general outer mantle surface and of the mantle isthmus which produce the inner layers of the valves and ligament respectively. The histological differentiation of the mantle epithelium in Anodonta is compared with that of Mytilus edulis and Ostrea edulis . The degree of zonation of the epithelium can be correlated with the extent to which the shell layers differ in composition from one another. It is suggested that the variation in histological appearance between epithelia from different areas of the mantle is partly determined by the relative amounts of non-calcareous material (conchiolin) which these epithelia produce and by the rate of their secretory activity. The different epithelial regions of the mantle in Anodonta and other lamellibranchs vary appreciably in their reactions to histochemical tests for alkaline phosphatase, ribonucleic acids, and glycogen. In Anodonta , the epithelial cells of the outer mantle fold differ markedly from those which secrete the inner layers of the shell in that they exhibit high alkaline phosphatase activity and appear to be rich in cytoplasmic ribonucleic acids. In contrast, the epithelium of the general outer surface of the mantle is distinguished by a high content of glycogen, a condition which also occurs in many other bivalves. It is interesting to note that alkaline phosphatase can be demonstrated in epithelia which secrete non-calcareous regions of the shell. In all the lamellibranchs investigated, ribonucleic acids can be detected in relatively higher concentration in epithelial cells concerned principally with the secretion of conchiolin.

Abstract: A comparison has been made between the staining reactions and histochemical properties of the non-calcareous material (conchiolin) from the different layers of the shells of Anodonta cygnea , Mytilas edulis , and Ostrea edulis . Acid hydrolysates of the conchiolin protein have been analysed qualitatively by paper chromatography. The composition of the conchiolin in Anodonta confirms the view that corresponding layers of the valves and ligament represent modifications of the same layers of the shell. In this bivalve, the properties of the outer layers of the valves and ligament are closely comparable with each other, and also with those of the periostracum and the fusion layer of the ligament. All these regions consist of a quinone-tanned protein, hydrolysates of which are rich in phenolic amino-acids, especially tyrosine, and in glycine. Much of the periostracal conchiolin in Mytilus shows basically the same properties as the periostracum in Anodonta . However, the outer layers of the valves and ligament in Ostrea and Mytilus each exhibit progressively greater specialization compared with the situation in Anodonta . This is most marked in Mytilus where these components differ completely in character. The conchiolin in the inner shell layers differs markedly in composition from that in the outer layers in Anodonta and Ostrea and from the periostracum in Mytilus.Hydrolysates of its protein constituent contain appreciably more aspartic acid and glutamic acid but much smaller amounts of phenolic amino-acids. The protein is only lightly tanned. Although in these properties the corresponding inner layers of the valves and ligament appear fundamentally alike, each component has certain specialized features. It is suggested that the modifications shown by she protein of the inner ligament layer, which is characterized by a high content of proline and methionine, are correlated with the specialized function of this region of the shell.