Chiton tooth

The formation of the chiton tooth takes place on a tongue-like process, the radula, which comprises more than a hundred rows of teeth and is around 3 cm long. Two of the teeth in each row are mineralized (Fig. 1.2). The chiton uses its teeth to scrape the rocky substrate for algae and sponges living on the surface or just below the surface [11, 12]. Teeth are worn out at a rate of about a row every 12-48 h, and are continuously replaced [13, 14]. Thus the radula is in essence a conveyor belt, and every tooth row represents the product of 12-48 h worth of assembly and mineralization activities. The chiton radula is thus an ideal object to study assembly and mineralization. Its major drawback is that the teeth are small 100 pm across [6]. 

The chiton tooth radula is perhaps the best documented example of the general principle in biomineralization, namely that the organic matrix is formed first, and only then are the spaces within the framework filled by the mineral. 

The cells continuously withdraw as they form more dentin, and the pulp cavity decreases in size. The zone of formation between the cell surface and the final mineralized product is usually only a few tens of micrometers thick (Fig. 1.6). It is known as the predentin. Many rodents have continuously forming incisors. They, like the chiton teeth, are ground away at one end, and are continuously replaced. Thus all stages of tooth formation can be studied in a single rodent incisor. In the rat tooth the stages merge into each other, and are not separated, as is the case in the chiton tooth. 

The chiton tooth, dentin and the sea urchin larval spicule reflect the enormous diversity of the field of biomineralization. They differ with respect to the nature of their mineral and macromolecular components, as well as their structures. Few underlying common strategies can be recognized the delineation of a dedicated space in which the mineralized tissue forms, the formation of mineral in a preformed framework within this space, and the precipitation of mineral from a supersaturated phase. In this section we will reexamine some of these underlying issues, focussing in particular on the microenvironment in which mineralization occurs. 

In the chiton tooth, the organic framework components are synthesized and secreted by the cells into the extracellular space, and there they self assemble. By the time mineralization is about to occur the cells are tens of micrometers away from many of the mineralization sites. They must therefore operate by remote control. The mineralization sites themselves are within a complex chitin framework, the dimensions of which are in the nanometer range. The sea urchin larval spicule represents the exact opposite situation. Mineralization occurs in a vacuole defined by a membrane, and the entire apparatus is within a consortium of fused cells (the syncytium). The membrane of the syncytium tightly surrounds the growing spicule [74], Therefore, it has been proposed that the cells directly control spicule formation. The mineralization vacuole is subdivided by framework proteins. Nothing is known about the structure of the one nucleation site per spicule in the larvae, but in the adult a well-defined location, enclosed within a framework, has been identified as the nucleation site [83]. Dentin formation is intermediate between the two. It is an extracellular process, and the distances between cells or cell processes and mineralization sites are in the range of tens of micrometers or several micrometers respectively. Nucleation occurs within the fibril or at its surface and is associated with a site on the fibril surface some 7 or 8 nm wide [54]. The space available for crystal growth within the fibril is even smaller in one of the dimensions, namely 2 or 3 nm wide. 

Figure 1.2. Light micrograph of the radula of the chiton Acanthopleura haddoni, showing 46 tooth rows. On the right hand side of the figure the first seven tooth rows are totally transparent (not mineralized), while in the next four rows the first mineral deposits appear. The dark color is due to the iron oxide mineral, magnetite.

Figure 13 Chiton teeth. A sketch of the radular organ in the chiton mouth and a micrograph showing the shape and rows of magnetite teeth. Each tooth is less than 20 p,m in length (source Nesson and Lowenstam, 1985, figure 1, p. 336).

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