Extracellular calcification by epithelia

The extracellular formation of crystals of CaC03 by epithelia is the most common type of skeleton-forming system and occurs in corals (Vandermeu-len, 1975), molluscs (Wilbur, 1964), some annelids (see p. 84) brachiopods (Williams, 1971), and arthropods (Travis, 1970). In the arthropods, elaborate cellular extensions penetrate the mineralized carapace and are important in the calcification process. Formation of a mineralized skeleton in these taxa involves the movement of Ca and HCOj across a layer of cells from the body fluid (absent in corals). Nucleation and crystal growth take place in an organic milieu secreted by the epithelium. 

It is generally assumed that route (a) is the one involved in forming the 

The CO2/HCO3 system involved in calcification can be considered in a somewhat similar manner. The question as to whether the carbonate ion of the mineral is derived from extracellular fluids originating in the external medium or whether it arises from intracellular metabolic CO2 has been examined in corals (Pearse, 1970), molluscs (Campbell and Speeg, 1969 Wheeler et ah, 1975), and arthropods (Greenaway, 1974c) (see pp. 74, 80, 87). It may be that both sources contribute to the skeletal carbonate. The significance of determining the intracellular and extracellular sources of carbonate is not simply that it enables one to draw up a balance sheet of input or output but that it serves as an indication of the participation of cellular function in mineralization processes. 

The morphological classification adopted in an earlier section (see pp. 89— 91) emphasizes the important role of membrane systems in calcification. Membranes, both in intracellular and extracellular calcification, are thought to be involved in an active transport of calcium to the site of calcification. They may also be involved in facilitating the availability of bicarbonate ions and in removing protons released during calcification. Thus, all the main ion species involved in biological calcification may be controlled by membrane processes. The ions are related according to the empirical equation 

The deposition of mineral on one side of a cell membrane is affected by both the availability of calcium and carbonate ions and also by their interaction with other ions. Interfering ions not only impede the effective collisions of calcium and carbonate but they may also form ion pairs with the calcium and carbonate ions (Skirrow, 1975). Mg and PO may also interfere with the growth of the crystJil lattice so that, in their presence, the rate of calcification may be reduced and a particular crystal type may be favoured (Kitano et al., 1976). One of the functions of cellular membranes is probably to control the ionic composition of invertebrate skeletons and that of some intracellular mineral deposits. 

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It appears that, in most cases, the deposition of CaCOj by invertebrates is accomplished by one of three morphologically distinct systems which have common properties. The systems are (1) calcification within vesicles or vacuoles (2) extracellular calcification by single cells and (3) extracellular calcification by epithelia. We now briefly consider each of these systems in summarizing the detailed information on various invertebrate groups given on pp. 71-87. 

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