Extracellular calcification by single cells

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. 

Intracellular calcification within vesicles, or in vacuoles within syncytia, is present in a wide variety of tissues in every phylum examined thus far. Commonly, the calcium deposited takes the form of amorphous calcium spherules formed in vesicles associated with the endoplasmic reticulum or Golgi sys- 

The formation of the calcified structures involves movement of organic material from its presumed site of synthesis in the endoplasmic reticulum and Golgi regions into the vesicle or vacuole where it is joined by inorganic ions which may be pumped into this site. Some of these ions are presumably bound to the organic material. The organic material normally becomes incorporated into the calcified structures, although this is uncertain in skeletal structures of echinoderms (Wilbur, 1976). 

Single cells may deposit CaCOs extracellularly. This occurs in the formation of the test in protozoa (Pautard, 1970) and the spicules of calcareous sponges (Jones, 1970). Single cells also have the capacity to form portions or entire skeletons of echinoderm larvae in vitro (Okazaki, 1975) and cells which do not form distinct epithelia regenerate spines of echinoderms extracellularly (Heatfield and Travis, 1972). These results suggest the possibility that cells which are not in an epithelial layer may also deposit the meshwork of the echinoderm test and spines extracellularly. 

A mechanism which would accomplish extracellular mineralization by single cells may involve the entrance of Ca and HCOj from the medium and their movement through the cell and onto the site of deposition of CaCOs on the crystal surface. The details and metabolic costs of these movements remain obscure. 

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