Calcification extracellular

Extracellular Sites of Calcification 3.2.1 Calcium and Hydrogencarbonate Pump... 

IRVING s association of lipids with sites of calcification. On that basis, it is interesting to note the confirmation of lipids in mollusc shells629. Another site where extracellular vesicles might exist is in the crustacean cuticle630. The extrusion of granules from the epidermis has been described. 

The final type of calcification mechanism involves some form of extracellular transport system which avoids all the transcellular energy barriers. As a concept, it is extremely valuable but it needs redefining of its relation to the ions under consideration and the motive forces for their movement. 

It appears to be the case that most animals maintain the concentration of mineral ions at constant levels in their extracellular fluids. Perturbations with various forms of acidosis usually result in the animal re-establishing an equilibrium between its body fluids and the apparent solubility product of some mineral. Two important conclusions follow from this. First, it provides a theoretical basis for defining calcification. When there is a change of phase in the total extracellular fluids (i. e., mineralization occurs) then the fluids re-equilibrate to make good the ions which have been lost as minerals. 

Pita, J. C., Howell, D. S., Kuettner, K. Evidence for a role of lysozyme in endochondral calcification during healing of ricketts. In Extracellular matrix influence on gene expression. Slavkin, H. C., Greulich, R. E. (eds.). New York Academic Press 1975... 

Salomon, C. D. A fine structural study on the extracellular activity of alkaline phosphatase and its role in calcification. Calc. Tiss. Res. 15, 201—212 (1974). 

The deposition of limited quantities of hydroxyapatite in extracellular matrix has been observed without bounding cells. Cartilage calcification is such a case where local pH control and Ca2+ are dependent upon diffusion and the rate of mineral deposition is driven by phosphate presentation. Chondrocytes produce alkaline phosphatase that generates the required phosphate, but cartilage is not delimited by any cellular structures and transfer of Ca2+ and H+ is by diffusion from extracellular fluid. 

Once inside the cell, HCO3 is converted to CO2 by the enzyme, carbonic anhydrase. CO2 is then fixed by carboxydismutase and OH is excreted to maintain ionic balance. Carbonic anhydrase is also associated with the extracellular carbonate dissolution by boring organisms (Schneider, 1976) and with the C02-transfer system for intracellular calcification. It represents a key enzyme in the biological cycling of carbonate (Degens, 1976 Raven, 1974). 

Anatomical aspects. The skeletons of corals are formed of crystals of aragonite in an organic matrix. Calcification begins when the free-swimming larva attaches to the substratum. The larva then metamorphoses into a polyp which continues to deposit extracellularly CaCOs and organic matrix through the secretory activity of the calicoblastic epithelium. The epithelium is a layer of cells at the base of the polyp adjoining the skeletal area at which deposition takes place. 

Calcification mechanisms. The mechanisms of CaCOs deposition have not been clearly defined. Two major questions are involved first, the relation of the organic portion of the skeleton to the initiation and control of crystal growth and, second, the role of the symbiotic photosynthetic zooxanthellae, the algae which grow within the tissue of hermatypic corals (Chapman, 1974). Another persistent question has been the site of crystal initiation, whether it is entirely extracellular or whether it is partly intracellulfir as well (Muscatine, 1971). 

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. 

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. 

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 microenvironment of calcification. In extracellular calcification, crystallization occurs within a volume of fluid which is isolated from the externEil medium. The fluid in most cases is a thin layer between the cells responsible for calcification and the skeletal surface on which CaCOs is deposited. Cells can alter the composition and concentration of the ions in the fluid by creat-... 

Van Rooijen N (1993) Extracellular and intracellular action of clodronate in osteolytic bone diseases a hypothesis. Calcif Tissue Int 52 407... 

Extracellular calcification is also commonly found in the separated into markers of bone resorption (Table 49-5),... 

Nucleation of calcium phosphate precipitation within the matrix vesicles is mediated by phosphatidylserine, which comprises about 8% of the phospholipids of the inner cytosolic membrane surface (Fig. 9.5a). Calbindin in the vesicle (Fig. 9.5b) may also contribute. Rapid mineral growth within the vesicle keeps the concentration of dissolved calcium and inorganic phosphate ions so low that additional Ca2+ and Pi ions spontaneously enter from the extracellular fluid via their respective transporters. Attached type II and type X collagens from cartilage in the growth plate enhance calcium ion transport and calcification during endochondral ossification (Fig. 9.5b). 

The free calcium ions in blood and extracellular fluid are critical for building and maintaining an adequate bone mass, and also for preventing excessive calcification. The sensor that regulates the free calcium ion concentration of plasma is within the parathyroid glands, where it controls the secretion of parathormone (PTH). This 84 amino acid peptide is split from a large, precursor protein and retained in secretory vesicles. If the concentration of free calcium ions drops below a critical level in blood plasma, the gland is activated to secrete PTH into the bloodstream. 

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