Calcification membranes

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 avian egg shell membrane seems to be necessary for the formation of the biocrystalline layer of the egg shell. Under experimental conditions no egg shell is formed in the absence of the membrana testacea (MT)360). It is generally accepted, however, that the initial processes of calcification are located in the mammilary layer of the shell28). 

Fig. 9. A generalized form of Anderson s membrane vesicle theory of biomineralization. The vesicles break away from the cells adjacent to the site of calcification.

The Golgi membraneous system also occupies a central position in mollusc calcification297. However, different from the coccolithophoridae, the actual CaC03 deposition does not take place inside the Golgi vesicles which are abundant in epithelial cells lining the surface of the outer mantle fold. Instead, the calcification site is an organic matrix released from the extrapallial fluid separating the mantle from the shell. 

Vardaxis, N. J., Boon, M. E., and Ruijgrok, J. M. (1996). Calcification of cross-linked collagen-elastin membrane implants in vivo and their proposed use in bone regeneration. Biomaterials 17, 1489-1497. 

Vitamin D may regulate calcification of cartilage by regulating chondrocyte synthesis of proteoglycans. l,25(OH)2D3, an active metabolite of vitamin D has been shown to produce a concentration-dependent reduction in an immortalized rat chondrocyte cell line. Both vitamin D and l,25(OH)2D3 are considered secosteroids with vitamin D receptors, which includes cytosolic/nuclear proteins that bind specifically to the secosteroid as it transits the plasma membrane. 

OI type V is now widely recognized as a distinct OI phenotype with characteristic clinical and radiological features, such as predisposition to formation of hypertrophic callus at sites of fractures or surgical interventions, early calcification of the interosseous membrane of the forearm, and appearance of dense metaphyseal bands in radiographs. Patients have moderate... 

When the calcium plaque is thick, it can be removed by scraping with a scalpel or by performing a superficial keratectomy. Other reported methods include the use of a diamond burr, neodymium-yttrium aluminum garnet (Nd YAG) laser, lamellar keratoplasty, and PTK. A recent treatment option described the combined use of superficial lamellar keratectomy, NaEDTA chelation, and amni-otic membrane transplantation. In this procedure the calcific lesions were treated with NaEDTA and a blunt superficial lamellar keratectomy was performed. Once a smooth ocular surface was achieved, an amniotic membrane was transplanted to replace the excised epithelium and stroma. The procedure resulted in the... 

Tympanic membrane calcification can lead to severe, irreversible conductive deafness (25). 

In general, calcification is governed by photosynthetic and non-photosynthetic CO2 fixation and, one may recognize an evolutionary sequence from simple chemical precipitation caused by the environmental effects of procaryotes to the highly organized membranous calcification systems found in eucaryotic algae and animals. 

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. 

ALP activity is present in most organs of the body and is especially associated with membranes and cell surfaces located in the mucosa of the small intestine and proximal convoluted tubules of the kidney, in bone (osteoblasts), liver, and placenta. Although the precise metabolic function of the enzyme is not yet understood, it appears that ALP is associated with lipid transport in the intestine and with the calcification process in bone. 

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). 

Other reports (27-29) have focused on the role of citric acid, as a source of carboxylate anions, during precipitation of calcium phosphates from electrolyte solutions. It has been found that citrate anions inhibit the ciystal growth of calcium phosphates and hinder their transformation into hydroxyapatites. This was attributed to the adsorption of citrate anions into the crystals and the displacement of an equivalent amount of phosphate anions. Interestingly, Rhee and Tanaka (30) found that the presence of a collagen membrane in the medium changed the behavior of citrate anions from being an inhibitor to becoming a promoter of calcification, provided that the molar ratios of calcium to citric acid were between 2 and 12. 

Calcification by bacteria. Lime-water application seems not to guarantee sufficient consolidation. Precipitation of calcite by microorganisms is known to occur in freshwater and marine environments, as well as in soil, and this led to trials of bioremediation on heritage surfaces. Very recently, approaches using calcification due to bacterial activity have even been carried out particularly on stone and architectural surfaces (see page 236). Remediation by several bacteria was shown to result in the formation of newly developed calcite precipitations. For this, chemoorganotrophic bacteria, as well as nutrients, were supplied to the surfaces. Up to now the role of bacteria is not completely understood, however, studies on marine bacteria revealed three main mechanisms (i) calcium-binding membrane proteins, (ii) ionophores, and (iii) extracellular materials on cell surface. 

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