Microscopic and X-Ray Investigations on Calcification of Tissue

Abstract

An attempt has been made to determine exactly the nature of the solid inorganic phase in several types of calcified tissues, including normal bone, dental enamel, dentine, and in pathological conditions such as rachitis, salivary calculus and calcified tubercular lung tissue. Measurements were made of the mean index of refraction, n, by the oil immersion method of Becke as used by mineralogists in identifying rock constituents. The crystals present are too small to show characteristic cleavages or angles but are seen to be present in small particles apparently encased in a thin organic sheath of low refractive index. The smaller the relative amount of organic matter, the higher is the refractive index: dental enamel, which is the maximum of this group (n = 1.62), has only 1 or 2% of organic material. Thus the index of refraction may be taken as a measure of the degree of calcification. The following are typical values of refractive indices: human cheek-bone 1.561, dentine 1.577, enamel 1.625, rachitic rat bone 1.560, salivary calculus 1.563, and calcified lung 1.585, ±0.003. The upper limit, 1.625, which comes the nearest to representing pure inorganic crystals, is practically the index of refraction of the minerals of the apatite series 3Ca₃(PO₄)₂. CaX₂, where X₂ may be CO₃, O, (OH)₂, F₂ or SO₄. Typical minerals of this formula are fluorapatite, dahllite and podolite. We are, therefore, of the opinion that apart from the variability of X₂ the crystal phase is essentially the same in all the cases of calcification examined. No evidence was found for either CaHPO₄.2H₂O or the so-called tri-calcium phosphate, Ca₃(PO₄)2. The latter is unknown as a mineral and has never been prepared in a pure state in the laboratory.