Fine structure of the osteocyte capsule and of the wall of the lacunae in bone

Summary

Bone tissue from femura of adult and old rats and from young mice, as well as from dogs, were fixed in osmium tetroxide, potassium permanganate, in an osmium tetroxide — potassium permanganate combination, and in glutaraldehyde followed by osmium tetroxyde — potassium permanganate. The results of the different fixatives were found to complement one another in such a way that existing controversies and uncertainties concerning the fine structure could be settled. This was especially true of the question whether or not the so-called capsule of the osteocyte contains collagen fibrils. Notwithstanding considerable variations in the structure of the capsule it was definitely shown that cross-banded fibrils are present in a mucopolysaccharide-containing ground substance. The material of the capsule corresponds, therefore, to the matrix of connective tissue in general, and its ground substance is, as in any connective tissue, the medium of transport between the blood and the tissue. In respect to the organic structures the “intralacunar” matrix is similar to the “interlacunar” mineralized matrix. In sections of demineralized bone, especially after osmium tetroxide fixation, the wall of the lacuna and canaliculi is marked by a dark line which is described as a special osmiophilic lamina. Since the same line, although thinner and less distinct, was found also in tissue fixed with agents other than osmic acid it was concluded that the osmiophilic lamina is a true structure which must be permeated by substances passing to and from the interlacunar matrix. The osmiophilic lamina belongs to a wider border zone which differs from the bulk of the mineralized matrix by its thinner and less tightly packed fibrils. Accordingly, the bone crystals were found to be less orderly arranged than those deeper inside the mineralized matrix. Bordering directly on the intralacunar pathway they were described as the coastal crystals and are believed to represent the labile bone minerals which are metabolically available without any change in the bone structure. The findings about the fine structure of the capsule of the osteocyte and of the wall of the lacunae were discussed in terms of the transport problems in bone. The osteocyte itself, by its fine structure and relationship to the intralacunar matrix seems to be engaged not only in the maintenance of the open pathways in bone but also in the transport mechanism itself.