Radioautographic visualization of collagen metabolism in the periodontal tissues of the mouse

The time sequence of collagen formation in periodontal structures of mice was studied by radioautography after injection of tritium-labelled proline or glycine. For comparison, some animals were injected with leucine-H³ or methionine-H³. The structures studied were periodontal membrane, cementum, and alveolar bone of molar teeth. Grains were counted in each component of the periodontium at the levels of root apex, root sides and alveolar crest.

Results showed that in all structures radioactivity appeared first in cells (fibro-,osteo-, and cementoblasts) and later in intercellular spaces. The fact that glycine-H³ or proline-H³ (the most abundant amino acids in collagen) labelled these collagen secreting cells far more strongly than any other cell type was taken as evidence that the label was mainly in collagen.

In the periodontal membrane, collagen formation proceeds at a rapid rate which is much more pronounced than in dense connective tissue elsewhere (tendons, ligaments, gingiva). Periodontal membrane around the root apex (apical ligament) or at the level of alveolar crest (crest ligament) incorporates more glycine-H³ than at other sites (horizontal and oblique ligaments).

In cementum only small areas were labelled, indicating that not all cementoblasts are actively secreting collagen at a given time. The label migrated from the cells to the pre-cementum and later to calcified cementum, with a small decrease in total radioactivity.

Alveolar bone showed labelled osteoblasts at 30 min after injection. Later the radioactivity was found in pre-bone and in bone proper. The radioactivity stayed a shorter time in pre-bone than in pre-cementum, before appearing in the calcified structures. This means that calcification proceeds faster in alveolar bone than in cementum. In bone the number of radioactive areas decreased with time, but the number of silver grains per unit area in regions of persisting radioactivity decreased only slightly or not at all.

The radioautographic evidence presented in this paper, showing that the periodontium is characterized by a constant renewal of collagen, may explain why this structure is sensitive to metabolic disturbances and food deficiencies.