The effect of beta‐aminoproprionitrile on the periodontal ligament: II. Radioautographic study of collagen secretion from fibroblasts

Fibroblasts are distributed evenly throughout the periodontal ligament (PDL) of normal mice. In mice fed beta‐aminoproprionitrile (β‐APN) the fibroblasts undergo aggregation to form palisades of closely juxtaposed cells abutting pools of acellular collagenous matrix. Individual fibroblasts within these aggregates retain their polarized cytoplasmic organization and continue to synthesize and secrete collagen. However, unlike normal PDL fibroblasts, the β‐APN‐treated cells appear immobilized by well‐developed cell‐to‐cell adherens‐type junctions along their lateral surfaces. We studied collagen secretion from β‐APN‐treated fibroblasts by light and electron microscopic radioautography after injection of ³H‐proline. Newly synthesized collagen was secreted from the distal ends of the β‐APN‐aggregated fibroblasts as a distinct band of labeled material, resembling the pattern of matrix deposition seen in osteogenesis and dentinogenesis. The radioactive band of collagenous matrix was displaced further away from the fibroblasts at 2 and 4 days after ³H‐proline injection as more collagen was secreted. This pattern of radiolabeled collagen secretion confirmed previous observations that PDL fibroblasts are highly polarized and that collagen secretory granules are extruded from the distal or secretory pole of the cell. In normal PDL the even distribution of fibroblasts and the complex interrelationship of their distal cell processes leads to a diffuse pattern of silver grain deposition, masking the oriented flow of new collagen from the distal ends of individual fibroblasts. Analysis of electron microscopic radioautographs revealed that newly synthesized collagen was packaged and secreted from β‐APN‐treated fibroblasts via the normal cytoplasmic pathways but at a slower rate.