Light and electron microscopic abnormalities in diastrophic dysplasia growth cartilage

Summary Light and electron microscopic studies of diastrophic dysplasia iliac crest growth cartilage performed on five occasions in two patients from 1 to 10 years of age reveal extensive cell and matrix abnormalities at each time period. Light microscopy shows atypical chondrocytes with extreme variation in size and shape, and premature cytoplasmic degeneration, and formation of target ghost cells. Promment, densely staining fibrotic foci are present throughout the cartilage. Ultrastructure reveals some structurally intact chondrocytes with a single large fat inclusion, slightly dilated rough endoplasmic reticulum, and abundant glycogen. As early as 1 year of age cystic degeneration of chondrocyte cytoplasm is evident with indistinct organelles seen. The cartilage matrix demonstrates a general increase in fibrous tissue as well as the fibrotic foci. The collagen in these foci is remarkably abnormal. It is composed of short, extremely broad fibrils ranging from 150 to 950 nm in width which are separated at their terminal ends but fused to each other centrally in random fashion. On cross-section there are very few round fibrils but rather a marked irregularity in shape giving the appearance of having fibrils randomly added to others to form enlarged nonuniform fibril aggregates. On longitudinal sectioning, regular cross-banding across the entire fibril width is seen but fibril splitting and aggregation are highly irregular. Though no specific molecular abnormalities of collagen have been identified, the disordered self-assembly process points to either a modification on one of the collagen molecules favoring the abnormal fibril aggregation or a defective noncollagenous matrix molecule which secondarily interferes with normal cartilage synthesis and allows for deposition of a broad, cross-banded collagen in what should be a strictly cartilage domain.