Ultrastructural features of chondrogenesis are correlated with the appearance of collagen and proteoglycan components of the cartilage matrix in the developing human hand (stages 16-18).

Prechondral mesenchymal cells comprising the digital plate (stage 16) exhibit euchromatic nuclei, sparse granular endoplasmic reticulum, few polysomes, broad cell to cell associations, little enhancement of electron density after postfixation with osmium tetroxide/potassium ferrocyanide and scanty incorporation of 3H-proline. Extracellular matrix is electron-lucent.

During stage 17, early chrondroblasts acquire affinity for OsO4/K4Fe(CN)6, develop filopodia, lose broad intercellular attachments, assemble cytoplasmic polysomes, incorporate 3H-proline and the extracellular matrix acquires electron-dense particles (10-20 nm in diameter).

By stage 18, chondrogenic cells of digital blastemata exhibit affinity for osmium-ferrocyanide, and particles (10-20 nm in diameter) and filaments (5 nm in diameter) are present in the extracellular matrix. Mature chondrocytes are present.

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