TEM and SEM observations on the extracellular matrix of the developing murine centra.

TEM and SEM application demonstrated that the shift from chondrification to ossification in the developing murine centra from day 15 to day 18 of gestational age is marked by typical structural variations of the extracellular matrix (ECM). During day 15 GA, typical matrix vesicles with crystalline contents appeared, as followed by single and fusing pleomorphic aggregates of a more regular crystalline structure. During days 17/18 GA, these structures disappeared, and the ECM now exhibited a network of collagen fibrils that had been less conspicuous before. During the time period studied, the ECM switched from a more acid (proteoglycans) to a rather neutral (glycoproteins) milieu.