Chondroblast cells contain highly organized intercellular F-actin fiber networks

At day 7, we observe a highly confluent population of chondroblast cells. One of the most striking features of these cells are the numerous intercellular F-actin fibers they contain in their cytoplasm. Fibers are distributed throughout the cytoplasm in a highly organized structural network that extends from the nuclear membrane to the periphery of the cell. The white globular structures in the cytoplasm are very likely collagen type II granules (3). This collagen will eventually be secreted to participate to the formation of the extracellular matrix of the cartilage. Black vacuoles are also present in the cytoplasm. These vacuoles are part of the endocytic pathway that the cell uses to degrade and recycle old cellular components.

Collagen accumulates during differentiation while vacuoles appear

As differentiation progresses, the amount of collagen and the number of vacuoles increases. The distribution of collagen granules reflects the position of the endoplasmic reticulum (ER), where collagen is massively produced.

The first step towards chondrogenic tissue structure?

By day 10, cells start to separate leaving small gaps between them. When fully differentiated, chondrocytes form chondrogenic centers surrounded by cavities in the matrix called lacunae. It is possible that the gaps we observe between cells is the first step towards cells assuming the structure of chondrogenic tissue, although this would need to be confirmed by longer experiments.

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