ESPE Abstracts

Background: During early bone formation, mesenchymal stem-cells condense and differentiate into collagen type 2-expressing chondrocytes which form the cartilaginous bone anlagen. This anlage becomes enclosed by the perichondrium. The perichondrium consists of two layers, the inner cambium layer containing chondrocyte- and osteo-precursor cells and the outer fibrous layer important for mechanical and structural support. While the regulation of the growth plate is increasingly well understood, little is known about the regulative capacity of perichondrium on the growth plate, as well as in bone growth.

Aim: To explore the cellular origins of perichondrium.

Methods: Perichondrium-specific markers were identified by microarray and real-time PCR analysis of microdissected growth plate and perichondrial tissues. High-density chondrocyte pellets were produced by isolating chondrocytes from epiphyseal cartilage. During the dissection, the perichondrium was carefully removed and its absence was confirmed by histological examination and by real-time PCR for perichondrium markers. Differentiation of pellet cultures was studied using real-time PCR and in situ hybridization for chondrocyte and perichondrium markers.

Results: Cultured chondrocyte pellets developed a surrounding perichondrium-like tissue by day 21 of culture. This surrounding tissue did not express chondrocyte markers, collagen 2 or 10, as assessed by in situ hybridization. Instead, perichondrium markers Periostin (Postn), Roundabout 2 (Robo2), Dickkopf 3 (Dkk3), Pleiotrophin (Ptn), Protein Tyrosine Phosphatase Receptor Type Z 1 (Ptprz1), Cadherin 2 (Cdh2), and L-galectin (Lgals) were upregulated during the formation of the perichondrium-like tissue by real-time PCR and in situ hybridization. Interestingly, only molecular markers for the cambium (Dkk3 and Lgals) and not the fibrous layer (Collagen 14) were upregulated in cultured chondrocyte pellets.

Conclusions: These findings support previous cell-lineage tracing studies suggesting that the perichondrium is formed from chondrocytes in the periphery of the bone anlagen. In addition, the results suggest that epiphyseal chondrocytes of postnatal animals are not terminally differentiated, but still retain the potential to transdifferentiate into perichondrial cells. In addition, this study presents evidence that not only epiphyseal cartilage, but also bone tissue and perichondrium are made up from collagen 2 expressing cells.