ESPE Yearbook of Paediatric Endocrinology

To read the full abstract: J Bone Miner Res 2019;34(8):1387–1392.

In brief: Cell tracing of the growth plate chondrocytes most adjacent to the perichondrium demonstrate that they transit into the metaphysis and contribute to the pool osteoblasts and reticular stromal cells of the bone marrow during growth in young mice.

Commentary: Borderline chondrocytes are the growth plate chondrocytes located as a ring around the growth plate and closest to the perichondrium. They form part of the ossification groove of Ranvier, which gives rise to the periosteal bone collar surrounding the growth plate. It was first described by Ranvier in 1889 (1) and its role, and the role of the borderline chondrocytes, have been discussed among developmental biologists ever since. Some have argued that they contribute to the appositional expansion of the growth plate, whereas others, including Ranvier argued that they move from the growth plate into the perichondrial groove, contributing to formation of the bone collar.

The current study authors used a pthrp-creER mouse line (allows for conditional labelling of Pthrp-expressing cells) to label and trace borderline chondrocyte. They showed that postnatal borderline chondrocytes transition into the metaphysis where they act as mesenchymal precursor cells, thus giving rise to osteoblasts forming metaphyseal bone and also to Cxcl12 positive reticular stromal “CAR” cells that are important to support haematopoietic stem cells in the bone marrow. Another recent report by Usami and co-authors (2) used an Axin2-creER which labelled postnatal borderline chondrocytes. After an extended chase, the labelled cells had populated the outer portion of the growth plate, including the outer part of the resting zone. Taken together, these studies indicate that the borderline chondrocyte niche is an important cell source, which contributes chondrocytes to the transverse appositional growth of the growth plate as well as new mesenchymal stem cells for the rapidly growing bone.

References:

1. Ranvier L, Traité technique d’histologie. second ed. 1889: Paris, F. Savy.

2. Usami Y, et al., Possible Contribution of Wnt-Responsive Chondroprogenitors to the Postnatal Murine Growth Plate. J Bone Miner Res, 2019;34(5):964–974.