Equine Interzone Cells: The Next Generation of Stem Cells?

Formation of stable articular cartilage is the ultimate goal and main challenge of cartilage tissue engineering. Since permanent articular cartilage is only generated during embryonic development, a better understanding of embryonic joint formation may provide essential clues for cartilage tissue engineering.

Embryonic joint formation first becomes morphologically evident when the tripartite interzone, consisting of 2 outer and an intermediate layer, forms at each prospective joint site. Microsurgical ablation of the interzone has been shown to result in joint ablation, thus proving the pivotal role the interzone plays in joint formation. In addition several lineage tracing experiments have demonstrated that interzone cells form all articular structures such as articular cartilage, ligaments and synovial lining and have a separate embryonic origin from the transient chondrocytes, which prefigure the bony skeleton.

However, to date layer selective analysis of interzone cells has not been reported and therefore significant controversy exists regarding the exact role of the different interzone layers in joint development. In this study, we used laser capture microdissection (LCM) to obtain homogenous samples from the intermediate (figure 1, outlined in red) and the outer (figure 1, outlined in green) femorotibial interzone layers and the transient embryonic cartilage (figure 1, outlined in blue) at gestational ages 13.5 and 15.5, as well as from the undifferentiated limb bud mesenchyme at gestational day 11.5 and from adult articular cartilage. Microarray-based differential gene expression analysis and functional analysis of the differentially regulated genes revealed a high representation of inflammatory pathways and functions in the differential gene expression profile specifically in the intermediate layer. In the outer interzone, genes with high differential up-regulation play roles in cartilage matrix formation, chondrocyte differentiation (hypertrophy) and endochondral ossification. Principal component analysis clustered the intermediate interzone at gestational age 15.5 closest to adult articular cartilage. Further studies are needed to look into the specific role of the inflammatory pathways in the interzone and to examine whether the differentially up-regulated genes relevant to chondrocyte hypertrophy and endochondral ossification in the outer interzone reflect its cell fate.  [...]