Development and regeneration of the elbow joint in the chicken embryo.

摘要

Synovial joints are among the most important structures that give us complex motor abilities as humans. Degenerative joint diseases, such as arthritis, cause loss of normal joint functioning and affect over 40 million people in the USA and approximately 350 million people worldwide. Therapies based on regenerative medicine hold the promise of effectively repairing or replacing damaged joints permanently. Here, we introduce a model for synovial joint regeneration utilizing the chick embryo. In this model, a block of tissue that contains the prospective elbow is excised, leaving a window with strips of anterior and posterior tissue intact (window excision, WE). In addition, we also slice out the same area containing the elbow and the distal piece of the limb is pinned back onto the stump (Slice Excision, SE). For making excisions precisely we first carried out a cell fate analysis for elbow forming tissue in the developing limb and carefully determined the tissue to be removed. Interestingly, when the elbow is removed via WE, regeneration of the joint takes place, whereas the elbow joint does not regenerate following SE. In order to investigate whether the regeneration response recapitulates the developmental program of forming joints, first we made a gene expression analysis for the elbow joint because a specific gene expression analysis for the elbow joint was not available in the literature. Among the genes analyzed, we used GDF-5 and Autotaxin (Atx) as joint tissue specific markers and Sox-9 and Col-9 as cartilage markers for in situ hybridization on sections at different time points after WE and SE surgeries. Re-expression of GDF-5 and Atx is observed in the WE samples by 60 hours after the surgery. In contrast, the majority of the samples that underwent SE surgery did not express GDF-5 and Atx. Also, in SE fusion of cartilage elements takes place and the joint interzone does not form. This is indicated by continuous Col-9 expression in SE, whereas Col-9 is down-regulated at the joint interzone in the regenerating WE samples. This order and pattern of gene expression observed in regenerates is similar to the development of a joint suggesting that regeneration recapitulates development at the molecular level. Various growth factors have been shown to trigger or enhance the regenerative response in different models and organs. The regeneration response we observe in WE is present in 50% of the embryos. In order to test the effect of growth factors on this response, we implanted window excised limbs with BMP-2, Noggin, or BSA beads as control. BMP-2 inhibits the joint regeneration, while Noggin does not improve regeneration of the joint tissue. On the other hand, Noggin treatment resulted in elongation of the cartilage elements from the amputated surface.;This model defines some of the conditions required for inducing joint regeneration in an otherwise nonregenerating environment. This knowledge can be useful for designing new therapeutic approaches for joint loss or for conditions affecting joint integrity in humans.