Fig. (4) The image shows two clusters, a small one with extensive BMP 2/4 expression at the right side and a larger one with poor BMP 2/4 expression on the left side. Because of high BMP expression, vital cells and a small rim of neo-matrix the right cluster was considered to be a young cluster. The left one with the large amount of non-vital cells, poor BMP expression and the large rim of neo-matrix was considered to be a mature cluster (BMP 2/4, 400x).