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ENDOCHONDRAL BONE, ENDOCHONDRAL OSSIFICATION---EARLY BONE DEVELOPMENT in the EPIPHYSEAL DISK region, 100X. The reddish color in the lower part of the image is NEWLY-FORMED BONE, and the deep purple color is CALCIFIED CARTILAGE. The upper part of the image is hyaline cartilage with numerous cartilage cells and a pink matrix. Originally the bones are made of Hyaline cartilage models of the long bone--this is gradually destroyed and replaced with bone in a complex process named Endochondral Ossification. In the upper part of the image you can see the changes in the cartilage cells including their degeneration and erosion of the cartilage. This image also shows many OSTEOBLASTS (BONE-FORMING CELLS), many of which line up on the surface of calcified cartilage spicules and lay down osteoid and bone--the reddish material covering the surface of the calcified cartilage (purple). Osteoblasts entrapped in the osteoid or bone become Osteocytes. BONE CELLS (OSTEOCYTES) are evident inside the newly-formed bone (reddish matrix). The EPIPHYSEAL DISK is also called the GROWTH PLATE and is in this region and is a very active area related to production of cartilage, destruction and erosion of cartilage, and replacement of the cartilage with bone as bones grow in length until the process stops and bones cease growing in length.