The interface between the two is called the calcification front. It is thought that matrix calcification is initiated by cell-derived structures called matrix vesicles, which are found lying free in the matrix. They are thought to bud directly from the cell membranes of osteoblasts (www.iupui.edu). Osteoblasts are responsible for both building and remodelling bone, they invade the cartilage and secrete the hard material that ossifies into bone. Mackie (2003) describes osteoblasts as mononucleated, and their shape varies from flat to plump, reflecting their level of cellular activity, and in later stages of maturity lines up along bone forming surfaces. He goes on to state that osteoblasts are responsible for regulation of osteoclasts and deposition of bone matrix.
The space occupied by a chondrocyte is known as a lacuna. When the chondrocytes die they leave spaces behind, the osteoblasts move into these spaces (lacunae) and start to put bone on what is left of the calcified cartilage. Capilliaries also get through the cavities and bring food and O2 to the osteoblasts. The osteoblasts eventually become stuck in the matrix that they have released and become an osteocyte. These occupy lacunae and are linked to each other through long cytoplasmic extensions that live in minute passageways called canaliculi. The canaliculi is utilised for the exchange of nutrients and waste through gap junctions. New bone matrix is formed utilising calcium and phosphate ions from the blood (workbook). Both ends of the bone shaft are called epiphyses, ossification starts in the middle of the bone shaft and then the two ends. Ossification continues between the three points until there is only a thin band of cartilage left below the epiphysis called the epiphyseal plate. The bones are different shapes and can be thicker in places than other parts, some have to be continually added to while other areas are reshaped.
