The backbone vertebrae, skull, and rib cage make up the axial skeleton. Among other types of cells, bone marrow produces red blood cells, which carry oxygen in the blood. Osteoblasts can eventually become osteocytes. If bones rapidly deconstruct faster than new bone tissue grows, this can lead to less dense and more fragile bones. When severe, this condition is called osteoporosis. Ligaments such as the MCL and ACL connect bone to other bone at joints. Osteoblasts, osteoclasts, and osteocytes are three types of bone cells that work together to maintain and remodel bone tissue. Osteoblasts are responsible for bone formation and the secretion of collagen and other proteins that make up the bone matrix. Osteoclasts are responsible for bone resorption, which involves breaking down and removing old bone tissue. Osteocytes are mature bone cells that are derived from osteoblasts and are responsible for maintaining bone tissue by sensing and responding to mechanical stress. The activities of osteoblasts and osteoclasts are tightly regulated to maintain bone tissue homeostasis. Osteoblasts and osteoclasts work together in a process called bone remodeling, which involves the coordinated resorption of old bone tissue by osteoclasts and the subsequent formation of new bone tissue by osteoblasts. This process allows bone tissue to adapt to changes in mechanical stress and repair damage. If the activities of osteoblasts and osteoclasts become imbalanced, bone tissue homeostasis can be disrupted, leading to various bone disorders such as osteoporosis, osteomalacia, and Paget's disease. For example, if osteoclast activity is higher than osteoblast activity, more bone tissue will be resorbed than formed, leading to bone loss and decreased bone density. Conversely, if osteoblast activity is higher than osteoclast activity, more bone tissue will be formed than resorbed, leading to increased bone density.