ossifikaatioon

Ossifikaatioon, also known as ossification, is the biological process by which bone tissue forms. It occurs during embryonic development, in postnatal growth, and in the replacement of cartilage or other tissues during bone remodeling and fracture repair. There are two main modes of ossification: intramembranous ossification and endochondral ossification.

Intramembranous ossification involves mesenchymal cells in a loose connective tissue differentiating directly into osteoblasts, which secrete osteoid that mineralizes to form bone. This pathway forms most of the flat bones of the skull, parts of the clavicle, and some facial bones. Endochondral ossification builds bone by replacing a hyaline cartilage model with bone tissue. A cartilage template grows, becomes calcified, and is invaded by blood vessels and osteoblasts, leading to the formation of bone. This mode forms most long bones, as well as vertebrae and the ends of certain bones.

Regulation of ossification depends on hormones, signaling pathways, and mechanical factors. Growth hormone, thyroid hormone, and sex steroids influence bone growth, while local factors such as bone morphogenetic protein (BMP), Wnt signaling, PTH-related peptide, and Indian hedgehog coordinate the activities of osteoblasts, osteoclasts, and chondrocytes. Adequate mineral nutrition, including calcium, phosphate, and vitamin D, supports proper mineralization.

Disorders of ossification can affect skeletal development and integrity. Examples include osteogenesis imperfecta, rickets or osteomalacia due to defective mineralization, skeletal dysplasias, heterotopic ossification, and increased risk of osteoporosis with aging.