Final answer:
Non-skull bones generally fuse through a process called endochondral ossification, which continues into early adulthood. The clavicle, on the other hand, fuses through intramembranous ossification, completing around age 25. Cranial sutures fuse in a process known as synostosis, occurring at various stages throughout life.
Step-by-step explanation:
Non-skull bones, besides the clavicle, typically fuse during a process called endochondral ossification. This process begins with the formation of a hyaline cartilage model of the bone during fetal development. The model is eventually replaced by bone tissue as the child grows. Most of the bones in the human body are formed this way, and this process of ossification and subsequent fusion continues into early adulthood.
The clavicle, however, is unique as it is the only appendicular skeleton bone that develops through intramembranous ossification. This process involves the direct conversion of mesenchymal tissue into bone, bypassing the cartilage stage. While ossification of the clavicle begins as early as the fifth week of fetal development, it is not fully completed until around the age of 25.
Additionally, the cranial sutures in the skull undergo a process called synostosis, where the fibrous connective tissue ossifies and adjacent bones fuse after birth. This allows the skull to expand for brain growth during infancy and childhood. Early in life, certain sutures in the skull may ossify and fuse, such as the frontal and maxillary bone sutures, which fuse into a single bone by around eight years old. Later in life, fusions such as those of the sagittal, coronal, and lambdoid sutures may occur, leaving no trace of the original suture lines.