What Bones Fuse From Infancy To Adulthood?

The decrease from ~300 bones to 206 is a result of bone fusion in several key areas of the body. This process continues from infancy through adolescence and into the early twenties.

• The Skull: The cranial bones of an infant are separate, with gaps called fontanelles (soft spots). These bones fuse along joints called sutures, which become rigid in adulthood.
• The Spine (Vertebral Column): This is a major site of fusion.
  • Sacrum: An infant is born with five separate sacral vertebrae (S1-S5). In the late teens, these fuse to become the sacrum, a single, solid, triangular bone at the base of the spine.
  • Coccyx (Tailbone): A child starts with three to five small coccygeal vertebrae. These fuse to form the coccyx, a single tailbone.
• The Pelvic Girdle: The hip bone (innominate bone) in a child consists of three separate bones: the ilium, ischium, and pubis. These fuse together in the mid-teen years to form a single, solid pelvic bone on each side.
• Long Bones: While they don't change the count, the epiphyseal plates (growth plates) at the ends of long bones (like the femur and humerus) are initially cartilage and later ossify, fusing the ends of the bone (epiphysis) to the main shaft (diaphysis) once growth is complete.