Bone or cartilage?

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The Birds and Bees of Baby Bones

While adults normally have 206 bones, babies start out with a skeletal mix of about 300 different bones and cartilage elements. Various adult bones, such as the cranium, start off in life as several different fragments. At birth, the cranium has three unfused plates, allowing for passage of the baby's head through the birth canal. Over time, these plates fuse into one piece.

All bones start off as cartilage, but many are still cartilage at the time of birth. Cartilage turns into bone over time through a process called ossification. As cartilage develops, a nutrient artery grows into it. This in turn prompts cells called osteoblasts to develop along the lining of the cartilage. These osteoblasts in the cartilage begin producing compact bone, which covers the cartilage. Next, blood vessels begin to spread throughout the cartilage, branching off the nutrient artery and enabling marrow and other nutrients to be dispersed throughout the developing bone. When this occurs, it prompts the development of a primary ossification center, which will continue producing cells that dissolve the cartilage and replace it with new bone.

The patella is a sesamoid bone. That means that it's a bone that exists in the middle of a ­tendon, as we discussed earlier. The patella is the largest sesamoid bone in your body.

As such, it takes a little longer than some bones to, well, become bone.

Although it doesn't show up on X-rays, your baby does in fact have kneecaps. They're just not bony kneecaps. At birth, these kneecaps are still cartilage, and remain so for a few years. So all those spills and falls your toddler is taking aren't going to be knee-breakers, just sponge-compressors. By the time your child is anywhere from 3 to 5 years old, those cartilage plates will have fully ossified into big-kid kneecaps, made of real bone. And every year after that, as that bone loses its bounce, those thrills and spills will get progressively less fun until those grown-up babies are creaking and groaning with the rest of us.

For lots more information on bones, cartilage and babies, see the next page.