|LESSON 4: Bone Tissue|
Ever noticed how a baby's bones are very small and fragile compared to adult bones? The fingers and toes on a baby are so very much smaller than ours yet they become quite huge over time. How do bones get so much bigger over time?
Bones grow longer over time but they also get thicker. There are mechanism that allow for bones to grow in both length and width.
To make a bone longer just add bone tissue to the ends. The problem is, we can't add directly to the end of a bone, it has to move at the joints. If tissue is added at the end of the bone the skeleton could not move properly. It's like closing both lanes on an interstate until the bridge is repaired.
For this to work, bone tissue must be added below the joint somewhere along the length of the bone. This occurs at the epiphyseal plate, or growth plate. Here chondrocytes first produce hyaline cartilage. The cartilage then becomes calcified or ossified to form hard bone tissue (involves addition of Ca+ and Phosphorous ions). The condrocytes produce cartilage on one side of the plate and push the end of the bone up. The other side of the epiphyseal plate gradually becomes calcified.
Once a person reached adulthood and the bones have reached maximum length, and the whole plate gets calcified. It forms a visible line called the epiphyseal line.
Making a bone grow in diameter is a more straightforward process. To make a bone thicker, just add new bone tissue to the outside. It's like taking a pipe and adding layer after layer of duct tape to it. The pipe would get thicker and heavier as you go. The problem is the bone gets thicker and heavier as you go. In fact if you started with a bone the size of a baby's and make it as large as an adult's, you would have a bone that's essentially solid. Baby bone have a very small marrow cavity.
Since we don't want to weigh an extra couple hundred of pounds more than we do now, it's important to take excess bone tissue away from the inside. That way the bone can grow in diameter, get thicker and not completely fill up with bone tissue.
Using proper terms, we then add tissue to the periosteal side of bone and remove it from the endosteal side. In growing bone we find Osteoblasts on the periosteal side adding bone, and Osteoclasts on the endosteal side removing some bone tissue.