Anatomy and Physiology- It's Going Tibia Okay: Bone Growth, Development, and Remodeling

Bones are very interesting in both their growth and function, as well as their recovery after damage has occurred. We are going to explore this further, beginning with bone growth and development.

Bone Growth and Development Overview
When babies are born, their skeletal system contains 300 components. These components are actually pieces of cartilage, which undergo a process called ossification, that allows them to become bone. The bony skeletal structure begins to develop at 8 weeks, endochondral ossification begins at the second month, and bone growth continues even after birth.

Bony skeleton development:

http://images.agoramedia.com/wte3.0/gcms/fetal-development-skeletal-system-article.jpg

Cartilage contains no blood vessels or nerves, and are surrounded by perichondrium, which prevents outward growth. Some of the cartilage remains, however, such as that found in the nose and ear. Cartilage can be classified as hyaline, elastic, or fibrocartilage. Matrix is secreted both inside and against the outside of the bone, through appositional and interstitial processes. This calcifies, or hardens the bone. Cartilage calcifies during normal bone growth, as well as during old age, which causes arthritis.

The pieces that do ossify often fuse together, creating a skeletal system of 206 bones by the end of development. Bones can be classified into two categories: axial skeleton and appendicular skeleton. The axial skeleton contains the bones of the skull, vertebral column, and rib cage. The appendicular skeleton contains the bones of the upper and lower limbs, as well as the shoulders and hips. Bones can also be classified as long bones, short bones, flat bones, and irregular bones. Long bone growth generally stops around the end of puberty, which prevents you from getting taller.

Fetal skull before fusion:

http://bio.sunyorange.edu/updated2/embry.html/2.The%20Skeletal%20System_files/45%20FRONTAL.jpg

Skull after fusion:

https://shortnotesinplasticsurgery.files.wordpress.com/2012/04/1289-and-1290.jpg

The Ossification Process

There are two types of ossification: intramembranous ossification, which replaces the thin connective tissue membrane with bone, and endochondral ossification, which replaces fetal cartilage with bone. Both types, however, rely on the thyroid hormone calcitonin for the regulation of calcium metabolism.

In bone production, there are three really important cells to consider: osteoblasts, osteoclasts, and osteocytes. Since your bones are always either growing or adapting to support your body, there are cells that will actually destroy bone, and others that will repair the bone to support its new requirements. Osteoclasts originate in bone marrow, and move to the bone surface where they will eat away old bone or protuberances that are out of place. Osteoblasts also come from the marrow and make their way to the surface to fill in cavities and restore bone. After their job is done, they flatten and line the surface of the bone. They then work to regulate calcium, as well as activate osteoclasts through the production of special proteins. Osteocytes come from osteoblasts and can be found inside the bone. As the osteoblasts build the bone, some surround themselves with new material while extending "arms", called dendrites, to other osteocytes. Osteocytes sense pressures and cracks, and help direct osteoclasts to where the bone needs to be dissolved.

The process of ossification: