Puberty Bone Growth

Puberty is one of the most complex processes in human physiology. Specifically, the process is a combination of chemical signals, nutrition, genetics, age, and probability. I will try to briefly explain the general interactions of these elements without sounding too much like a biology textbook.

Bone Structure

There are several different kinds of bones in your body, with each of them performing somewhat different tasks for your body. While most of them are involved in protecting your organs (e.g. your ribs shield your heart, lungs, liver, etc.), some specialize in making new blood cells (e.g. sternum). The 3 smallest bones in your body, the ear ossicles, do neither. They simply transmit sound virations from your outer ear to your inner ear, but I digress. In this section, we will only focus on bones that contribute significantly to your stature.

Bones are primarily made of two substances, collagen and hydroxyapatite (a type of calcium phosphate crystal). Collagen is the framework protein that is laid out during bone synthesis, and hydroxyapatite is the hard mineral deposited on the collagen framework. Using a skyscraper as an analogy, the steel structure is like collagen, which determines the basic shape and size of the building; the bricks are the hydroxyapatite, which make up the bulk of the building, and once in place, they become the permanent structure of the building. This hardening process is called ossification. Once a bone is ossified, its shape is pretty much set. But just because something is set, it does not mean that it is dead. Bones are highly active tissues in your body. Within the hard collagen and hydroxyapatite matrix, small blood vessels and cells are scattered all over. These cells constantly dissolve and rebuild the hydroxyapatite in response to physiological changes. Remodeling is the term used to describe this process. Remodeling generally does not change a bone’s shape or length, but the density of the bone. To get a better picture of this process, imagine that you just bought a beautiful house with a kitchen that is always being remodeled. Now, imagine a contractor who keeps taking apart the sink then making a new one. As long as the sink is being replaced, the kitchen remains functional. But what if one day the contractor forgets to bring the parts to build a new sink? You no longer can wash your dishes in the kitchen. The house is now flawed on the inside, even though it still looks great on the outside. In the case of the bones, the density depends on calcium. If the calcium levels in your body is off-balance, your bone health may be jeopardized, even though everything appears OK on the outside.

Different bones look and grow differently, but let’s simplify things a bit by looking at the structure of a typical long bone (arms & legs). A bone itself can be divided into two distinct parts: diaphysis and epiphysis. Diaphysis is the main portion of the bone that extends from the middle to approximately the two ends of the bone. Epiphysis is the region the end of a bone, and a bone has two epiphyses (two ends). In young individuals, epiphyses are composed of hyaline cartilage, which is a kind of flexible cartilage. Because epiphyses are made out of a different bone tissue, the visible band between the diaphysis and epiphysis is called the epiphyseal plate. The epiphyseal plates can be very visible on X-ray films. Doctors can determine if a young individual is finished growing by taking a wrist X-ray and see if the epiphyseal plates are still there.

 

Bone Structure[1]

Bone Growth

A long bone adds to its length by the process called endochonral ossification. Chondrocytes, or bone cells, multiply by cell division at bone epiphysis, which extends the bone by laying down cartilage. Another kind of bone cells (osteoblasts) laid down hydroxyapatite in place of the old chondrocytes, thereby making the newly synthesized sections permanent. This process is called ossifcation as mentioned before. The rate of ossification is controlled by hormones (more on these later), and because the ossification process is faster than chondral growth, ossification eventually takes over the growing epiphysis. When this happens, we say that the growth (epiphyseal) plate is fused.

Epiphyseal Plate[2]

Epiphyseal fusion usually occurs in early twenties, and after that, the bones are unable to elongate without surgical procedures. Remember that just because the bones lost their ability to lengthen, it does not mean they are dead. The cells within the bone tissue continuously maintain the bones in response to nutrition, stress, and other physiological conditions. Therefore, it is important to maintain a healthy lifestyle both before and after puberty.

[1] Adapted from Wikipedia http://en.wikipedia.org/wiki/Long_bones
[2] Adapted Merck from http://www.merck.com/mmpe/sec21/ch309/ch309b.html

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