Re: Human Skeletal Growth / Human Skeletal ReGrowth

1. Do bones in the human body stop growing at different times (ages) ? If so which bones at which ages ? (More importantly/in detail, which in the later years age 15/ 16+)

Bones do stop growing at different ages. Forensic anthropologists use this information to determine the age of archaeological finds, and this technique is also used in forensic crime labs. According to this page on forensic anthropology, the elbow is the first to stop growing at age 14, followed by the hands and feet around age 15, the ankles around age 16, the femurs around age 17, followed by the knees and wrists around 18-19, the shoulders and hips around age 20-21, and the clavicle around age 28.

There are a couple different ways to answer this question, so I'll give a couple different perspectives on the issue, but you should understand that there's no clear answer yet regarding exactly which theory is correct.

It has been proposed by Baron et al. that growth of the skeleton and fusion of the epiphyseal plates occurs when the proliferating chondrocytes of the growth plate undergo a process known as programmed senescence. The basic idea is that there are cells in the embryo which are able to divide a certain fixed number of times. The cells initially divide rapidly as the young child grows, but eventually slow down and lose their replicative ability around adulthood. The signal to stop dividing may be encoded in the cells themselves through epigenetic mechanisms such as DNA methylation, but is affected by steroid hormones, which is why exposure of young children to anabolic hormones can result in stunting of their growth. In that respect, the reference point would be the structure of portions of the nucleus of the cell - once demethylation proceeds to a certain point, cell division stops and growth plate fusion occurs. It's kinda like there's a timer within the cells that is set to a certain time when the baby is conceived, and runs down until it hits zero and bone growth stops.

If you are more interested in why some bones are bigger than others, instead of why some people or animals are bigger than others, the same explanation can be used, but taking into account the influence of factors that determine how fast the clock ticks away time. These factors may be produced by other cells within the developing embryo, so that once the chondrocytes(the bone-forming cells) get a certain distance away from the growth factor-producing cells, the growth-inducing factor no longer reaches them, and thus are provided clues on where and how to grow. The major proteins involved in this process are Bone Morphogenetic Proteins 2, 4 and 6, Fibroblast Growth Factors 4, 8, and 10, and related signaling molecules named sonic hedgehog and gremlin. You can read much more about this in this short review of developmental morphogenesis.

The current technique practiced to add length to bone is called distraction osteogenesis. To do this, the bone is cut through the middle and held apart. The bone begins to heal the gap, and is continually stretched to keep the gap about 1 mm wide. The bone isn't being stretched, but rather is maintained in a broken state which the body continually attempts to heal. Approximately 7 cm in height can be gained in this way, but the patient is unable to walk and is in considerable pain throughout the procedure, which takes about a year. There's a fairly lengthy rundown of the process on answers.com.

With regards to my qualifications to answer the questions, I have no more qualifications than a stranger you may meet on the street. I have been engaged in academic research on bone regeneration for some years, but please note that I am not a doctor, I do not give medical advice, and therefore if you make any decision based on the information I have provided, you are making a potentially very serious mistake and should consult a medical professional.