Just as potholes develop in the street in front of your house, damage occurs in bones that are subjected to frequent use.  Workers are able to patch potholes in pavement by applying new asphalt.  Occasionally, parts of the road become so damaged that a worker breaks up the pavement with a jackhammer before replacing the damaged area.  A very similar process occurs in bones. 

How are bones repaired?

Bones are under constant repair and remodeling in the body.  About 10% of the bone tissue in an adult is replaced each year.  Bone cells called osteoblasts build bone... while bone cells called osteoclasts break down bone.  Osteoblasts and osteoclasts work in constant balance throughout a person's life.  When a child is growing taller, osteoblasts work harder than osteoclasts.  After age 40 (more or less) the osteoclasts break down bone faster than the osteoblasts build bone.  If too much bone is broken down, a person will develop osteoporosis.

How does an osteoblast do its job?

Osteoblasts are the body's bone builders.  They build bone by first putting together a framework of collagen... have you ever seen a caoncrete worker set down an iron mesh before laying the concrete?  This makes the bone much stronger.  Then, the osteoblasts deposit calcium phosphate (i.e., similar to concrete) that is hardened by the addtion of hydroxide and bicarbonate ions from the blood.  But here's the kicker!!! The osteoblast builds new bone all around itself.  Once the bone hardens, the osteoblast is trapped by its own bone walls!  The osteoblast is then called an osteocyte ("a bone cell").

How does an osteoclast do its job?

An osteocyte is a relatively large bone cell (but you still need a microscope to see it!) with multiple nuclei.  It is formed from multiple precursor cells that combine to form one "giant cell."  What was that robot movie where several robots combined to form one giant killing machine?  Anyway, bone is normally filled with small channels that provide access to its deepest inner parts.  Osteoclasts attach to the walls of these channels (called "canaliculi") and burrow into the hard bone.  Calcium, other minerals, and collagen are broken down by special chemicals and the byproducts are removed by the bloodstream.

How do the osteoblasts and osteoclasts know how hard to work?

The balance between bone breakdown and construction is complex.  Local chemical signals are sent from bone cells to neighboring cells to coordinate this process.  Also, a hormone secreted from the parathyroid glands called "PTH" helps regulate this balance.  PTH plays a critical role in controlling the amount of calcium and phosphate floating around in the bloodstream.  Most of the body's calcium is stored in bones.  When not enough calcium is found in the diet, osteoclasts are instructed to remove calcium from the bones.

So why is milk "vitamin D fortified"?

Vitamin D and vitamin D byproducts play an important role in regulating bone growth and resorption.  A deficiency of vitamin D may cause rickets, which causes weakening of the bones.

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