To understand how osteogenesis imperfecta affects the way skeletal tissue forms, it's important to understand how a normal human body grows new bone. The task largely falls to cells called osteoblasts, which create new bone tissue, while cells called osteoclasts break down old bone. This ongoing process is called bone remodeling.
Osteoblasts form new bones and increase the size of growing bones. This process is called ossificiation.
Think of workers constructing a building -- the process has a lot in common with the way osteoblasts build new bone. First, both need strong building blocks with which to form the finished product. Construction workers use bricks or concrete blocks to form walls. Osteoblasts form bone from inorganic mineral salts, mainly calcium carbonate and calcium phosphate. This is the reason a calcium-rich diet is important to maintain strong bones.
But you can't erect a building using only blocks and bricks. You need a metal framework to give it flexibility and tensile strength, the amount of stress a construction can endure without falling apart. An earthquake or strong wind could destroy a skyscraper, no matter the strength of the pieces, without a flexible steel framework. In bones, fibers of collagen, a protein produced by all vertebrates, supply this framework. Osteoblasts secrete these collagen fibers to form the framework and then initiate calcification -- calcium fills the flexible framework, providing strength.
In cases of OI, the body either creates poor-quality collagen or doesn't create enough of it. The resulting bones lack flexibility and tensile strength, making them far more susceptible to fracture than normal, healthy bones.
However, OI doesn't just affect bone tissue. Collagen also forms the framework for cartilage, teeth and various forms of connective tissue. Poor or too little collagen can also render these features brittle. Collagen shows up in sclera, the visible white portion of the eye. As a result, someone with OI may develop a blue or gray hue in this portion of his or her eyes. The condition can even lead to early hearing loss if the patient suffers a fractured stapes. The stapes, or stirrup, is one of the smallest bones in the body and plays a central role in transmitting sound waves to the eardrum.
OI is a genetic condition. Either a parent passes the gene on to his or her child or the genes mutate spontaneously. Doctors have identified eight types of OI, and six involve a dominant mutation. This means only one parent has to pass on the OI gene. Two additional varieties involve a recessive mutation, meaning both parents have to pass on the gene. While considered a rare condition, OI affects 20,000 to 50,000 people in the United States alone.
On the next page, we'll look at the different types of OI and the symptoms associated with each.