Osteoporosis

Osteo- refers to bones and -porosis means pores. So, osteoporosis is when there’s a higher breakdown of bone in comparison to the formation of new bone which results in porous bones, meaning a decrease in bone density to the point of potential fracture.

Looking at a cross-section of a bone, there’s a hard-external layer known as the cortical bone and a softer internal layer of spongy bone or trabecular bone that is composed of trabeculae. The trabeculae are like a framework of beams that give structural support to the spongy bone. The cortical bone, in turn, is made up of many functional, pipe-like units called osteons, which run through the length of the bone. In the center of these osteons, there are hollow spaces called Haversian canals, which contain the blood supply and innervation for the bone cells. Around the Haversian canals, there are concentric lamellae, which look a bit like tree rings. The lamellae have an organic part, which is mostly collagen, and an inorganic part called hydroxyapatite, which is mostly calcium phosphate. In between neighboring lamellae, there are spaces called lacunae, which contain bone cells called osteocytes.

At first glance, bone may appear inert and unchanging, but it’s actually a very dynamic tissue. In fact, spongy bone is replaced every 3 to 4 years and compact bone is replaced every 10 years, in a process called bone remodeling, which has two steps: bone resorption, when specialized cells called osteoclasts break down bone, and bone formation, which is when another type of cells called osteoblasts form new bone.

Bone remodeling as a whole is highly dependent on serum calcium levels, which, in turn, are kept in the normal range by a balance between parathyroid hormone, or PTH, calcitonin and vitamin D. Parathyroid hormone is produced by the parathyroid glands in response to low serum calcium, and it increases bone resorption to release calcium into the bloodstream. On the other hand, calcitonin is produced by the thyroid gland in response to high serum calcium, so it opposes the action of PTH - therefore promoting bone formation and decreasing bone resorption. Finally, vitamin D promotes calcium absorption in the gut, so it increases serum calcium, promoting bone formation and decreasing bone resorption.