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Microscopic Structure of the Skeletal System: What makes our bones Strong?

We have previously discussed the structure of our bone macroscopically, and we have discussed how strong compact bones are compared to cartilage bones. However, we have yet to delve into why our bones are strong. Hence, this post is focused on further explaining the bone matrix, how bones are formed, and the concept of bone remodeling or resorption.

Bone Matrix

The bone matrix is composed of both organic and inorganic materials. Organic be it carbon containing molecules such as collagen and proteins. Inorganic be it free flowing ions like minerals such as sodium, magnesium, and phosphate.

the Haversian systems or osteons are ordered in the bone matrix. As shown in the figure on the left, the osteon is used to describe the lamellae which circulate around the the Haversian canal. The separation between the lamellae is called lacunae, which is interconnected by canaliculi.

Haversian canal – contains the blood vessels, nerve, and lymph that provides the bone nutrients with interaction with the canaliculi

Lacunae – houses osteocytes (mature bone cells)

Canaliculi – small canals that allow nutrient and wastes exchange between themselves and the Haversian canals.

Ossification (Bone Formation)

Bone formation occurs in two ways: endochondral (endo-inside chondral-cartilage) ossification and intramembranous (intra – within) ossification.

Endochondral Ossification

In our pervious post, we briefly mentioned that the bones in our body were once cartilage. The process in which the cartilage hardens is called endochondral ossification.

Intramembranous Ossification

Intramembranous ossification is another way in which bones are formed. It occurs in undifferentiated embryonic connective tissue (mesenchymal tissue) which becomes bone.

Bone Remodeling or Bone Resorption

Our bone is constantly being broken down and built back up by these cells called: osteoclasts and osteoblasts (used in ossification).

Bone resorption is best described as the breakdown of the bone by osteoclasts which causes bone minerals (calcium and phosphate) to be released into the blood. Bone resorption can occur anywhere and it can happen during exercising or seen in patients with periodontitis. Stimulation of blood resorption is detected by the calcium receptors in the blood, which is monitored by the parathyroid. With low levels of calcium in the blood, parathyroid hormone is secreted from the parathyroid gland to encourage bone resorption.

You’ve guessed it, our body is smart. So smart, that as it notices bone resorption occurs, ossification takes over. Of course, there are situations in which our body loses its imbalance, such as in patients with osteoporosis (increased osteoclast resorption and decrease bone formation). Therefore, it is encouraged for growing children, well all of us, to consume adequate amounts of calcium rich foods (MILK!) to help with bone formation. The logic is, I presume, with high levels of calcium in our system, it is less likely the need of our parathyroid gland to secrete the parathyroid hormone to encourage bone resorption or breaking down our bone to sustain the desired level of calcium in our blood.

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