1. Microscopic Osteology and Bone Formation

2. Compact Bone We know that compact bone is very dense It is also very complex when viewed under a microscope

3. Mature bone cells, osteocytes, are found in tiny cavities within the matrix called lacunae. The lacunae are arranged in concentric circles called lamellae around Haversian canals.

4. Each complex consisting of central canal and matrix rings is called an osteon or Haversian system. Haversian canals run lengthwise through the bone matrix and carry blood vessels and nerves to all areas of the body.

5. Tiny canaliculi radiate outward from the Haversian canals to all lacunae. The pathway from the outside of the bone to its interior features is completed by perforating (Volkmann’s) canals.

6. It is due to this intricate network of canals that nutrients are able to go to all parts of the bone. As you can see these vessels and nerves also continue into the spongy bone matrix of bones as well.

7. Bone Formation, Growth and Remodeling The skeleton is formed from two of the strongest and most supportive tissues in the body. In embryos the skeleton is primarily made up of hyaline cartilage.

8. As an embryo grows and matures the hyaline cartilage is replaced slowly by bone This process is called ossification and occurs throughout the body until only the bridge of the nose, parts of the ribs, and the joints are cartilage.

9. There are two major steps in human bone development The first is the hyaline cartilage model that is completely covered with bone matrix by bone forming cells called osteoblasts.

10. This gives us our fetal bones that have “cartilage bones” enclosed by “bony bones”. By birth or shortly after most of the hyaline cartilage model has been converted to bone except for two regions -- the articular cartilages and the epiphyseal plates.

11. The articular cartilages will exist through out life in a normal healthy individual At the epiphyseal plates is where we grow longitudinally.

12. How is bone growth controlled Bone growth is controlled by growth hormone and during puberty, the sex hormones.

13. Bone is an ever-changing tissue Your bones do not stop being active once you have reached adulthood. They are constantly remodeled in response to changes in two factors 1) Calcium levels in the blood 2) The pull of gravity and muscles on the bone

14. Calcium levels in blood When blood calcium levels drop below homeostatic levels, the parathyroid glands are stimulated to release parathyroid hormone (PTH) PTH activates osteoclasts which break down bone and release calcium ions into the blood

15. If the blood calcium levels are too high, a condition called hypercalcemia exists When you become hypercalcemic the body deposits calcium in the bone matrix as hard calcium salts

16. OSTEOCYTES It is important to understand that bone is constantly being remodeled and rebuilt. When this happens the osteoblasts lay down new bone in the bone matrix and get trapped inside These osteoblasts are where mature bone cells, osteocytes, come from.

17. Effects of improper bone growth and formation Rickets is a disease of children whose bones do not harden and ossify. This is usually due to a lack of calcium in the diet or of Vitamin D. Vitamin D is a vitamin that helps the body properly absorb and use calcium from foods.