1. What are bones made of? Bone Matrix:
1/3 organic and 2/3 inorganic by weight.
Organic part consists of mainly collagen fibers. These fibers provide the bone with the ability to resist stretching and twisting.
Inorganic part consists of mostly two salts, small amounts of magnesium, fluoride and sodium. These minerals give bone its hardness and the ability to resist compression.

2. Osteon
A circular unit of bone containing a central Haversian canal surrounded by rings of bone matrix
Osteon

3. Central Haversian Canal
■ Opening in the center of an osteon
■ Carries blood vessels and nerves

4. Volkmann’s Canals
Running perpendicular () to the haversian canals are Volkmann’s canals. They connect the blood and nerve supply in the periosteum to those in the haversian canals and the medullary cavity.

5. Osteoblasts Osteoblasts Bone-building cells
They make and secrete collagen fibers and other organic components of bone matrix.
They begin the process of calcification (bone hardening).
The blue arrows indicate the osteoblasts. The yellow arrows indicate the bone matrix they’ve just secreted.

6. Osteocytes Osteocytes Mature bone cells.
Yellow arrows indicate osteocytes – notice how they are surrounded by the pinkish bone matrix.
Blue arrow shows an osteoblast in the process of becoming an osteocyte.
Osteocytes
Mature bone cells.
Osteoblasts that have become trapped by the secretion of matrix.
No longer secrete matrix.
Responsible for maintaining the bone tissue.
On the right, notice how the osteocyte is “trapped” within the pink matrix

7. Osteoclasts
Large cells made from the fusion of as many as 50 white blood cells
These cells digest bone matrix – this process is called bone resorption and is part of normal bone growth, development, maintenance, and repair.
Osteoclasts secrete digestive enzymes to digest the bone matrix.
They also pump out hydrogen ions to create an acid environment that eats away at the matrix.
Osteoclasts reshape new bone growth after a break to look like the original bone shape.

8. Microscopic Anatomy of Bone
Lacunae
Cavities containing bone cells (osteocytes)
Arranged in concentric rings
Lamellae
Rings around the central canal
Sites of lacunae

9. Microscopic Anatomy of Bone
Canaliculi
Tiny canals
Radiate from the central canal to lacunae
Form a transport system connecting all bone cells to a nutrient supply