What are bones made of?What are bones made of?

Bone Matrix:Bone Matrix:– 1/3 organic and 2/3 inorganic by 1/3 organic and 2/3 inorganic by

weight.weight.Organic part consists of mainly Organic part consists of mainly collagen fibers. These fibers provide collagen fibers. These fibers provide the bone with the ability to resist the bone with the ability to resist stretching and twisting.stretching and twisting.

Inorganic part consists of mostly two Inorganic part consists of mostly two salts, small amounts of magnesium, salts, small amounts of magnesium, fluoride and sodium. These minerals fluoride and sodium. These minerals give bone its hardness and the ability give bone its hardness and the ability to resist compression.to resist compression.

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

Osteon

Central Haversian CanalCentral Haversian Canal

■ ■ Opening in the center of an osteonOpening in the center of an osteon

■ ■ Carries blood vessels and nervesCarries blood vessels and nerves

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

OsteoblastsOsteoblasts

OsteoblastsOsteoblasts

Bone-building cellsBone-building cells

They make and secrete They make and secrete collagen fibers and other collagen fibers and other organic components of organic components of bone matrix. bone matrix.

They begin the process They begin the process of calcification (bone of calcification (bone hardening).hardening).

The blue arrows indicate the osteoblasts. The yellow arrows indicate the bone matrix they’ve

just secreted.

OsteocytesOsteocytes

OsteocytesOsteocytesMature bone cells.Mature bone cells.Osteoblasts that Osteoblasts that have become have become trapped by the trapped by the secretion of matrix.secretion of matrix.No longer secrete No longer secrete matrix.matrix.Responsible for Responsible for maintaining the bone maintaining the bone tissuetissue..

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.

On the right, notice how the osteocyte is “trapped” within the pink matrix

OsteoclastsOsteoclasts– Large cells made from the fusion of as many as 50 white blood Large cells made from the fusion of as many as 50 white blood

cellscells– These cells digest bone matrix – this process is called These cells digest bone matrix – this process is called bone bone

resorptionresorption and is part of normal bone growth, development, and is part of normal bone growth, development, maintenance, and repair.maintenance, and repair.

– Osteoclasts secrete digestive enzymes to digest the bone Osteoclasts secrete digestive enzymes to digest the bone matrix. matrix.

– They also pump out hydrogen ions to create an acid They also pump out hydrogen ions to create an acid environment that eats away at the matrix. environment that eats away at the matrix.

– Osteoclasts reshape new bone growth after a break to look like Osteoclasts reshape new bone growth after a break to look like the original bone shape.the original bone shape.

Microscopic Anatomy of BoneMicroscopic Anatomy of Bone

LacunaeLacunae– Cavities containing Cavities containing

bone cells bone cells (osteocytes)(osteocytes)

– Arranged in concentric Arranged in concentric ringsrings

LamellaeLamellae– Rings around the Rings around the

central canalcentral canal– Sites of lacunaeSites of lacunae

Microscopic Anatomy of BoneMicroscopic Anatomy of Bone

Canaliculi Canaliculi – Tiny canalsTiny canals– Radiate from the Radiate from the

central canal to central canal to lacunaelacunae

– Form a transport Form a transport system connecting all system connecting all bone cells to a bone cells to a nutrient supplynutrient supply