Page 107 - Anatomy and Physiology of Farm Animals, 8th Edition

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Microscopic Anatomy cells) surrounded by an intercellular matrix
Mature bone consists of osteocytes (bone
and Formation of Bone
VetBooks.ir bout a third of the weight of bone composed of calcified osteoid material. The
osteocytes are in small cavities in the bone
A consists of an organic framework called lacunae (meaning little lakes) (Fig. 5‐1).
of fibrous tissue and cells. This organic A system of tiny canals called canaliculi con-
matter mainly consists of collagen and nects the lacunae within the substance of the
polysaccharides called glycosaminogly- bone. Even though bone is highly vascular,
cans (GAGs), which contain chondroitin with capillaries close together, the canaliculi
sulfate. They give resilience and toughness transmit tissue fluid that is essential for main-
to bones. The remaining two‐thirds of taining the life of the osteocytes.
bone weight consists of inorganic calcium Both the lacunae and canaliculi are
and phosphorus salts in the organic frame- formed because cytoplasmic processes
work. About 80% of these salts are calcium connect the osteoblasts (bone‐forming
phosphate, and the remainder is primarily cells) at the time the osteoid material is laid
calcium carbonate and magnesium phos- down. Thus the cells and their processes
phate. The calcium phosphate is primarily act as a mold until the osteoid tissue is set
found in hydroxyapatite crystals formed and mineralized. The cytoplasm is then
with calcium hydroxide. These salts give partially withdrawn, leaving the cells, now
hardness and rigidity to bones and make known as osteocytes, in the lacunae, which
them resist the passage of x‐rays. If the are connected by canaliculi containing
inorganic salts are removed by soaking a cytoplasmic extensions.
bone in dilute acid, the resulting decalci- Cancellous bone, or spongy bone,
fied bone will retain its original form but consists of a network of fingerlike bony
will be flexible enough to be tied in a knot. spicules, or trabeculae. This type of bone
On the other hand, if the organic matter is is found in the extremities of long bones,
removed by charring in a furnace so that where resistance to compression without
only the inorganic salts remain, the bone excessive weight is needed. Flat bones
will retain its form but be brittle and break between two layers of compact bone, as in
unless handled with extreme care. the skull, are also cancellous. The spicules

Figure 5-1. Unstained ground bone. Osteocytes in lacunae (L) and fine canaliculi extend from each
lacuna. The central (C) and perforating (P) canals contain blood vessels, nerves, and lymphatics. Source:
Dellmann and Eurell, 1998. Reproduced with permission of John Wiley & Sons, Inc.