Page 108 - Anatomy and Physiology of Farm Animals, 8th Edition

Microscopic Anatomy and Growth and Development of Bone / 93

of bone are arranged so as to resist stresses but only a portion of the new cells actually
secretes osteoid substance and forms bone;
and strains imposed on the bone by weight
VetBooks.ir or pull of muscles. the rest is held in reserve as the osteogenic
Compact bone, found in the shafts of
layer of the periosteum and endosteum
long bones, consists primarily of many within the marrow cavity and central
laminated tubes known as osteonal sys- canals. These reserve cells divide and form
tems (formally termed haversian systems). more osteoblasts whenever more bone is
Each osteon consists of one central canal needed, as in repair of fractures, response
containing vessels and nerves surrounded to stress, or growth. Because the intercel-
by circular plates of bone (osteonal lamellae) lular matrix is unyielding, bone can be
forming the laminated cylinder (Fig. 5‐2). added only on the surface, and the osteo-
These plates are laid down in a centripetal cytes (mature osteoblasts) probably have
fashion (from the periphery toward the lost the ability to divide.
center). After the bone is formed, the oste- The shaft of a long bone enlarges in
oblasts that became embedded in the bone diameter as a result of the activity of the
substance are called osteocytes. In general osteogenic layer of the periosteum, while
these osteons are added on the periphery bone along the inner surface is normally
of the shaft of a bone as the bone increases resorbed by the endosteum to increase the
in diameter. Blood vessels extend from the size of the marrow cavity (Fig. 5‐3).
periosteum to central canals through per- Resorption of bone may also occur under
forating canals (also known as Volkmann’s abnormal conditions, such as during a
canals), which often travel at right angles period of calcium deficiency. Whenever
to the central canals (Fig. 5‐2). bone is resorbed (under normal or abnor-
Osteoblasts usually come from mesen- mal conditions), large multinucleated cells
chymal cells, the parent cells of all connec- called osteoclasts (bone‐destroying cells)
tive tissues. The osteoblasts divide readily, are usually found (Fig. 5‐4). These cells,

Inner circumferential lamellae
Blood vessel in central canal
Interstitial lamellae

Osteon
Osteonal lamella

Central canal
Outer circumferential
lamellae
Periosteum
Perforating canal

Figure 5-2. The structural unit of compact bone is the osteon. An osteon is telescoped to show the
concentric layers of bone that surround a central canal. Interstitial lamellae of bone fill the space between
osteons, and the inner and outer surfaces are formed by inner and outer circumferential lamellae. Source:
Dellmann and Eurell, 1998. Reproduced with permission of John Wiley & Sons, Inc.