Biology Reference
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FIGURE 13.4 Differences
in primary and secondary
osteons.
development and continuing into the first stages of adulthood ( Martin and Burr, 1989; Carter
and Beaupre, 2001 ). Initial endochondral osteogenesis takes place in hyaline cartilage starting
with the development of the bone collar, or model. The bone collar is initially formed via
intramembranous ossification. Once blood enters the mixture, with the surrounding peri-
chondrium, the membrane of fibrous connective tissue covers the surface of cartilages except
at the joints ( Martin and Burr, 1989; Junqueira et al., 1998; Carter and Beaupre, 2001 ).
Following this process, a primary growth center appears at the rudimentary base of the
diaphysis and secondary ossification centers form at the epiphyses (or ends) of the bone ( Leeson
and Leeson, 1981; Carter and Beaupre, 2001 ). Longitudinal growth occurs by chondrocytes (carti-
lage-forming cells) in an area near the ends of bone called the growth plate or physis.Thephysis
serves as a boundary between the bone epiphysis and metaphysis (zone between the epiphysis
and diaphysis) ( Martin et al., 1998 ). The growth plate is divided into several zones: resting,
proliferative, hypertrophic, and provisional calcification. Chondrocytes progress through these
above-mentioned subsequent phases during their life cycle ultimately resulting in a calcified
cartilage matrix ( Martin et al., 1998 ) followed by a zone of ossification ( Leeson and Leeson,
1981 ). During this process, osteoblasts accumulate at these calcified areas to lay down bone.
Intramembranous Ossification
Intramembranous bone development occurs within the flat bones of the skeleton,
including the neurocranium, the pelvis, and some bones of the face. It is also responsible
for the thickening of long bones undergoing endochondral growth ( Junqueira et al., 1998;
Carter and Beaupre, 2001 ). This process is seen as early as the eighth week of gestation.
During this time, undifferentiated mesenchymal cells directly differentiate into osteoblasts
and begin to deposit osteoid. This unmineralized bone matrix eventually surrounds the oste-
oblast and mineralizes. The encased osteoblasts are now known as osteocytes, which are
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