Bone and Cartilage – its Structure and Physical Properties - Biomechanics of Hard Tissues - page 15

Figure 1.7 Cross section of the young rat tibia in develop-
ment. The trabecular bone network with medullary cavity in
the center is exposed. To the left, a smaller cross section of
fibula is seen. Courtesy of Litwin and Gajda.
Endochondral ossification begins with points in the cartilage called '' primary
ossification centers .'' They mostly appear during fetal development, though a few
short bones begin their primary ossification after birth. They are responsible for
the formation of the diaphyses of long bones, short bones, and certain parts of
irregular bones. Secondary ossification occurs after birth, and forms the epiphyses
of long bones and the extremities of irregular and flat bones. The diaphysis and
both epiphyses of a long bone are separated by a growing zone of cartilage (the
epiphyseal plate), cf. Figures 1.6 and 1.7.
Epiphyseal plates (growth plates) are located in the metaphysis and are respon-
sible for growth in the length of the bone, cf. Figure 1.10. Because of their rich
blood supply, metaphysis of long bones are prone to hematogenous spread of
Osteomyelitis in children.
When the child reaches skeletal maturity, all of the cartilage is replaced by the
bone, fusing the diaphysis and both epiphyses together (epiphyseal closure).
Exterior shape of the bone is characteristic of every species and is revealed
by different roughnesses, spikes, spicules, openings, and holes; it is an effect of
modulating influence from the side of the soft components of the organism. This
paradoxal observation is explained by the fact that bones develop relatively late
when soft parts are formed, and the growing bone has to match its form to the
shape of soft components.