Page 110 - Anatomy and Physiology of Farm Animals, 8th Edition

Microscopic Anatomy and Growth and Development of Bone / 95

VetBooks.ir

Figure 5-5. The stages of endochondral ossification of a long bone. (A) The chondrocytes in the center
of the model hypertrophy. (B) A bony collar begins to form around the cartilage model. (C) Blood vessels
from the periosteum (periosteal bud) invade the cartilage model, bringing bone‐forming cells to initiate
the primary center of ossification. (D) The physis and secondary centers of ossification are established at
either end of the bone (only shown at one end for image simplicity). (E) The growth plate closes in the
mature bone, and a confluent marrow cavity from the epiphysis to the diaphysis is formed. Source: adapted
from Dellmann and Eurell, 1998. Reproduced with permission of John Wiley & Sons, Inc.

cartilage to provide this separation and where the diaphysis and an epiphysis meet
thus allow for continued growth in the is a metaphysis (Fig. 5‐5).
length of the long bone. Several hormones affect the rate of
The midshaft region of a long bone that growth of long bones, but growth hor-
contains the primary ossification center is mone and sex hormones (androgens and
the diaphysis; each end that contains a estrogens) are key regulators. In general,
secondary ossification center is an epiphy- growth hormone promotes elongation of
sis (Fig. 5‐5). As animals grow and mature, long bones, and the sex hormones promote
the region of cartilage that separates the growth and epiphyseal closure. Growth
bony diaphysis and epiphyses continues to hormone itself has little direct effect on
narrow. The epiphyseal plate, another chondrocytes within epiphyseal plates. It
term used to describe this region of carti- stimulates other cells within the area of the
lage in growing animals, emphasizes how plates and in the liver to produce peptides,
narrow it may become. insulin‐like growth factors (IGFs), which
When the cartilage in the epiphyseal in turn stimulate the chondrocytes to pro-
plates is completely replaced by bone, liferate and increase their rate of cartilage
increases in length, hence growth in stat- production. This provides more cartilage
ure of the animal, is impossible. This is epi- in which bone can form to increase the
physeal closure. An epiphyseal line can length of the long bone. The critical role
often be seen on bones where this closure of IGFs has been confirmed in some con-
has occurred. The region of a long bone ditions of human dwarfism, including the