Page 112 - Anatomy and Physiology of Farm Animals, 8th Edition

Microscopic Anatomy and Growth and Development of Bone / 97

when pressure is constant and excessive Fractures and Fracture Healing
VetBooks.ir or when there is little or no stress, as in A fracture of bone is simply a break in the
weightlessness in space or when a limb is
continuity of a bone. Among the many
immobilized and not bearing weight.
Proliferation of bone may occur in types of fractures described are the follow-
response to concussion or intermittent ing (Fig. 5‐6):
pressure. Thus, pressure can cause A simple fracture is one in which the
either atrophy or proliferation, depend- skin over the fracture site is unbroken.
ing on the degree and duration of stress An open fracture is one in which a
and the maturity of the bone. Excessive wound from the exterior contacts the bone
pressure on growing bone slows or stops at the point of the fracture. This may be
growth, while in mature bone it may caused by a broken end of bone perforating
stimulate either excessive growth or the skin or by a penetrating object, such as
rearrangement of structure. a bullet, causing the fracture.
A greenstick fracture is one in which
one side of the bone is broken or splintered
Calcium of Bone and the other side only bent. This type of
fracture usually is found only in young
In 100 cm of bone there is 10 g of calcium, animals.
3
as compared with 6 mg per 100 cm for most A complete fracture is one in which the
3
tissues and about 10 mg per 100 mL for bone is broken entirely across.
blood. Thus, bone serves as a reservoir of A physeal fracture (formally known as
minerals (especially calcium), which are epiphyseal fracture) is one that occurs at
constantly being either replenished or the junction of an epiphysis and the dia-
depleted. Through the action of osteoclasts physis of a bone. This type of fracture is
and osteocytes, calcium can be taken from limited to young animals.
this reservoir when serum calcium levels A comminuted fracture is one in which
are low. The reservoir can be replenished by the bone was splintered or crushed, pro-
the action of osteoblasts and osteocytes. ducing small fragments.
The activity of the cells within bone is If the broken ends of a fractured bone
subject to regulation by the hormones par- are brought into apposition (touch) and are
athyroid hormone (PTH) and calcitonin. immobilized (prevented from moving),
The sources of these hormones and the the normal process of healing will take
regulation of their secretion are discussed place (Fig. 5‐7). When the fracture occurs,
in Chapter 13. The overall effect of PTH is some blood vessels are ruptured, releasing
to increase serum calcium by increasing blood around the broken ends of the bone.
the net release of calcium salts from bone. This forms a clot, or hematoma, that is
This effect is due in part to increased oste- invaded by connective tissue cells forming
oclast activity and inhibition of osteoblast granulation tissue (term for mass of tis-
activity. PTH also affects osteocytes in sue consisting largely of fibroblasts and
mature bone, and these cells also play a capillaries). At first, periosteal and bone
role in the rapid release of calcium salts in marrow elements die off from lack of vas-
response to PTH. However, the mecha- cular supply, and then the osteoblasts from
nism by which osteocytes promote a rapid the surface of the bone, from the perios-
release of calcium is still being elucidated. teum, and from the endosteum lining the
The primary effect of calcitonin is to marrow cavities and central canals begin to
reduce osteoclast activity, which tends to divide rapidly and produce a massive
lower serum calcium. Thus, the normal amount of osteoid tissue called a callus. As
regulation of serum calcium concentra- the hematoma is reorganized, the osteoid
tions involves a balancing of the effects of tissue fills the gap between the broken ends
PTH and calcitonin on the cells of bone. of the bone and fills the bone marrow