HORMONE plays a key role in regulating this
release. The process of bone resorption leaves
vacant lacunae on the surface of the bone that
subsequently fill with new bone structure.In health, osteoblasts and osteoclasts function
in relative balance so the rate of new bone forma-
tion matches the rate of old bone destruction. This
process of bone remodeling is one of maintenance,
not growth. Bone growth, in which the bones
increase in size, occurs through ossification (con-
version of CARTILAGEcells to bone cells). Imbalance
may result from disease processes that alter HOR-
MONElevels in the body or when calcium levels in
the blood circulation are too low. Calcium is vital
to numerous cellular activities and crucial for MUS-
CLEcontraction and the conduction of NERVEsig-
nals; when its levels in the blood circulation are
inadequate, the body accelerates bone resorption
so it can withdraw calcium from the bones.
Bone StructureWere they solid, bones would weigh more than
the body could support or the muscles could
move. So instead they are a combination of densi-
ties that provide a balance between STRENGTHand
mass. Though all bones contain the same elements
of structure, the particular combination of those
elements varies according to the bone’s role.
The outermost layer of bone, called compact
bone or cortical bone, is made of multiple thin
layers, called lamellae, that contain tightly packed
osteocytes. Each lamella contains a somewhat dif-
ferent structure of cells, altering the density and
orientation of the bone structure for maximum
strength. Compact bone is heavily mineralized and
very dense; tooth enamel is the only other sub-
stance in the body that is harder than compact
bone. An intricate network of canals, called the
Haversian systems, bring blood vessels and nerves
through the lamellae to nourish and support com-
munication among the osteocytes. Compact bone
protects the inner bone structures and provides
the stiffness necessary to leverage the muscles for
movement.
The middle layer of bone is cancellous, or
spongy, bone, also called trabecular bone, where
mineralized filaments form intricate networks of
walls and spaces. The spaces contain osteocytes,
fluids, and other cells. The structure of cancellous
bone is more elastic than that of compact bone,
allowing the bones to absorb compression such as
occurs with walking, running, and jumping. Can-
cellous bone has less than half the density of com-
pact bone but many times more the surface area.
Some bones contain a center channel, the
medullary canal, that houses BONE MARROW. In
children every bone contains red bone marrow,
the type of bone marrow that produces new blood
cells. By adulthood only the long bones, sternum,
and hip bones contain appreciable amounts of red
bone marrow. Yellow bone marrow, a mix of col-
lagen and fatty tissues, occupies the innermost
layer of most other bones. Some bones do not
contain any marrow.
A thin but tough membrane called the perios-
teum covers the surface of the bones except at the
joints. It forms the attachment surface for tendons
and ligaments. The periosteum contains a rich
network of blood vessels and nerves that help
nourish the compact bone. Osteoblasts in the
periosteum are “first responders” when there is
injury to the bone, rapidly forming new bone for
repair. The nerves in the periosteum are largely
responsible for PAINsignals when there is injury to
the bone.Types of Bones
The SKELETONcontains four basic types of bones:
long, short, flat, and irregular. Long bones, such as
those in the arms and legs, must support the body’s
weight and mass. Their length and structure also
allows them to function as levers to make move-
ment possible. A thin layer of compact bone pro-
vides the rigidity the long bones require; a
substantial middle layer of cancellous bone pro-
vides added bone mass for strength and stability.
The intricate trabecular structure of cancellous
bone makes it much stronger for supporting
weight, though more vulnerable to impact. At each
end of a long bone is the epiphysis, or growth plate,
where ossification takes place during growth in
childhood. The shaft of a long bone is its diaphysis.
Lengthwise through the center of a long bone is a
medullary canal that contains bone marrow.
Short bones, such as those that form phalanges
(metacarpals in the fingers and metatarsals in the
toes), are structurally long bones on a much308 The Musculoskeletal System