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U N I T 1 2
Musculoskeletal Function
and an inner, more cellular layer of osteoprogenitor cells
which can differentiate into osteoblasts (bone-forming
cells, to be discussed). The periosteum contains blood
vessels and acts as an anchorage point for blood vessels
as they enter and leave the bone. The
endosteum
is the
membrane that lines the spaces of spongy bone, the mar-
row cavities, and the haversian canals of compact bone.
It is considerably thinner than the periosteum and is
composed of a single layer of flattened osteoprogenitor
cells and small amounts of connective tissue. The prin-
cipal functions of the periosteum and endosteum are the
nutrition of bone tissue and continuous supply of new
osteoblasts for repair and growth of bone.
Bone marrow
occupies the medullary cavities of the
long bones (see Fig. 42-2C) throughout the skeleton and
the cavities of spongy bone in the vertebrae, ribs, sternum,
and flat bones of the pelvis. The cellular composition of
the bone marrow varies with age and skeletal location.
Red bone marrow contains developing red blood cells
and is the site of blood cell formation. Yellow bone mar-
row is composed largely of adipose cells. At birth, nearly
all of the marrow is red and hematopoietically active. As
the need for red blood cell production decreases during
postnatal life, red marrow is gradually replaced with yel-
low bone marrow in most of the bones. In the adult, red
marrow persists in the vertebrae, ribs, sternum, and ilia.
BoneTissue
Bone, or
osseous
, tissue is perhaps one of the most dis-
tinctive tissues in the body. It is typical of other con-
nective tissue in that it consists of cells, fibers, and an
extracellular matrix. However, its extracellular compo-
nents are hard and calcified, a feature that enables it
to serve both supportive and protective functions. The
extracellular bone matrix can be divided into two parts:
the organic matrix and inorganic mineral matrix.
The organic matrix of bone and other connective tis-
sues is a composite of collagenous fibers and an amor-
phous mixture of protein and polysaccharides called
ground substance
. The ground substance provides
support and adhesion between cellular and fibrous ele-
ments and serves an active role in many metabolic func-
tions necessary for bone growth, repair, and remodeling.
The inorganic mineral matrix of bone is composed
largely of calcium phosphate in the form of hydroxyapa-
tite crystals. It comprises about 25% of the bones’ vol-
ume, but because of its high density, it is responsible for
about half its weight. In addition to calcium and phos-
phate, bone contains considerable amounts of the body’s
content of carbonate, magnesium, and sodium. Bone
may also take up lead and other heavy metals, thereby
removing these toxic substances from the circulation.
Laminar and Woven Bone.
There are two distinct
types of bone tissue based on their stage of develop-
ment: laminar (mature) and woven (immature).
Laminar
or mature bone is composed largely of cylinder-
shaped units of calcified matrix, called
osteons
, that are
oriented parallel to the long axis of the bone. Functionally,
osteons can be thought of as tiny weight-bearing pillars.
Running through the core of each osteon is a central canal,
called a
haversian canal
, which contains the blood ves-
sels and nerves that supply the osteon (Fig. 42-4A). Canals
of a second type called
perforating
, or
Volkmann canals
,
lie at right angles to the long axis of the bone, connect-
ing the vascular and nerve supplies of the periosteum and
medullary cavity. Spider-shaped osteocytes (mature bone
cells) occupy small cavities, or
lacunae
, at the junctions of
the lamellae (Fig. 42-4B). In compact bone (e.g., diaph-
ysis of long bones), the lamellae exhibit a characteristic
organization of outer circumferential lamellae, inner cir-
cumferential lamellae, and interstitial lamellae.
The outer
circumferential lamellae
are located just beneath the peri-
osteum and the
inner circumferential lamellae
are arranged
concentrically around a central haversian canal. Between
the two circumferential lamellae are the irregularly shaped
interstitial lamellae.
These are the remnants of haversian
systems that have been destroyed during bone growth or
remodeling.
Trabecular lamellar bone
forms the coarse
spongy bone of the medullary cavity. It exhibits plates of
lamellar bone perforated by marrow spaces.
In contrast to compact bone,
immature or woven
bone
, consisting of trabeculae, looks like poorly orga-
nized bone. It is deposited more rapidly than lamellar
bone, has low tensile strength, and serves as temporary
scaffolding for support. It is found in the developing
fetus, in areas surrounding tumors and infections, and
as part of a healing fracture.
Blood Supply
Bones are richly supplied with blood from nutrient and
perforating arteries (see Fig. 42-2C). The nutrient arter-
ies enter the bone through a nutrient foramen and sup-
ply the marrow space and the internal half of the cortex.
The perforating arteries are small arteries that extend
inward from the periosteal arteries on the external sur-
face of the periosteum and anastomose in the cortex
with branches of the nutrient arteries coming from the
bone marrow. The distribution of blood in the cortex
occurs through the haversian and Volkmann canals
(see Fig. 42-4A). Veins accompany arteries through the
nutrient formania. Many large veins also leave through
openings near the articulating ends of bones. Bones con-
taining red bone marrow have numerous large veins.
Bone Cells
Five types of cells participate in the formation and main-
tenance of bone tissue: osteoprogenitor cells, osteo-
blasts, osteocytes, osteoclasts, and bone-lining cells
(Table 42-1). With the exception of the osteoclast, each
type of bone cell originates from the same basic cell
type, undergoing transformation as it matures.
Osteoprogenitor Cells.
Osteoprogenitor cells are undif-
ferentiated or resting cells that are found in the perios-
teum, endosteum, and epiphyseal plate of growing bone.
Derived from stem cells in the bone marrow, they have
the potential to differentiate into many different cell
types, including adipocytes, fibroblasts, and osteoblasts.
