The Skeletal System and Its Movements 5This is termed the epiphyseal plate (G. epi, upon +
physis, growth) or “growth plate” (see Bone Devel-
opment and Growth for more information). In the
adult these epiphyseal plates have been replaced with
bone, and the diaphysis has fused with the epiphysis.
The bone used in this fusion is very dense and is vis-
ible as an epiphyseal line on X rays.
With the exception of the portion of the epiphyses
covered with articular cartilage, the whole outside of
the bone is covered by a fibrous membrane called
the periosteum (G. peri, around + osteon, bone). The
inner layer of the periosteum contains cells that are
capable of laying down new bone (osteoblasts). The
periosteum is richly supplied with blood vessels,
which are essential for bone nutrition. It also pro-
vides a site for attachment of muscles and ligaments
to the bone. Muscles generally do not attach right
into the bone; rather their connective tissue exten-
sions, such as the tendon, attach to the periosteum,
which in turn has small fibers that penetrate into
the bone (Sharpey’s fibers). The periosteum can be
readily injured, and due to its abundant nerve supply
may be responsible for much of the pain associated
with shin splints, bone bruises, and fractures.
The endosteum (G. endon, within + osteon, bone)
is a membrane lining the internal bone surfaces
including the medullary cavity and the canals pass-
ing through the compact bone. Like the periosteum,
it contains cells that can help with bone growth
and repair. These cells, located in the endosteum
and periosteum of growing bones, are particularly
important for growth of bones in terms of girth
versus length (see Bone Development and Growth
for more information).
Structure of Other Types of Bones
Similar to long bones, the short bones, irregular
bones, and flat bones have an outer layer of compact
bone covered by periosteum. Underneath this layer
of compact bone lies cancellous bone that is covered
by endosteum. These types of bones are not cylin-
drical in shape and so have no epiphyses, diaphysis,
or medullary cavity. However, they do contain bone
marrow between their trabeculae. Some of the flat
bones contain red marrow, the type of marrow
capable of generating red blood cells.
Bone Development and Growth
During fetal development, specialized connective
tissue (mesenchyme) can be directly turned into
bone, termed intramembranous ossification (L. within
a membrane; os, bone + facia, to make), but more com-
monly is turned into cartilage models of bones that are
then mostly replaced with bone as the child develops.
This latter type of ossification is termed endochondral
ossification (G. endon, within + chondros, cartilage),
and it is this type of ossification that is responsible for
the increase in length of long bones. Endochondral
ossification originates at a site near the center of the
shaft of the cartilage model that is called the primary
ossification site. This ossification begins at the end of
the second month of intrauterine life (Hall-Craggs,
1985) and proceeds in both directions away from the
center to form the diaphysis. Shortly before or after
birth, one or more secondary ossification centers
appear toward the extremities of the long bone,
which ossify the epiphysis.
As growth proceeds, a plate of cartilage remains
between the diaphysis and epiphyses: the epiphyseal
plate or “growth plate” previously described. This
epiphyseal plate maintains its thickness by balancing
the growth of cartilage on its epiphyseal side with the
replacement by bony tissue on its diaphyseal side.
This process prevents fusion and allows growth in
length of the bone to continue until the adult size
of that particular bone is achieved. At this time,
the production of new cartilage declines; the carti-
laginous epiphyseal plates are replaced by bone; and
the diaphysis fuses with the epiphyses. The “growth
plates” are now considered “closed.” This closure of
the epiphyseal plates generally occurs progressively
from puberty to maturity. Although there is much
individual variability, most of the long bones of the
limbs achieve closure between approximately age
15 and 25 (Goss, 1980); it occurs as much as four
years earlier in a female than a male (Kreighbaum
and Barthels, 1996).
In addition to growth in length, long bones also
undergo remodeling and growth in circumference,
termed appositional growth. The osteoblasts in the
deep layer of the periosteum lay down new bone
(intramembranous ossification), while cells in the
endosteum (osteoclasts) resorb bone. This process
allows the bone to grow “outward,” increasing its
girth while slightly expanding the medullary cavity to
make a thicker and stronger bone while preventing
the bone from becoming too heavy. Although this
expansion in girth occurs at the greatest rate before
maturity, it continues throughout adulthood. A sum-
mary of this growth of long bones in circumference
and length appears in figure 1.3.Bone Remodeling
In addition to growing in length and width, bone is
also continually remodeling. Approximately 5% to