are attached to the bone. The periosteum anchors
these structures and contains both the blood vessels
that enter the bone itself and osteoblasts that will
become active if the bone is damaged.
EMBRYONIC GROWTH OF BONE
During embryonic development, the skeleton is first
made of cartilage and fibrous connective tissue, which
are gradually replaced by bone. Bone matrix is pro-
duced by cells called osteoblasts(a blastcell is a “grow-
ing” or “producing” cell, and osteomeans “bone”). In
the embryonic model of the skeleton, osteoblasts dif-
ferentiate from the fibroblasts that are present. The
production of bone matrix, called ossification, begins
in a center of ossificationin each bone.
The cranial and facial bones are first made of
fibrous connective tissue. In the third month of fetal
development, fibroblasts (spindle-shaped connective
tissue cells) become more specialized and differentiate
into osteoblasts, which produce bone matrix. From
each center of ossification, bone growth radiates out-
ward as calcium salts are deposited in the collagen of
the model of the bone. This process is not complete
at birth; a baby has areas of fibrous connective tissue
remaining between the bones of the skull. These
are called fontanels(Fig. 6–2), which permit com-
pression of the baby’s head during birth without
breaking the still thin cranial bones. The fontanels
also permit the growth of the brain after birth. You
may have heard fontanels referred to as “soft spots,”
and indeed they are. A baby’s skull is quite fragile and
must be protected from trauma. By the age of 2 years,
all the fontanels have become ossified, and the skull
becomes a more effective protective covering for the
brain.
The rest of the embryonic skeleton is first made of
cartilage, and ossification begins in the third month of
gestation in the long bones. Osteoblasts produce bone
matrix in the center of the diaphyses of the long bones
and in the center of short, flat, and irregular bones.
Bone matrix gradually replaces the original cartilage
(Fig. 6–3).
The long bones also develop centers of ossification
in their epiphyses. At birth, ossification is not yet com-
plete and continues throughout childhood. In long
bones, growth occurs in the epiphyseal discsat the
junction of the diaphysis with each epiphysis. An epi-
physeal disc is still cartilage, and the bone grows in
length as more cartilage is produced on the epiphysis
side (see Fig. 6–3). On the diaphysis side, osteoblasts
produce bone matrix to replace the cartilage. Between
the ages of 16 and 25 years (influenced by estrogen or
testosterone), all of the cartilage of the epiphyseal
discs is replaced by bone. This is called closure of the
epiphyseal discs (or we say the discs are closed), and
the bone lengthening process stops.
Also in bones are specialized cells called osteo-
clasts(a clastcell is a “destroying” cell), which are able
to dissolve and reabsorb the minerals of bone matrix,
a process called resorption. Osteoclasts are very
active in embryonic long bones, and they reabsorb
bone matrix in the center of the diaphysis to form the
marrow canal. Blood vessels grow into the marrow
canals of embryonic long bones, and red bone marrow
is established. After birth, the red bone marrow is
replaced by yellow bone marrow. Red bone marrow
remains in the spongy bone of short, flat, and irregu-
lar bones. For other functions of osteoclasts and
osteoblasts, see Box 6–1: Fractures and Their Repair.FACTORS THAT AFFECT BONE
GROWTH AND MAINTENANCE- Heredity—each person has a genetic potential for
height, that is, a maximum height, with genes
inherited from both parents. Many genes are
involved, and their interactions are not well under-
stood. Some of these genes are probably those for
the enzymes involved in cartilage and bone pro-
duction, for this is how bones grow. - Nutrition—nutrients are the raw materials of
which bones are made. Calcium, phosphorus, and
protein become part of the bone matrix itself.
Vitamin D is needed for the efficient absorption of
calcium and phosphorus by the small intestine.
Vitamins A and C do not become part of bone but
are necessary for the process of bone matrix forma-
tion (ossification). Without these and other nutri-
ents, bones cannot grow properly. Children who
are malnourished grow very slowly and may not
reach their genetic potential for height. - Hormones—endocrine glands produce hormones
that stimulate specific effects in certain cells.
108 The Skeletal System
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