Once completely surrounded by the bony matrix, osteoblasts
are called osteocytes. As ossif
cation continues, osteoblasts
beneath the periosteum deposit compact bone around the
Parts oF the skeletal system begin to Form during the f
Few weeks oF prenatal development, and bony structures
continue to grow and develop into adulthood. Bones Form
by replacing existing connective tissue in one oF two ways.
Some bones originate within sheetlike layers oF connective
tissues; they are called
begin as masses oF cartilage later replaced by bone tissue;
they are called
The broad, Flat bones oF the skull are
¯nz). During their develop-
ment (osteogenesis), membranelike layers oF unspecialized,
or relatively undiFFerentiated, connective tissues appear at
the sites oF the Future bones. Dense networks oF blood ves-
sels supply these connective tissue layers, which may Form
around the vessels. These partially diFFerentiated progenitor
cells enlarge and Further diFFerentiate into bone-Forming cells
te-o-blasts), which, in turn, deposit
bony matrix around themselves. As a result, spongy bone
Forms in all directions along blood vessels within the layers
oF connective tissues. Later, some spongy bone may become
compact bone as spaces f
ll with bone matrix.
As development continues, the osteoblasts may become
completely surrounded by extracellular matrix, and in this
manner, they become secluded within lacunae. At the same
time, extracellular matrix enclosing the cellular processes oF
the osteoblasts gives rise to canaliculi. Once isolated in lacu-
nae, these cells are called
Cells oF the connective tissue that persist outside the
developing bone give rise to the periosteum. Osteoblasts on
the inside oF the periosteum Form a layer oF compact bone
over the surFace oF the newly Formed spongy bone.
This process oF replacing connective tissue to Form an
intramembranous bone is called
lists the major steps oF the process.
Most oF the bones oF the skeleton are
¯nz). They develop From masses oF hya-
line cartilage shaped like Future bony structures. These car-
tilaginous models grow rapidly For a time and then begin to
change extensively. Cartilage cells enlarge and their lacunae
grow. The surrounding matrix breaks down, and soon the
cartilage cells die and degenerate.
As the cartilage decomposes, a periosteum Forms From
connective tissue that encircles the developing structure.
Blood vessels and partially diFFerentiated connective tissue
cells invade the disintegrating tissue. Some oF the invading
cells Further diFFerentiate into osteoblasts and begin to Form
spongy bone in the spaces previously housing the cartilage.
Fetal skeleton. (
) Note the stained bones of this
fourteen-week fetus. (
) Bones can fracture even before birth. This
fetus has numerous broken bones (arrows) because of an inherited
defect in collagen called osteogenesis imperfecta, which causes