Biomedical Engineering Reference
In-Depth Information
deposit it in the bone matrix (the cartilage). As a result, layers of spongy bone
are formed around the original cartilage.
Later in development, spaces among the spongy bone are filled with bone
matrix and become compact bone. Osteoblasts continue to deposit calcium
supplements into the matrix until it is totally surrounded by bone. Once
this occurs, the osteoblasts are considered to be encased in a lacuna and
are now called osteocytes. The original connective cells first formed around
the network of blood vessels are now called the periosteum. Osteoblasts still
not isolated in lacunae can emerge from beneath the layer of compact bone
and form layers of spongy bone over compact bone. Examples of membra-
nous bones are bones of the skull and facial bones.
A.1.2 Cartilaginous Bones
Cartilaginous bones ossify in cartilage (intracartilaginous or endochondral
ossification) and are thus derived from preformed cartilaginous models.
Endochondral ossification forms bone by replacing a cartilaginous model,
or precursor, that appeared there earlier in embryonic development. The
cartilaginous models first undergo rapid changes as the connective tissue
cells enlarge, which in turn destroys the surrounding matrix. Soon after,
the connective tissue cells die.
While the cells disintegrate, a periosteum is formed on the outside of the
developing structure (a membrane with many blood vessels). Next, blood
vessels and undifferentiated cells raid into the disintegrating tissue. Certain
connective tissue cells differentiate and form spongy bone around the
previous template of cartilage. Examples of cartilaginous bone are the bones
of limbs, the vertebral column, and the thoracic cage. A defect in endochon-
dral ossification causes a common type of dwarfism called achondroplasia,
in which the limbs are short but the trunk is normal. It is transmitted as a
Mendelian dominant character.
A.1.3 Membrocartilaginous Bones
There is a third type of ossification, in which a bone ossifies partly from
membrane and partly from cartilage. These bones are known as membro-
cartilaginous bones. In the process of bone formation, thickness in cartilage
bones is achieved by intramembranous ossification. Just beneath the layer
of periosteum, yet above the newly developed spongy bone, compact bone
is formed and hardened with the help of osteoblasts filling portions of the
porous spongy bone with calcium phosphate crystals (apatite). Sometimes
compact bone is formed on the surfaces of existing bone tissue and must be
eroded away by specialized cells called osteoblasts. The crystals of apatite
extracted from the bone tissue are delivered to blood and tissues on demand.
Examples of this class of bone include the clavicle, mandible, occipital,
temporal, and sphenoid bones.
