Bone and Cartilage – its Structure and Physical Properties - Biomechanics of Hard Tissues - page 36

• Osteoblasts are the bone-forming cells. They secrete osteoid, which forms the
bone matrix. They also begin mineralization. Osteoblasts arise from osteopro-
genitor cells located in the periosteum and the bone marrow. Osteoblasts, when
entombed within the osteoid, become osteocytes, with cytoplasmic processes that
communicate with each other.
• Osteocytes are the mature osteoblasts that no longer secrete matrix, but being sur-
rounded by it maintain metabolism, and participate in nutrient/waste exchange
via blood. Osteoblasts and osteocytes develop in the mesenchyme. (Mesenchyme
is the meshwork of embryonic CT in the mesoderm from which the CTs of the
body and the blood and lymphatic vessels are formed.)
• Osteoclasts function in the resorption and degradation of the existing bone; in
this role, they are the opposite of osteoblasts. Monocytes (white blood cells) fuse
together to create these huge cells, which are concentrated in the endosteum.
Osteoclasts play a key role in bone remodeling: they destroy bone cells and
reabsorb calcium.
• Bone lining cells are essentially inactive osteoblasts; they cover all of the available
bone surface and function as a barrier for certain ions [157, 158].
Bone is a dynamic tissue. It is constantly being reshaped by osteoblasts, which
build bone, and osteoclasts, which resorb bone. An osteoblast (Gr. bone and germ) is
a mononucleate cell, responsible for bone formation. Osteoblasts produce osteoid,
which is composed mainly of Type I collagen, and osteoblasts are responsible for
mineralization of the osteoid matrix. Osteoblast cells tend to decrease as individuals
become older, thus decreasing the natural renovation of the bone tissue, cf. [158,
159].
Osteocytes are networked to each other via long cytoplasmic extensions that
occupy tiny canals called canaliculi , which are used for exchange of nutrients
and waste. Hence, osteocytes in vivo possess a distinctive morphology - that of
dendricity - connecting osteocyte to osteocyte creating the osteocyte syncytium
and also connecting osteocytes with cells on the bone surface. It is thought that
bone fluid surrounding the dendrite within the canaliculi is responsible for the
transmission of mechanical strain through fluid flow shear stress. Dendrites may
be essential for osteocyte function, viability, and response to load [160-162].
Osteocytes, dendritic or star-shaped cells, are the most abundant cells found in a
compact bone, cf. Figures 1.22 and 1.23. There are about 10 000 cells per cubic
millimeter and 50 processes per cell.
Cell contains a nucleus and a thin ring of cytoplasm. When osteoblasts get trapped
in the matrix they secrete, they become osteocytes. The space that an osteocyte
occupies is called a lacuna . Although osteocytes have reduced synthetic activity and,
like osteoblasts, are not capable of mitotic division, they are actively involved in the
routine turnover of bony matrix, through various sensory mechanisms. They de-
stroy the bone through a rapid, transient (relative to osteoclasts) mechanism called
osteocytic osteolysis . Hydroxyapatite, calcium carbonate, and calcium phosphate are
depositedaroundthecell.
Interesting images of osteocyte lacuno-canalicular network have been obtained.
For example, in [160] one can see morphology of osteocytes, osteoblasts, and