Biomedical Engineering Reference
In-Depth Information
calcium, built outward from the marrow. Bone is a dynamic tissue
that is constantly being reshaped by osteoblasts, in charge of pro-
ducing matrix, mineral, and osteoclasts, which remodel the tissue.
The number of osteoblast cells tends to decrease with age, affecting
the balance of formation and resorption in bone tissue.
• Osteocytes. Osteocytes are mature osteoblasts, but they have a dif-
ferent function altogether. Some osteoblasts turn into osteocytes dur-
ing the process of bone formation. Osteocytes situated deep in the
bone matrix maintain contact with newly incorporated osteocytes in
osteoid, and with osteoblasts and bone lining cells on bone surfaces,
through an extensive network of cell processes (canaliculi). They are
thought to be ideally situated to respond to changes in physical forces
upon bones and to transmit messages to the osteoblastic cells on
the bone surface, directing them to initiate resorption or formation
responses. They also maintain metabolism and participate in nutri-
ent and waste exchanges through the blood. In general, an osteo-
cyte cell contains a nucleus and a thin ring of cytoplasm. When
osteoblasts become 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 osteo-
blasts, are not capable of mitotic division, they are actively involved
in the routine turnover of bony matrix through various mechano-
sensory mechanisms. They destroy bone through a rapid, transient
(relative to osteoclasts) mechanism called osteocytic osteolysis.
Osteoblasts/osteocytes develop in mesenchyme. Hydroxyapatites,
calcium carbonates, and calcium phosphates are deposited around
the osteocytes.
• Osteoclasts. Osteoclasts were discovered by Kolliker in 1873 [1]. An
osteoclast is a large cell, 40 μm in diameter. Osteocytes contain 15-20
closely packed oval-shaped nuclei. They are found in pits in a bone
surface, called resorption bays, or Howship's lacunae (a groove or
cavity, usually containing osteoclasts, that occurs in bone under-
going reabsorption). They are large multinucleated cells, like mac-
rophages, derived from the hematopoietic lineages. Osteoclasts
function in the resorption of mineralized tissue and are found
attached to the bone surface at sites of active bone resorption. Their
characteristic feature is a ruffled edge, where active resorption takes
place with the secretion of bone-resorbing enzymes, which digest
bone matrix. They are the cells responsible for the breakdown of
bone tissue. In particular, they release calcium and are much larger
than other bone cells. Osteoclasts release an enzyme that destroys
bone tissue when deemed necessary. These cells are an important
part of bone growth and bone repair, and they also help in rebuilding
bone when it is damaged. Osteoclasts and osteoblasts are together
