Biomedical Engineering Reference
In-Depth Information
instrumental in controlling the amount of bone tissue: Osteoblasts
form bones and osteoclasts resorb bones. Osteoclasts are formed by
the fusion of cells of the monocyte-macrophage cell line (monocytes
are a type of white blood cell). Osteoclasts are regulated by several
hormones, including PTH from the parathyroid gland, calcitonin
from the thyroid gland, and growth factor interleukin 6 (IL-6). The
last hormone, IL-6, is one of the factors in the disease osteoporosis,
which is an imbalance between bone resorption and bone forma-
tion. Osteoclast activity is also mediated by the interaction of two
molecules produced by osteoblasts: osteoprotegerin and the RANK
ligand. Note that these molecules also regulate differentiation of the
osteoclast [1].
• Osteogenics. Osteogenic cells are responsible for trauma response.
They are found in the bone tissue, which contacts endosteum and
periosteum. These cells are also responsible for the healing pro-
cess starting when trauma is experienced. Osteogenic cells heal by
calling on other cells, such as osteoblasts (bone-forming cells) and
osteoclasts (bone-destroying cells). When they all work together,
they repair damage to the bones. It is noted that osteoblasts arise
from osteoprogenitor cells located in the deeper layer of periosteum
and the bone marrow. Osteoprogenitors are immature progenitor
cells that express the master regulatory transcription factor Cbfa1/
Runx2. Osteoprogenitors are induced to differentiate under the
influence of growth factors, in particular the bone morphogenetic
proteins (BMPs) [2]. Aside from BMPs, other growth factors includ-
ing the fibroblast growth factor (FGF) [2], the platelet-derived growth
factor (PDGF), and the transforming growth factor beta (TGF-β) may
promote the division of osteoprogenitors and potentially increase
osteogenesis.
• Bone lining. Lining cells are a type of bone cell that creates the lining
of bones. These cells come from osteoblasts. Cell lining can be found
in the surface of adult bones. Lining cells regulate calcium and
phosphate in bones. These minerals must be regulated to maintain a
healthy mineral content.
It should be mentioned that bone surfaces that are not under remodeling
or modeling are covered by elongated, thin cells. These bone lining cells are
thought to be either inactive osteoblasts, which perhaps can be activated to
produce bone matrix, or a cell type of its own. In Gardner staining, osteo-
cytes are seen as a row of nuclei along bone surfaces, since their cytoplasm
does not contain many cell organelles and is therefore only weakly stained
in this procedure.
An important aspect concerning bone lining cells is that the retraction
or removal of these cells is a necessary step in starting osteoclastic bone
