Page 128 - Visions of the Future Chemistry and Life Science

Skeletal structure 117

forces morphogens

mesenchymal stem cells

osteogenesis chondrogenesis myogenesis ligamentagenesis

osteoblast chondroctye myoblast

osteocyte hypertrophic myotube ligament
chondrocyte fibroblast
bone cartilage muscle ligament

Figure 7.3. All skeletal tissues arise from a single cell type, the mesenchymal stem
cell. Differentiation into bone, cartilage, muscle, or ligament occurs in response to
the mechanical and biochemical stimuli of the stem cell’s environment.

osteoblasts. Upon activation, osteoblasts secrete osteoid, the organic extra-
cellular matrix into which mineral crystals are deposited. The organic
matrix contains 90 per cent collagen and a ground substance consisting of
large protein polysaccharides and a variety of matrix proteins. Gaps in the
collagen fibrils serve as mineral nucleation sites for calcium phosphate,
which forms the inorganic phase. The result is a composite of a ductile
polymeric (collagen) phase and a strong ceramic phase. This combination
gives bone its high mechanical strength and toughness.
Approximately 15 per cent of osteoblasts become entrapped in their
own matrix to become osteocytes. Osteocytes have a vast three-dimen-
sional network of cell processes (canaliculi), providing nourishment and
cell–cell interactions. Because they are located throughout bone tissue and
have an extensive canalicular network, osteocytes are assumed to be a vital
component of sensing mechanical signals. Nutrients are essential for the
vitality of bone tissue and are obtained from the blood supply, limiting
most osteocytes to lie within 150 m of a blood vessel, resulting in a high
cellular density: 25000 osteocytes within a square millimetre of bone