SCAFFOLD CONSIDERATIONS FOR OSTEOCHONDRAL TISSUE ENGINEERING - Intelligent Scaffolds for Tissue Engineering and Regenerative Medicine - page 785

Beneath the deep zone lies the calcified cartilage layer that forms
the interface between bone and cartilage both morphologically and
mechanically. Important to note is that cartilage is an avascular and
aneural tissue comprising only a single cell type that resides in an
oxygen-poorenvironment.
Bone itself is a complex tissue comprising several cell types
(osteoblasts, osteocytes, osteoclasts, and bone marrow cells) and
constantly undergoing remodeling. Osteoblasts are the bone-
forming cells that are responsible for laying down the organic
component of bone, which subsequently becomes mineralized.
During development and bone formation some of these cells
become trapped in their own matrix and differentiate further to
become osteocytes. These cells are responsible for bone mainte-
nance and also sensing of mechanical strain. Osteoclasts are the
bone-resorbing cells of the body and play an integral role in tissue
maintenance, working with the osteoblasts to remodel the bone in
response to mechanical signals or lack thereof. According to Wolff's
law, bones will adapt to the loads to which they are exerted, either
increasingordecreasingtheirdensityandabilitytowithstandloads
in a particular direction. 20 This is made possible by the inter-
play between the bone-forming osteoblasts and the bone-resorbing
osteoclasts. Several modifications to this law have been proposed
since, but the essence of it remains. 21 - 23 These same cells play a
role in the alterations in the subchondral bone of an osteoarthritic
or injured joint. Lying beneath the bone-cartilage interface is the
subchondral bone, which is comprised of a thin layer of dense cor-
tical bone. This bone is not heavily vascularized or porous com-
pared with the trabecular (spongy) bone directly beneath it. The
trabecular bone is filled with marrow and penetrated with blood
vessels. Mechanically, bone is much stiffer than cartilage, with vary-
ing reports on the Young's moduli of the different bone regions
but all in the giga-Pascal range. Cartilage by contrast measures in
the tens or hundreds of mega-Pascals (for review see Ref. 24). The
differences in mechanical properties enable cartilage to withstand
and dampen impacts and to transfer the load to the underlying
bone. Both cartilage and bone have a critical role to play in this
process.