Biomedical Engineering Reference
In-Depth Information
these signals are integral to their function. A gradient is also observed in
physical properties such as stiffness, porosity, and topology. A gradual transition
in physico-chemical and biological properties is required to connect these
mechanically mismatched tissues for smooth coordination of their functions.
The cues present in the ECM guide major cellular processes such as cell-cell
interaction, proliferation, migration, and differentiation. Cells in turn interact and
remodel the surrounding ECM to maintaintherespectivetissuephenotypeat
the interface.
Studies have also shown that tissue interfaces undergo gradual changes in their
properties with age and play important roles in diseases [ 8 ]. For example, the zone
of calcified cartilage forms the interface between cartilage and bone for transmit-
ting force, attaching cartilage to bone, and limiting diffusion of nutrients from
bone to the deep layers of cartilage [ 9 ]. The structure and the height of this
interface is a relatively constant percent of articular cartilage. The permeability
of the bone-cartilage interface to water and solutes varies with age [ 10 ].
Advancement of the calcified region toward the articular surface is observed
with age and has been associated with traumatic osteochondral defects [ 11 ]and
in diseases like osteochondritis dissecans (OCD) and osteoarthritis [ 12 ]thataffect
both the articular cartilage and bone. Damage to interfaces affects the function of
two integrated tissues and efforts to restore the interface using artificial implants
have not been successful to a great extent. Tissue engineering offers a promising
approach to recreate the interface using a combination of tissue-specific cells,
three-dimensional scaffolds, and biological signals. In vitro engineering of
these gradient interfaces requires creation of biomimetic materials with controlled
spatial and temporal features to direct cell response, tissue formation, and tissue
14.2 Major Tissue Interfaces
14.2.1 Cartilage-Bone Interface
Cartilage is a smooth, white, glistening tissue that lines diarthroidal joints and
helps in the very low friction movement of joints and in resisting compression. It
is a highly hydrated, avascular, aneural tissue, consisting of chondrocytes dis-
persed in an interpenetrating network of type II collagen and proteoglycans. Bone,
on the other hand, is a highly organized rigid connective tissue composed of
osteoclasts, osteocytes, and osteoblasts with abundant intercellular matrix in the
form of type I collagen fibers and stiffening inorganic substances. The articular
cartilage-bone transitional junction possesses a complex, zonal architecture that
varies in composition, structure, and biomechanical properties to allow for
smooth transition from articulation at the joint surface to rigid attachment at the
subchondral bone. This strong and stable interface, termed the “tidemark”, is
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