The Cell–Surface Interaction - Tissue Engineering: Cell-Surface Interactions for Tissue Culture - page 23

(a) (b)

Fig. 6 The ability of a biomaterial to retain fibrin attachment during the retraction phase of
wound healing is crucial in determining if migrating cells will reach the device. Owing to the lack
of anchorage, smooth surfaces do not retain fibrin matrix during wound healing contraction. In
contrast, microrough surfaces withstand the forces of wound healing contraction, thereby
retaining the fibrin matrix, which supports continuous cell migration and direct tissue contact.
(Modified from 111 ])

supported by both in vitro [ 112 , 113 ] and in vivo [ 114 ] experiments. Initially,
differentiating osteogenic cells secrete a non-collagenous organic matrix con-
taining both osteopontin and bone sialoprotein, which serve as CaP nucleation
sites where crystals can grow in size. Concomitant with this growth at the
boundary is the introduction of collagen fibre assembly. Collagen is thus deposited
onto this layer, which subsequently mineralizes (Fig. 7 ), but is separated from the
substratum by a collagen-free calcified tissue layer (approximately 0.5 lm thick).
It is suggested that the presence of a heterogeneous population at the bone-implant
boundary and an afibrillar interfacial zone is comparable to cement lines and the
lamina limitans [ 115 ].
As previously outlined, the biopassivity of an implant is partly attributable to
the oxide layer. It is this layer, often only a few nanometres thick, which is crucial
to the application of metal devices in vivo. The major differences between the
oxide layers of EPSS and titanium and its alloys involve the actual thickness of the
layer and the chemistry, both of which result in evoking distinct biological
responses. This distinction is highlighted most poignantly by an early investigation
by Albrektsson and Hansson [ 116 ] which studied in much detail the ultrastructural
differences evoked by EPSS and titanium screws in rabbit bone. Specifically, they
reported a continuous one to two cell thick layer separating the EPSS device from
bone, whereas titanium had direct anchorage to the bone. These observations
are also supported by recent studies that have observed fibro-osseointegration
at the tissue-EPSS interface versus osseointegration at the titanium interface
[ 78 , 80 , 81 ]. Albrektsson and Hansson [ 116 ] also described the presence of
inflammatory cells adjacent to EPSS devices, and a proteoglycan coat void of
collagen filaments was observed within the interface. However, in contrast, tita-
nium had a proteoglycan layer at the interface with collagen bundles in close
proximity. The importance of this direct apposition of the proteoglycan layer on
titanium is believed to directly result in the accelerated osseointegration properties
of this material due to the enhanced degradation of the hyaluronan network which
is formed as a result of the wound healing response [ 117 ].