Recruitment, development and cell expression
The formation of the neo-formed layer is a determining element of biological
activity. The neo-formed nanocrystals of apatite are extremely reactive and have the
property to bond specific bone proteins (osteopontin, osteocalcin, etc.) which later
promote adhesion, multiplication and cell expression. The formation processes of
this layer are, however, quite different from those that take place during primary
ossification, which can be described as the mineralization of a pre-formed collagenic
matrix. Furthermore, this layer does not have the characteristic organization of a
bone tissue with apatite crystals, well oriented with respect to the collagen fibers,
and should rather be considered as an uncontrolled mineralization. The events which
occur at the surface of bioactive minerals, however, have a certain analogy with
those which come into play during the bone remodeling in the cementing zone
which limits the bone osteons [MCK 93]. The same phenomena and the same
proteins seem to be involved. The neo-formed layer is favorable to the adhesion of
osteoblastic cells, which then multiply and form the collagenic matrix at first and
mineralize it later (see Figure 12.4). We must mention that the osteoblasts can also
attach and develop and form bone on materials which are not necessarily bioactive,
for example, cell culture dishes in polymer materials, normally more effective than
apatite substrate [HOT 97]. However, the chemical bond between the substratum
and the tissue is missing in these systems. To deserve the adjective bioactive, a
material should, at the same time, be able to establish a bond with the tissue and help
in its renewal.
The cells generally settle on a substratum by means of adhesive proteins
characterized by the presence of amino acids sequences (particularly the tripeptide
sequence arginine-glycine-aspartic acid (RGD)). On the other hand, the relationship
between secretion and/or adsorption of these proteins and the surface characteristics
remain unknown. The energy and the surface load appear to play a preponderant
role; thus, the proteins fixed on the substratum with different loads exhibit different
characteristics and can influence cell recruitment and development [SHE 88].
Generally, the positively charged substrate are more favorable for recruitment and
adhesion of negatively charged cells. Similarly, the surfaces which have a high
bipolar energy encourage cell development [HOT 97]. However, when over a period
508 Ceramic Materials
of time the protein film develops on the surface of the material, these initial factors
no longer have an effect on cell activity. It appears therefore that the kinetic factors
of the protein film formation and the competitive adsorption of different proteins,
probably determined by the characteristics of the surface, play a decisive role on cell
activity [DEW 99].
Figure 12.4. Photograph of cytoskeleton of an osteoblast
(after dissolution of the plasmic membrane) fixed on an HAP ceramic
12.3.2.3. Cell rehabitation
Some porous materials may be reoccupied by the cells and invaded by a newly
formed bone tissue. These materials should have at the beginning a definite pore
size. Very large pores do not encourage bone growth, while the very small ones do
not allow an adequate vascularization. The pore diameter should be a minimum of
100 µm, but could go up to 500 µm (see below). Moreover, tissue rehabitation
requires interconnected pores.
12.3.3. Biodegradation
Biological environments are particularly corrosive: they can generate relatively
acidic pH values, associated with the formation of oxide radicals (superoxides,
hydroperoxides) and molecules with strong chelating capabilities. These phenomena
may lead to a biodegradation of ceramics, at times desired and at times accidental.
Overall, biodegradation appears connected to the solubility of these ceramics. Thus,
very insoluble calcium phosphates, corresponding to stoechiometric apatites are non-
Bioceramics 509
biodegradable, while those that are relatively more soluble are biodegradable. It
seems that this general rule is equally valid for other types of ceramics.
We can distinguish between the phenomena of passive biodegradation not
involving cells and the phenomena of biodegradation activated by the cells.
المادة المعروضة اعلاه هي مدخل الى المحاضرة المرفوعة بواسطة استاذ(ة) المادة . وقد تبدو لك غير متكاملة . حيث يضع استاذ المادة في بعض الاحيان فقط الجزء الاول من المحاضرة من اجل الاطلاع على ما ستقوم بتحميله لاحقا . في نظام التعليم الالكتروني نوفر هذه الخدمة لكي نبقيك على اطلاع حول محتوى الملف الذي ستقوم بتحميله .
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