Biomedical Engineering Reference
In-Depth Information
functionally adequate epithelialization of tracheal scaffolds is of critical importance
for the biofunctionality of implants and constructs produced following the princi-
ples of tissue engineering. The research on mechanisms of regeneration and
differentiation of respiratory epithelial cells in contact with tissue-engineered con-
structs started only recently. Before that, the research concerning the differentia-
tion mechanisms of respiratory epithelial cells was focused on their differentiation
in the embryonic phase [76] and on the development and differentiation of epithe-
lial cells from precursor/stem cells [77]. It was shown that basal cells of the human
trachea probably are precursors of respiratory epithelial cells [77, 78]. The tracheal
epithelium is mainly composed of ciliary cells, goblet cells, and basal cells [79-81].
Basal cells are essential for the generation of precursor cells which are fundamen-
tal for the regeneration of epithelial damage [77, 78, 82-84].
Nomoto
et al
. seeded the scaffold material used by Omori with tracheal epithelial
cells of rats
in vitro
. These epithelial cells expressed
in vitro
the cytokeratines 14
and 18 as typical intermediate fi laments of epithelial cells as well as occludin, a
constituent of tight junctions in epithelial cells which is a main component of the
barrier against diffusion of soluble substances into the intercellular space. The
cell-seeded scaffolds were applied for the reconstruction of cervical tracheal defects
of 3 mm length in rats. Over the whole period of observation (30 days)
in vivo
, the
artifi cial trachea was covered with epithelium. Partially, a single- or double-layered
epithelium was found not carrying cilia, whereas other parts displayed prismatic
epithelial cells with functional cilia [85]. In a further development of this tech-
nique, a thin collagen matrix (Vitrigel
) was applied for 3D growth of cells in the
scaffold. This 3D matrix enhanced the growth of epithelial cells as well as the
invasion of mesenchymal cells. There was a clearly accelerated regeneration of
functional epithelial cells carrying cilia after tracheal reconstruction in rats using
Vitrigel-coated scaffolds compared to noncoated scaffolds [86].
The importance of epithelial-mesenchymal interactions for morphogenesis,
homeostasis, and regeneration of the epithelium are well known from literature
since several years [87-89]. During epithelial regeneration, epithelial precursors
arrived from the borders of epithelial damage to proliferate and differentiate there.
Mesenchymal cells situated below the epithelium regulate epithelial growth and
differentiation through generation of an appropriate biomatrix and through syn-
thesis and release of growth relevant factors [90, 91]. Fibroblasts are also important
participants in the interactions between epithelial and mesenchymal cells and
strongly infl uence epithelial regeneration in wound healing. They are able to
secrete a variety of growth factors like keratinocyte growth factor, epidermal
growth factor, and hepatocyte growth factor [92, 93]. The importance of fi broblasts
was shown already for epidermal wound healing [93], oral [94] and corneal epithe-
lial regeneration [95], and also for tracheal epithelial regeneration [96]. The cocul-
tivation of epithelial cells and tracheal fi broblasts
in vitro
induced the generation
of a layered epithelium containing epithelial cells with cilia, goblet cells, and basal
cells. Moreover, a basal membrane was constituted
in vitro
between epithelial cells
and fi broblasts where the presence of integrin-
4 was demonstrated, which is a
specifi c marker of basal membranes and of epithelial mucin secretion [96].
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