Biomedical Engineering Reference
In-Depth Information
5.2.3
Structure of Basement Membranes
Below, we briefly review the structure of the BM region in skin in order to follow
the details of the relevant synthetic processes. The terms “basal lamina” and “BM”
have been used interchangeably in the literature, leading to considerable confusion,
until it was realized that all three layers seen with the electron microscope repre-
sent the single layer (lamina densa or basal lamina) seen with the light microscope
(Fig.
5.1
, Burkitt et al. 1993). It has been recommended that the term
basal lamina
should be confined to its meaning as just one of the layers, i.e., lamina densa, as
originally employed (Martinez-Hernandez 1988; Burkitt et al. 1993).
As mentioned in an earlier chapter, the BM of an organ is an avascular, cell-free
tissue layer interspersed between a layer of avascular epithelia (tissues that cover or
line all body surfaces, cavities, and tubes) and a layer of stroma (vascularized con-
nective tissue, or “supporting” tissue). Several roles of BMs have been identified
in different organs. These include functions such as that of a boundary that restricts
transfer of cells and molecules (Farquhar 1981), an anchorage matrix for epithelial
cells and a mechanically competent adhesive-like layer that binds the epithelia to
the stroma (Furthmayr 1988; Uitto et al. 1996), a scaffold that facilitates tissue re-
pair after injury (Vracko 1974; Woodley and Briggaman 1988), as well as several
specialized roles during differentiation and growth (Hay 1981). Although subtle
variations in both composition and assembly of components have been observed
in BMs of different organs and species (Kefalides and Alper 1988; Breitkreutz
et al. 2009), there are strong similarities that appear to overshadow the differences
(Burkitt et al. 1993). There are significant thickness differences depending on ana-
tomical site or species (Furthmayr 1988). Also, the mature glomerular BM in the
kidney, as well as segments of the alveolar BM in the lung are three layered (trilami-
nar) (Martinez-Hernandez 1988). Furthermore, there is evidence that BM defects
that are lethal during development vary between tissues and organs (Breitkreutz
et al. 2013). However, there are strong similarities in composition between BMs of
species as different as
Drosophila
and mouse (Fessler et al. 1984). The BMs in skin
and peripheral nerves, the two organs that are treated in detail in this volume, are
very similar in composition and structure.
The first layer of the BM, 20-40 nm in thickness, is next to the cell membrane
of the innermost (basal cell) layer of the epithelia, and is the electron-lucent lamina
lucida that consists primarily of the glycoprotein laminin (Fig.
5.1
, bottom).The
intermediate layer is electron-dense, about 40-50 nm in thickness (lamina densa);
it consists primarily of type IV collagen. Adjacent to it is an electron-lucent reticu-
lar (fibroreticularis) layer that merges with the fibers of the underlying stroma; in
skin, this layer comprises fibers of type VII collagen (anchoring fibrils) that are
connected to the dermis by specific structures (anchoring plaques) (Briggaman and
Wheeler 1975; Carver et al. 1993b). Hemidesmosomes are discrete plaques inside
the layer of epithelial cells closest to the BM (basal cells); they serve to anchor the
basal cells to the BM by means of keratin filaments (tonofilaments) and by con-
nections to junctions in the BM (sub-basal plates) (Burkitt et al. 1993). Although
the BM is frequently described as consisting of three zones (lamina lucida, lamina
