Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

THE EXTRACELLULAR MATRIX OF ANIMALS
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(A)
nidogen
perlecan
(B)
type IV
collagen
laminin
perlecan
nidogen
type IV collagen
laminin
plasma membrane
integrin
the basal lamina is important for this function: when its GAG chains are removed
by specific enzymes, the filtering properties of the lamina are destroyed. Type IV
collagen also has a role: in a human hereditary kidney disorder (Alport syndrome),
mutations in a type IV collagen gene result in an irregularly thickened and dysfunctional
glomerular filter. Laminin mutations, too, can disrupt the function of
the kidney filter, but in a different way—by interfering with the differentiation of
the cells that contact it and support it.
MBoC6 m19.43/19.54
The basal lamina can act as a selective barrier to the movement of cells, as
well as a filter for molecules. The lamina beneath an epithelium, for example, usually
prevents fibroblasts in the underlying connective tissue from making contact
with the epithelial cells. It does not, however, stop macrophages, lymphocytes, or
nerve processes from passing through it, using specialized protease enzymes to
cut a hole for their transit. The basal lamina is also important in tissue regeneration
after injury. When cells in tissues such as muscles, nerves, and epithelia are
damaged or killed, the basal lamina often survives and provides a scaffold along
which regenerating cells can migrate. In this way, the original tissue architecture
is readily reconstructed.
A particularly striking example of the role of the basal lamina in regeneration
comes from studies of the neuromuscular junction, the site where the nerve terminals
of a motor neuron form a chemical synapse with a skeletal muscle cell
(discussed in Chapter 11). In vertebrates, the basal lamina that surrounds the
muscle cell separates the nerve and muscle cell plasma membranes at the synapse,
and the synaptic region of the lamina has a distinctive chemical character,
Figure 19–53 A model of the molecular
structure of a basal lamina. (A) The
basal lamina is formed by specific
interactions (B) between the proteins
laminin, type IV collagen, and nidogen,
and the proteoglycan perlecan. Arrows
in (B) connect molecules that can bind
directly to each other. There are various
isoforms of type IV collagen and laminin,
each with a distinctive tissue distribution.
Transmembrane laminin receptors
(integrins and dystroglycan) in the
plasma membrane are thought to
organize the assembly of the basal lamina;
only the integrins are shown. (Based on
H. Colognato and P.D. Yurchenco, Dev.
Dyn. 218:213–234, 2000. With permission
from Wiley-Liss.)