330
Cell Junctions, Structure, Function, and Regulation
Tab. 3
Tissue distribution of laminin chains. (Adapted from Patarroyo et al. (2002)
Seminars in
Cancer Biol.
,
12
, 197–207).
Laminin chain
Tissue distribution
α
1
Early embryo, neuroretina, kidney proximal tubes, salivary, and mammary
glands
α
2
Skeletal and cardiac muscle, peripheral nerve, a few capillaries,
trophoblast, brain, and other tissues
α
3
Skin and most other epithelia
α
4
Fetal skeletal muscle, cardiac and visceral smooth muscle, nerve, blood
vessel endothelia, bone marrow, and other tissues
α
5
Diverse epithelia, kidney, blood vessels, bone marrow, developing muscle
and nerve, synaptic BM
β
1
Most tissues
β
2
Neuromuscular junction, blood vessels, kidney glomerulus
β
3
Skin and most other epithelia
γ
1
Most tissues
γ
2
Skin and most other epithelia
γ
3
Kidney, lung, reproductive tract, nerve, and brain
subunit containing laminins play a crit-
ical role in this fltration Function. Mice
with a null mutation in the laminin
β
2
gene maintain high levels oF expression oF
the
β
1
laminin chain; nonetheless, these
mice die aFter birth because oF massive
proteinuria, because oF the Failure oF the
glomerular basement membrane to Func-
tion as a fltration barrier. Thus, laminins
play isoForm-specifc roles in tissue diFFer-
entiation and Function.
1.2.3
Other Extracellular Matrix Proteins
In addition to collagens and laminins,
there are other ECM proteins that play im-
portant roles in regulating cell behavior, in-
cluding fbronectin and fbrinogen/fbrin.
±ibronectin
is
a
large
disulfde-linked
dimer that is present in a soluble Form
in blood. ±ibronectin is also secreted by
fbroblasts and other cells into the ECM
where it Forms fbrils associated with the
cell surFace. Like laminin-1, fbronectin
is essential For early vertebrate develop-
ment. Characterization oF fbronectin-null
embryos indicated a requirement For f-
bronectin in the development oF meso-
dermal structures, blood vessels, and the
heart. ±ibronectin also plays an important
role in wound healing in the adult. During
cutaneous wound healing, a provisional
matrix is Formed From plasma proteins
released From injured vessels. These pro-
teins include fbronectin and fbrinogen.
±ibrinogen is proteolytically processed at
the site oF blood vessel injury to Form a
fbrin clot. This provisional matrix sup-
ports the Formation oF new blood vessels,
the proliFeration oF fbroblasts and the
reepithelialization oF the wound. The pro-
visional matrix is eventually replaced by
the normal ECM oF the skin.
In summary, extracellular matrices are
essential to tissue and organ structure
and development not only because oF
their scaFFolding Function but also be-
cause they provide important regulatory
inFormation
that
guides
cell
behavior
and diFFerentiation. As discussed in the
next
sections,
much
oF
this
inForma-
tion is relayed by integrin receptors that
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