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
1069
MUSCLE
basal lamina
connective tissue
EPITHELIUM
LUMEN OR
EXTERNAL SURFACE
KIDNEY GLOMERULUS
BLOOD
endothelial cell
URINE
basal lamina
muscle cell plasma membrane
connective tissue
basal lamina
epithelial cell
and Schwann cells (which wrap around peripheral nerve cell axons to form
myelin). The basal lamina thus separates these cells and epithelia from the underlying
or surrounding connective tissue and forms the mechanical connection
between them. In other locations, such as the kidney glomerulus, a basal lamina
lies between two cell sheets and functions as a selective filter (Figure 19–50).
Basal laminae have more than simple structural and filtering roles, however. They
are able to determine cell polarity; influence cell metabolism; organize the proteins
in adjacent plasma membranes; promote cell survival, proliferation, or dif-
MBoC6 m19.39/19.51
ferentiation; and serve as highways for cell migration.
The mechanical role is nevertheless essential. In the skin, for example, the epithelial
outer layer—the epidermis—depends on the strength of the basal lamina
to keep it attached to the underlying connective tissue—the dermis. In people
with genetic defects in certain basal lamina proteins or in a special type of collagen
that anchors the basal lamina to the underlying connective tissue, the epidermis
becomes detached from the dermis. This causes a blistering disease called
junctional epidermolysis bullosa, a severe and sometimes lethal condition.
Figure 19–50 Three ways in which
basal laminae are organized. Basal
laminae (yellow) surround certain cells
(such as skeletal muscle cells), underlie
epithelia, and are interposed between two
cell sheets (as in the kidney glomerulus).
Note that, in the kidney glomerulus, both
cell sheets have gaps in them, and the
basal lamina has a filtering as well as a
supportive function, helping to determine
which molecules will pass into the urine
from the blood. The filtration also depends
on other protein-based structures, called
slit diaphragms, that span the intercellular
gaps in the epithelial sheet.
Laminin and Type IV Collagen Are Major Components of the Basal
Lamina
The basal lamina is synthesized by the cells on each side of it: the epithelial cells
contribute one set of basal lamina components, while cells of the underlying bed
of connective tissue (called the stroma, Greek for “bedding”) contribute another
set (Figure 19–51). Although the precise composition of the mature basal lamina
varies from tissue to tissue and even from region to region in the same lamina, it
epithelial cells
basal lamina
collagen
10 µm
Figure 19–51 The basal lamina in
the cornea of a chick embryo. In this
scanning electron micrograph, some of
the epithelial cells have been removed to
expose the upper surface of the matlike
basal lamina. A network of collagen fibrils in
the underlying connective tissue interacts
with the lower face of the lamina. (Courtesy
of Robert Trelstad.)