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

BLOOD VESSELS, LYMPHATICS, AND ENDOTHELIAL CELLS
1235
Summary
Skeletal muscle fibers are one of four main categories of vertebrate cells specialized
for contraction, and they are responsible for all voluntary movement. Each skeletal
muscle fiber is a syncytium and develops by the fusion of many myoblasts. Myoblasts
proliferate extensively, but once they have fused, they can no longer divide. Fusion
generally follows the onset of myoblast differentiation, in which many genes encoding
muscle-specific proteins are switched on coordinately. Some myoblasts persist
in a quiescent state as satellite cells in adult muscle; when a muscle is damaged,
these cells are reactivated to proliferate and to fuse in order to replace the muscle
cells that have been lost. They are the stem cells of skeletal muscle, and exhaustion of
their regenerative capacity is responsible for some forms of muscular dystrophy as
well as for the decline of muscle mass in old age.
Blood Vessels, Lymphatics, and Endothelial
Cells
Almost all tissues depend on a blood supply, and the blood supply depends on
endothelial cells, which form the linings of the blood vessels. Endothelial cells
have a remarkable capacity to adjust their number and arrangement to suit local
requirements. They create an adaptable life-support system, extending by cell
migration into almost every region of the body. If it were not for endothelial cells
extending and remodeling the network of blood vessels, tissue growth and repair
would be impossible. Cancerous tissue is as dependent on a blood supply as is
normal tissue, and this has led to a surge of interest in endothelial cell biology,
in the hope that it may be possible to block the growth of tumors by attacking the
endothelial cells that bring them nourishment.
loose connective
tissue
smooth
muscle
basal lamina
elastic lamina
(elastin fibers)
lumen of
artery
endothelial lining
100 µm
Figure 22–21 Diagram of a small artery
in cross section. The endothelial cells
form the endothelial lining, which although
inconspicuous, MBoC6 is the m23.30/22.21 fundamental
component. Compare with the capillary in
Figure 22–22.
Endothelial Cells Line All Blood Vessels and Lymphatics
The largest blood vessels are arteries and veins, which have a thick, tough wall
of connective tissue and many layers of smooth muscle cells (Figure 22–21). The
inner wall is lined by an exceedingly thin single sheet of endothelial cells, the
endothelium, separated from the surrounding outer layers by a basal lamina. The
amounts of connective tissue and smooth muscle in the vessel wall vary according
to the vessel’s diameter and function, but the endothelial lining is always present.
In the finest branches of the vascular tree—the capillaries and sinusoids—the
walls consist of nothing but endothelial cells and a basal lamina (Figure 22–22),
together with a few scattered pericytes. Related to vascular smooth muscle cells,
pericytes wrap themselves around the small vessels and strengthen them (Figure
22–23).
basal lamina
nucleus of
endothelial cell
basal lamina
2 µm
lumen of
capillary
tight junction
red blood cell
Figure 22–22 Capillaries. Electron
micrograph (left) of a cross section of a
small capillary in the pancreas. The wall
is formed by a single endothelial cell
surrounded by a basal lamina, as seen
most clearly in the drawing to the right.
(From R.P. Bolender, J. Cell Biol. 61:269–
287, 1974. With permission from The
Rockefeller University Press.)