Vascular
Histology And Smooth Muscle Cell Ultrastructure
Larger blood
vessels share a common three-layered structure. Figure 4a illustrates the
arrangement of these layers, or tunics, in a muscular artery.
A thin inner layer, the tunica intima, comprises an endothelial
cell monolayer (endothelium) supported by connective tissue. The
endothelial cells lining the vascular lumen are sealed to each other by tight junctions,
which restrict the diffusion of large molecules across the endothelium. The
endothelial cells have a crucial role in controlling vascular permeability,
vasoconstriction, angiogenesis (growth of new blood vessels) and regulation of
haemostasis.
The intima is relatively thicker in larger arteries, and contains some
smooth muscle cells in large and medium-sized arteries and veins.
The thick middle layer, the tunica media, is separated from the
intima by a fenestrated (perforated) sheath, the internal elastic lamina,
mostly composed of elastin. The media contains smooth muscle cells embedded
in an extracellular matrix (ECM) composed mainly of collagen, elastin
and proteoglycans. The cells are shaped like elongated and irregular spindles
or cylinders with tapering ends, and are 15–100 µm orientated circularly or in a low-pitch spiral, so that the
vascular lumen narrows when they contract. Individual cells are long enough to
wrap around small arterioles several times.
Adjacent smooth muscle cells form gap junctions. These are areas
of close cellular contact in which arrays of large channels called connexons
span both cell membranes, allowing ions to flow from one cell to another.
The smooth muscle cells therefore form a syncytium, in which
depolarization spreads from each cell to its neighbours.
An external elastic lamina separates the tunica media from the
outer layer, the tunica adventitia. This contains collagenous tissue
supporting fibroblasts and nerves. In large arteries and veins, the adventitia
contains vasa vasorum, small blood vessels that also penetrate into the
outer portion of the media and supply the vascular wall with oxygen and
nutrients.
These three layers are also present in the venous system, but are less
distinct. Compared with arteries, veins have a thinner tunica media containing
a smaller amount of smooth muscle cells, which also tend to have a more random
orientation.
The protein elastin is
found mainly in the arteries. Molecules of elastin are arranged into a network
of randomly coiled fibres. These molecular ‘springs’ allow arteries to expand
during systole and then rebound during diastole to keep the blood flowing
forward. This is particularly important in the aorta and other large elastic
arteries, in which the media contains fenestrated sheets of elastin separating
the smooth muscle cells into multiple concentric layers (lamellae).
The fibrous protein collagen is present in all three layers of the
vascular wall, and functions as a framework that anchors the smooth muscle
cells in place. At high internal pressures, the collagen network becomes very
rigid, limiting vascular distensibility. This is particularly important in
veins, which have a higher collagen content than arteries.
Exchange vessel structure
Capillaries and postcapillary venules are tubes formed of a single layer
of overlapping endothelial cells. This is supported and surrounded on the
external side by the basal lamina, a 50-100 nm layer of fibrous proteins
including collagen, and glycoproteins. Pericytes, isolated cells that
can give rise to smooth muscle cells during angiogenesis, adhere to the outside
of the basal lamina, especially in postcapillary venules. The luminal side of
the endothelium is coated by glycocalyx, a dense glycoprotein network
attached to the cell membrane.
There are three types of capillaries, and these differ in their locations
and permeabilities. Their structures are illustrated in Chapter 20.
Continuous capillaries occur in skin, muscles, lungs and the
central nervous system. They have a low permeability to molecules that cannot
pass readily through cell membranes, owing to the presence of tight junctions
which bring the overlapping mem- branes of adjacent endothelial cells into
close contact. The tight junctions run around the perimeter of each cell,
forming a seal restricting the
paracellular flow of molecules of molecular weight (MW) >10 000. These
junctions are especially tight in most capillaries of the central nervous
system, and form an integral part of the
blood–brain barrier (see Chapter 20).
Fenestrated capillaries are much more permeable than continuous
capillaries. These are found in endocrine glands, renal glomeruli, intestinal
villi and other tissues in which large amounts of fluid or metabolites enter or
leave capillaries. In addition to having leakier intercellular junctions, the
endothelial cells of these capillaries contain fenestrae, circular pores
of diameter 50–100 nm spanning areas of the cells where the cytoplasm is
thinned. Except in the renal glomeruli, fenestrae are usually covered by a thin
perforated diaphragm.
Discontinuous capillaries or sinusoids are found in liver,
spleen and bone marrow. These are large, irregularly shaped capillaries with
gaps between the endothelial cells wide enough to allow large proteins and even
erythrocytes to cross the capillary wall.
Smooth muscle cell ultrastructure
The cytoplasm of vascular smooth muscle cells contains thin actin and
thick myosin filaments (Figure 4b). Instead of being aligned into
sarcomeres as in cardiac myocytes, groups of actin filaments running roughly
parallel to the long axis of the cell are anchored at one end into elongated dense
bodies in the cytoplasm and dense bands along the inner face of the
cell membrane. Dense bodies and bands are linked by bundles of intermediate
filaments composed mainly of the proteins desmin and vimentin to
form the cytoskeleton, an internal scaffold giving the cell its shape.
The free ends of the actin filaments interdigitate with myosin filaments. The
myosin crossbridges are structured so that the actin filaments on either side of
a myosin filament are pulled in opposite directions during crossbridge cycling.
This draws the dense bodies towards each other, causing the cytoskeleton, and
therefore the cell, to shorten. The dense bands are attached to the ECM by
membrane-spanning proteins called integrins, allowing force development
to be distributed throughout the vascular wall. The interaction between the ECM
and integrins is a dynamic process which is affected by forces exerted on the
matrix by the pressure inside the vessel. This allows the integrins, which are
signalling molecules capable of influencing both cytoskeletal structure and
signal transduction, to orchestrate cellular responses to changes in pressure.
The sarcoplasmic reticulum (SR, also termed smooth endoplasmic
reticulum) occupies 2–6% of cell volume. This network of tubes and flattened
sacs permeates the cell and contains a high concentration
(∼0.5
mmol/L) of free Ca2+. Elements of the SR closely approach the cell
membrane. Several types of Ca2+-regulated ion channels and transporters are
concentrated in these areas of the plasmalemma, which may have an important
role in cellular excitation.
The nucleus is located in the central part of the cell. Organelles
including rough endoplasmic reticulum, Golgi complex and mitochondria are
mainly found in the perinuclear region.
