Chapter 2
2 The Dermal Blood Vasculature
2.1
General Architecture of Blood Vessels
To grasp the delicacy of the microvasculature of the
skin it is necessary to review the organization of the
body vasculature, and also the construction of arteries,
capillaries, and veins.
2.1.1
Arteries
Arteries are classified with respect to their caliber and
structure of their walls as:
• Large arteries of elastic type or conducting arteries
(arteria pulmonalis, aorta and its large branches)
• Medium-sized arteries of muscular type or distributing arteries (for example the radial artery)
• Small arteries including small unnamed arteries
and arterioles
Accordingly, arteries of elastic type give off
branches that become medium-sized muscular arteries, and these in turn give off small arteries, which end
up in arterioles of different sizes. Eventually arterioles
merge into capillaries. In consequence, the caliber of
an artery gradually decreases with increasing distance
from the heart, while the sum of the diameters of the
lumens of all of its branches greatly increases.
The wall of all arteries consists of three layers. These
are from inside out: tunica intima, tunica media, and
tunica adventitia. In all vessels the luminal surface is
covered with a single layer of endothelial cells, which
alone or together with a thin subendothelial sheet of
fibrous and elastic tissue forms the intima. Usually the
media is the thickest layer and its construction determines the type of artery. In arteries of elastic type both
the subendothelial layer of the intima and the media
are mainly built up of elastic membranes. In arteries of
muscular type the media consists mainly of concentric
layers of smooth muscle cells and the elastic tissue is
confined to two distinct elastic membranes: one internal beneath the endothelium and one external between
the media and adventitia. Depending on the size of
the vessel, the media of small arteries and arterioles is
composed of one or several layers of smooth muscle
cells. Small arteries and arterioles have only one elastic
membrane located directly below the endothelium. In
all types of arteries the adventitia consists mainly of
connective tissue, which merges with the connective
tissue of the neighborhood. The modification of vessels from large arteries to arterioles is insidious and
only gently progressive. It means that in each group
of vessels the difference between the largest and the
smallest is conspicuous.
2.1.2
Capillaries
Capillaries are tiny tubes composed of one layer of
endothelial cells, which on the abluminal surface is
covered with a basement membrane; between these
two laminae there is a discontinuous layer of pericytes, cells which are provided with long branching
processes and are able to contract. The caliber of the
capillary is on average 9–10 µm, just large enough to
permit the cellular components of the blood to pass.
The capillaries have one arterial and one venous part,
and in that way they connect the arterial and venous
system.
Under normal conditions not all capillaries are
open simultaneously. Arterioles act as functional
sphincters and control the flow through the capillary
bed with respect to the needs of the tissue of the area,
caused by, for example, muscular movements, regulation of the body temperature and digestion. However,
they also react to infections and other kinds of injury
(Sect. 5.2.1).
2.1.3
Veins
The blood in the capillary bed drains into venules, the
counterpart to arterioles, of the venous vascular system. Towards the heart the size of the veins gradually
increases, while the wall becomes thicker. Like arteries
they are categorized as large, medium-sized and small.
2.2 The Microvasculature of the Skin
Fig. 2.1 Arterioles at the dermal–subcutaneous interface. There
are only two ascending arterioles (arrows). Above and to the left
of the left arteriole there is a dilated small vein and to the right
there is a dilated lymphatic with valves. H&E, original magnification ×60
Usually veins accompany their corresponding arteries. The caliber and capacity of the vein are greater, but
the wall of the vein is thinner and less elastic than that
of the artery. Many medium-sized veins have valves,
which prevent flow away from the heart.
Also epithelioid smooth muscle cells (glomus cells) replace the subendothelial elastic membrane. The arterial part of the glomus then connects with a short thinwalled venule, which leaves the glomus and drains to
the subcutaneous or dermal venous plexus. Glomera
are mainly found in the nail bed, and finger and toe
pads, the central part of the face, and ears.
2.1.4
The Endothelium
The endothelium is a huge organ of the body with
many important properties. It has the capacity to control the passage of small and large molecules into the
arterial wall and through the walls of capillaries and
venules, to maintain the balance between coagulative
and fibrinolytic activities, to regulate the vascular tone
and blood flow, and to regulate immune and inflammatory reactions.
2.2
The Microvasculature of the Skin
Arteries in the skin belong to the group of small arteries, and are derived from vessels located below the
muscles. They penetrate the muscles and continue
through the subcutaneous connective tissue towards
the dermal–subcutaneous interface, where they appear as ascending arterioles.
In the deep dermis ascending arterioles give off
2.1.5
Arteriovenous Anastomoses
In addition to the connection between the arterial and
venous blood system at the capillary level there are in
many parts of the body arteriovenous anastomoses
at the level of small arteries and veins. The connecting branches are profusely innervated, have a special
architecture, and are provided with a contractile segment. When the segments are contracted the blood
passes through the capillaries; when they are relaxed
the blood is shunted directly into the venous system.
The arteriovenous anastomoses play an important role
in the regulation of body temperature.
Other temperature-regulating shunts are the abundantly innervated glomus structures (glomera) situated in the subcutaneous tissue and deep dermis. The
glomus, which has a connective tissue capsule, is penetrated by an arteriole. Inside the capsule the vessel
splits up into branches or becomes highly convoluted.
Fig. 2.2 Arterial vasculitis. A 4-mm punch biopsy specimen
shows two affected ascending arterioles (arrows) at the dermal–
subcutaneous border, one on each side of the specimen. The
structures above the left arrow are sweat glands. H&E, ×25
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2 The Dermal Blood Vasculature
branches, which create the deep vascular plexus. A
dense capillary network, originating form the deep
plexus, surrounds sweat glands and hair follicle bulbs.
In the mid-dermis ascending arterioles anastomose
with each other. On the way to the papillary dermis
they become progressively smaller, give rise to the
superficial plexus, and finally as terminal arterioles
transform into capillaries. Like the deep plexus, the
superficial plexus is arranged parallel to the skin surface. It gives rise to a capillary network around hair
shafts and sends capillary loops into the dermal papillae. Approximately one loop supplies one papilla
(Ryan 1983). The postcapillary venules drain into venules of the superficial plexus. Descending venules go
from the superficial plexus to the deep plexus, which
drains into veins in the subcutaneous tissue. Like arterioles, venules also anastomose. However, the venous
part of the microvasculature is more profuse than that
of the arterial part.
The dermal microvasculature has been studied by
electron and light microscopy (Yen and Braverman
1976; Braverman and Yen 1977; Higgins and Eady
1981). The walls of ascending arterioles, arterioles in
the superficial plexus, and descending venules contain
smooth muscle cells arranged in one or several layers. Ascending arterioles, but not terminal arterioles,
have an internal elastic membrane. In terminal arterioles, postcapillary venules and venules of the superficial plexus, muscle cells are replaced by a continuous
sheath of pericytes. Capillaries have a discontinuous
layer of pericytes. The capillary loop has three sections:
one extrapapillary ascending, one intrapapillary, and
one extrapapillary descending. The first two sections
have arterial, and the third venous, characteristics.
The main difference is that the arterial capillary has a
homogeneous basement membrane, while the venous
capillary has a multilayered one. This change appears
abruptly at the border between the papillary and subpapillary areas. Also there is a close relationship between the capillary loops and the overlying epidermis.
In areas of normal skin with high rete ridges the papillary loops are well developed and elongated, while in
areas with low ridges the loops are short. Regardless of
the length of the loop, the intrapapillary part is always
arterial in character (Braverman and Yen 1977).
In histologic sections of normal skin, vessels are
inconspicuous; also most observed vessels are venules. Figure 2.1 demonstrates the rather large space
between ascending arterioles at the dermal–subcutaneous interface. In this punch biopsy specimen the
distance between the lateral borders of the two present
arterioles is about 2.8 mm. In another specimen taken
with a knife, where three arterioles were identified, the
distance between each vessel was at least 3 mm. Consequently, a biopsy taken with a 4-mm punch will include at the most two ascending arterioles (Fig. 2.2);
using a 3-mm punch, affected vessels may be missed.
To identify elastic membranes in venules and small
arterioles it is necessary to use a special staining such
as vG-elastin, which stains the elastic membrane black.
Small arterioles in the upper dermis may be visualized
by immunohistochemical staining with antibodies
against smooth muscle α-actin or smooth muscle myosin heavy chain (Fig. 2.3). In routinely stained specimens thickened veins may be difficult or impossible
to differentiate from arterioles (Figs. 2.3e and 2.4c).
Medium-sized veins in the upper subcutis may have
valves (Fig. 2.4b).
References
1. Braverman IM, Yen A (1977) Ultrastructure of the human
dermal microcirculation. II. The capillary loops of the dermal papillae. J Invest Dermatol 68:44–52
2. Higgins JC, Eady RAJ (1981) Human dermal microvasculature. I. Its segmental differentiation. Light and electron
microscopic study. Br J Dermatol 104:117–129
3. Ryan TJ (1983) Cutaneous circulation. In: Goldsmith LA
(ed) Biochemistry and physiology of the skin. Oxford University Press, Oxford, pp 817–877
4. Yen A, Braverman IM (1976) Ultrastructure of the human
dermal microcirculation. The horizontal plexus of the papillary dermis. J Invest Dermatol 66:131–142
References
Fig. 2.3 Vessels in tissue with inflammatory changes. a A
small artery in which the internal elastic membrane is seen as
a pleated grayish band (arrow); H&E, ×200. b A small artery in
which the internal elastic membrane is seen as a distinct black
band (arrow); vG-elastin, ×200. c A small vein with several layers of disrupted elastic membranes; vG-elastin, ×200. d Arteri-
ole; vG-elastin, ×200. e A sclerotic venule is encircled by a thick
elastic membrane; vG-elastin, ×200. f Two venules and an arteriole (arrowhead); vG-elastin, ×100. g Small arteriole; H&E,
×200. h A small arteriole; vG-elastin, ×200. i Three small dilated
venules and two small arterioles (arrowheads); smooth muscle
α-actin, ×200
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2 The Dermal Blood Vasculature
Fig. 2.4 Vessels in tissue with inflammatory changes. a A
branching arteriole at the dermal–subcutaneous border (sole of
the foot); ×200. b A vein in the superficial part of the subcutis,
probably cut close to a junction with a smaller affluent branch.
The smaller vein contains a pleated valve (border of the foot);
×125. c In the lower dermis a thin-walled dilated venule drains
into a thick- walled one; ×200. d Dilated and winding venule in
the lower part of the dermis; H&E, ×100