Embryologic, Histologic, and Anatomic Aspects
Blood vessels in the skin of an embryo come into being when mesenchymal cells arranged in an intricate network arborize and encircle conduits that are seen through a microscope as mere spaces. The dermal vasculature consists of a superficial and a deep plexus of arterioles and venules, those plexuses being connected to one another by communicating vessels (Figs. 1.72 and 1.73) that arise from arteries and lead eventually to veins that reside within septa of the subcutaneous fat. The deep plexus of arterioles and venules is situated in the lower part of the reticular dermis, and the superficial plexus is positioned in the upper part of the reticular dermis, just beneath the papillary dermis. In times past, the superficial plexus was called the “subpapillary plexus.” Those two plexuses, the superficial and the deep, lie parallel to the skin surface, whereas communicating vessels are oriented perpendicular to the plexuses and to the surface of the skin. From the superficial plexus, an arcade of capillaries loops into each dermal papilla. Those capillaries can be demonstrated strikingly by stains that employ antibodies against antigens expressed by endothelial cells or by stains that reveal alkaline phosphatase in endothelial cells (Fig. 1.74). A capillary loop within a dermal papilla consists of an ascending arterial component that takes a hairpin turn at its crest to become a descending venous limb. The venous portion of a capillary loop empties into postcapillary venules of the superficial plexus and, successively, into dermal communicating venules, the large venules of the deep plexus, and the small veins of the subcutaneous fat. But capillaries are not limited to the dermal papillae; they envelop both epithelial structures of adnexa (folliculosebaceous-apocrine units and eccrine units) and nonepithelial structures of adnexa (smooth muscles, nerves, and medium-sized and larger blood vessels).
The dermal vasculature consists of a network of two plexuses that parallel the skin surface. The superficial plexus is situated in the upper part of the reticular dermis (subpapillary plexus), whereas the deep plexus is positioned in the lower part of the reticular dermis. Superficial and deep plexuses are connected by communicating blood vessels that are oriented perpendicular to those plexuses. Capillaries emanate from the vascular plexuses to supply the papillary-periadnexal (adventitial) dermis and the epithelial and nonepithelial structures of adnexa housed in the reticular dermis.
Schematic representation of the vasculature of the dermis and subcutaneous fat.
Arcades of capillaries from vessels of the superficial plexus loop into dermal papillae. (Alkaline phosphatase preparation.) (Courtesy of William Montagna, Ph.D.)
The superficial and deep vascular plexuses anastomose richly throughout the dermis. The arborizing nature of the cutaneous circulation can be demonstrated exquisitely by staining for activity of alkaline phosphatase in endothelial cells (Figs. 1.75 and 1.76).
Numerous capillaries of the adventitial dermis envelop sebaceous and eccrine glands. (Alkaline phosphatase preparation.) (Courtesy of William Montagna, Ph.D.)
Many capillaries in the perifollicular adventitial dermis supply blood to follicles. (Alkaline phosphatase preparation.) (Courtesy of William Montagna, Ph.D.)
Arteriovenous anastomoses, known as Sucquet-Hoyer canals, are present in skin of the digits especially (Fig. 1.77). Those specialized segments are surrounded by three to six rows of uniform, ovoid, specialized smooth muscle (glomus) cells that serve as sphincters. Each of these shunts, known as a “glomus,” enables blood to bypass the capillaries, thereby accelerating blood flow through skin on acral sites and conserving the core temperature of the body, a boon in frigid weather.
Glomus cells surround arterioles and venules of specialized shunts known as Sucquet-Hoyer canals. (x187)
Arteries in the subcutaneous fat and larger arterioles in the deep part of the dermis consist of three distinct layers: (1) an intima formed of endothelial cells and scant connective tissue, (2) a media that contains collagen, elastic fibers, and several concentric layers of smooth muscle cells which, in arteries, are bounded distally by an external elastic membrane, and (3) an adventitia composed of fibrocytes, collagen bundles, and elastic fibers. In arteries, an internal elastic membrane separates intima from media. Arterioles in the superficial dermis do not possess an internal or external elastic membrane; their wall harbors a discontinuous layer of elastic fibers and smooth muscle. The ascending arterial segment of a capillary loop in a dermal papilla is seen by electron microscopy to consist of a single layer of endothelial cells that line a narrow lumen, those cells being surrounded by a layer of pericytes (Fig. 1.78). The endothelial cells and pericytes are enveloped by a basal lamina. In the descending venous segment of a capillary loop, the lumen becomes wider, the pericytes become more numerous, and the basal lamina becomes multilayered. The wall of vessels that serve as channels for drainage of venules becomes thicker as the vessels themselves progress from postcapillary venules to subcutaneous veins. Postcapillary venules resemble capillaries, their wall consisting of endothelial cells, pericytes, and basal lamina and being surrounded by a thin zone made up of collagen bundles. The wall of larger venules contains smooth muscle and elastic fibers in variable amounts, but no elastic membrane. Large venules and veins of the lower extremities have valves. Some large veins actually possess an internal elastic membrane.
Cross section through a capillary in the adventitial dermis. Parts of five endothelial cells are depicted, in two of which nuclei (E) are seen. Basal lamina is observed to surround endothelial cells and a pericyte (P). Within the lumen (L) is a platelet (Pl). (x9000) (Courtesy of Helmut Wolff, M.D.)
All arterioles, capillaries, and venules in the dermis are encircled by flat adventitial cells termed “veil cells.” Those cells, whose exact nature is still shrouded, are situated entirely outside the wall of a vessel and demarcate it from the surrounding dermis. Veil cells sometimes are seen around small vessels in the subcutaneous fat.
Through the permeable wall of capillaries and venules, oxygen, water, nutrients, and hormones are delivered from the blood to the tissues, and carbon dioxide and other metabolic byproducts are transported from it to excretory organs. Postcapillary venules, the predominant type of vessel in the upper part of the dermis, are the most permeable of the cutaneous vasculature and are the site of changes pathologic in a variety of cutaneous inflammatory diseases, the stereotypical example being leukocytoclastic vasculitis. In a postcapillary venule, pericytes and endothelial cells make contact with one another at many points along the surface of it. A single pericyte forms tight junctions with two to four subjacent endothelial cells through breaks in a multilaminate basement membrane. These points of contact between pericytes and endothelial cells are replete with fibronectin. Pericytes are contractile cells that help to create gaps in postcapillary venules during inflammatory processes. Vasoactive substances, e.g., prostaglandins and histamine, released by leukocytes, mast cells, and platelets, increase permeability of venules by inducing endothelial cells and pericytes to contract, widening of intercellular spaces being the result. This process favors deposits of immune complexes within the wall of postcapillary venules in conditions such as leukocytoclastic vasculitis, and causes serum and inflammatory cells to move into surrounding connective tissue. Also relevant to aspects pathophysiologic of inflammatory processes, particularly in regard to vasodilation and vasoconstriction, is the ability of vascular endothelium to secrete biologically active substances, such as prostaglandins, plasminogen activator, angiotensin-converting enzyme, and Factor VIII.
The red color of eruptions in the skin of Caucasians is imparted by blood that flows through widely dilated vessels in the upper part of the dermis. Papules of many acute inflammatory diseases result, in large measure, from edema that comes into being because of an imbalance between leakage of serum into tissues and reabsorption of it by lymphatics. A cutaneous vasculature that functions properly is crucial, too, to resolution of inflammatory reactions and restitutio ad integrum.