The cutaneous vasculature is divided into a superficial, a deep, and a subcutaneous vascular plexus. The superficial vascular plexus is formed by parallel pairs of arterioles and venules connected via capillary loops that extend into the dermal papillae. These segments may individually or conjointly respond to exogenous or endogenous stimuli, thereby influencing skin disease expression.
Skin microvessels are formed from an endothelial cell lining that is supported by mural cells that are pericytes in most of the microvasculature but smooth muscle cells in the larger arterioles and venules and resident perivascular leukocytes, including T cells, macrophages, mast cells, and dendritic cells.
Skin microvessels, like other microvessels, perform three important constitutive functions: regulating fluidity of the blood, forming a barrier that separates and controls transfer of molecules and cells between circulating blood and tissue, and regulating local blood flow.
Control of blood flow through skin microvessels have a special and critical role in thermoregulation not performed by other segments of the vasculature.
Skin microvascular cell morphology and gene expression and function are altered by acute or chronic inflammatory skin diseases and in cancers. These processes may involve formation of new blood vessels (angiogenesis) or remodeling of preexisting vessels.
Skin-specific microvascular responses may be influenced by keratinocyte-derived and other environmental-derived factors.
The blood vascular system is a continuous series of hollow tubes that are lined by a one-cell-thick layer of epithelium-like mesenchymal cells, the endothelium, and are supported by various mural cells, typically pericytes (PCs) in microvessels and smooth muscle cells (SMCs) in larger caliber vessels. All vascular endothelial cells (ECs) share common features and functions and hence may be collectively described as one cell type. However, ECs from one segment of the vascular system may differ in significant ways from the ECs at other anatomic sites. Blood vessel ECs also differ from lymphatic ECs, which are not discussed in this chapter. Mural cells have distinct embryologic origins throughout the blood vasculature, but little is known about their variation with anatomic location. Consequently, these cells will be discussed as if they were homogeneous, but such descriptions should be regarded with caution.
STRUCTURE AND ORGANIZATION OF THE SKIN VASCULATURE
Approximately 3 decades ago, Irwin Braverman1 described the organization of the vascular network of human skin. The human dermal vasculature consists of two interconnected systems, a superficial and a deep vascular plexus (DVP) with additional vascular networks surrounding sweat glands and hair follicles (Fig. 9-1).
Schematic diagram of the architecture of the skin vasculature.
The superficial vascular plexus (SVP) is composed of paired arterioles and venules that form an interconnected network of vessels coursing on a plane parallel to and just beneath the epidermal surface. Capillaries arise from the arterioles, extend upward within the papillary dermis ...