Arterial Blood Supply and Venous Drainage
The architectonics of blood vessels in the skin follow a strict tiered principle. There are three main arterial networks:
- Fascial network — lies deepest, between the muscle fascia and the subcutaneous adipose tissue (hypodermis).
- Deep cutaneous network — forms at the border between the hypodermis and the reticular dermis.
- Superficial (subpapillary) network — is located at the border between the reticular and papillary dermis.
Branches of two types originate from each of these networks. Horizontal and small vertical arteries supply the layer where the plexus itself resides. Ascending branches direct upward toward the next vascular network. The terminal link of this system consists of capillaries that extend from the subpapillary network directly into the dermal papillae, forming characteristic capillary loops.
The venous bed generally mirrors the arterial supply, with vessels running parallel to one another. However, there is a key difference: two consecutive venous plexuses (the first and second subpapillary plexuses) form in the subpapillary zone, whereas there is only a single arterial network there. An important feature of cutaneous hemodynamics is the abundance of arteriovenous anastomoses (shunts), which are essential for effective thermoregulation.
Lymphatic System and Immunity
Lymphatic drainage begins in the papillary dermis, where blind-ended lymphatic capillaries originate. By fusing, they form lymphatic plexuses that accompany the blood vessels in deeper layers.
The lymphatic system is closely linked with the immune apparatus of the skin. T lymphocytes concentrate around lymphatic capillaries and postcapillary venules, forming structures analogous to the T-dependent zones of lymph nodes. The highest concentration of T cells is found in the superficial layers of the dermis and the epidermis itself.
Efferent Innervation (Autonomic)
Motor innervation of the skin is provided by the autonomic nervous system. Its primary role is to control smooth muscle structures and the secretory apparatus. Autonomic fibers innervate:
- The walls of blood vessels.
- Smooth muscle bundles of arrector pili muscles (m. arrector pili).
- Sweat glands.
Afferent Innervation: Classification of Receptors
Sensory receptors are classified based on their location within skin layers and the presence or absence of a connective tissue capsule.
In the epidermis, predominantly non-encapsulated structures are found:
- Free nerve endings — penetrate between keratinocytes and mediate pain and temperature sensation.
- Merkel discs — function as mechanoreceptors (tactile sensation).
In the papillary dermis, two main types of nerve endings are identified:
- Non-free (non-encapsulated) endings — dendritic branchings here are accompanied by glial cells (lemocytes/Schwann cells) and mediate pain and temperature.
- Meissner corpuscles — encapsulated mechanoreceptors consisting of terminal dendritic branches, modified lemocytes, and a thin connective tissue capsule. Upon light touch, the capsule deforms, transferring pressure via lemocytes to the dendrite to generate an impulse.
In the reticular dermis, deep receptors are localized:
- Lamellar corpuscles (Pacinian corpuscles) — the most common deep receptors. They possess a very thick capsule made of fibrous lamellae with fluid-filled layers in between, mediating static pressure and vibration.
- Ruffini corpuscles — respond to skin stretch due to the close intertwining of dendrites with collagen fibers; particularly numerous in the sole of the foot.
- Krause end bulbs — specialized mechanoreceptors of the external genitalia with heavily branching dendrites and a rounded capsule.
Notably, the hair follicle has an extensive nerve supply. Most receptors are duplicated in the intradermal portion of the hair. Merkel discs and free nerve endings are found in the hair bulb, whereas Pacinian and Ruffini corpuscles are located in the surrounding connective tissue sheath (hair follicle receptor apparatus).