Arterial blood supply and venous drainage
The angioarchitecture of blood vessels in the skin follows a strict "tiered" principle. Three main arterial networks are distinguished:
- Fascial network — lies deepest, between the muscle fascia and the subcutaneous adipose tissue (hypodermis).
- Deep cutaneous network — is formed at the border of the hypodermis and the reticular layer of the dermis.
- Superficial (subpapillary) network — is located at the border of the reticular and papillary layers.
Branches of two types depart from each of these networks. Horizontal and small vertical arteries supply the layer in which the plexus itself is located. Ascending branches head upward toward the next vascular network. The terminal link of this system is represented by capillaries that extend from the subpapillary network directly into the dermal papillae, forming characteristic loops there.
The venous bed generally mirrors the arterial bed, with vessels running parallel to each other. However, there is a key difference: two consecutive venous plexuses (the first and second subpapillary) are formed immediately in the subpapillary zone, whereas there is only one arterial network there. An important feature of skin hemodynamics is the abundance of arteriovenous anastomoses (shunts). They are essential for the effective thermoregulation of the body.
Lymphatic system and immunity
Lymph drainage begins in the papillary layer of the dermis, where blind-ended lymphatic capillaries are located. By merging, they form lymphatic plexuses that accompany blood vessels in the deeper layers.
The lymphatic system is closely connected with the immune apparatus of the skin. T-lymphocytes concentrate around the lymph capillaries and postcapillary venules, forming structures similar to the T-dependent zones of lymph nodes. The greatest number of T-cells is concentrated 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 main task is the control of smooth muscle structures and the secretory apparatus. Autonomic fibers innervate:
- The walls of blood vessels.
- Smooth muscle bundles of the arrector pili muscles (m. arrector pili).
- Sweat glands.
Afferent innervation: classification of receptors
Sensory receptors are classified depending on which layer of the skin they are located in, as well as on the presence or absence of a capsule.
In the epidermis, predominantly unencapsulated structures are found:
- Free nerve endings — penetrate between keratinocytes and are responsible for the perception of pain and temperature.
- Merkel tactile discs — perform the function of mechanoreceptors (tactile sensitivity).
In the papillary layer of the dermis, two main types of endings are distinguished:
- Non-free (unencapsulated) endings — dendritic branches here are accompanied by glial cells (lemmocytes). They perceive pain and temperature.
- Meissner corpuscles — encapsulated mechanoreceptors. They consist of terminal dendritic branches, modified lemmocytes, and a thin connective tissue capsule. Upon light pressure, the capsule deforms, and the pressure is transmitted through the lemmocytes to the dendrite, which triggers the generation of an impulse.
In the reticular layer of the dermis, deeply situated receptors are localized:
- Pacinian corpuscles — the most frequent receptors of this layer. They have a very thick capsule made of fibrous lamellae with fluid layers between them. They are responsible for the perception of static pressure and vibration.
- Ruffini corpuscles — respond to skin stretching due to the close interweaving of dendrites with collagen fibers. They are especially common in the skin of the sole.
- Krause end bulbs — specific mechanoreceptors of the skin of the external genitalia with heavily branching dendrites and a rounded capsule.
The innervation of the hair follicle is worthy of separate note. Most receptors are duplicated in the intraepidermal/intradermal part of the hair. Merkel discs and free nerve endings are located in the hair bulb, while Pacinian and Ruffini corpuscles are localized in the hair follicle (in the connective tissue).
Briefly: answers to topical questions
Which neurotransmitters are released by autonomic fibers during the innervation of sweat glands and skin vessels?
Acetylcholine is released during the innervation of sweat glands, and norepinephrine is released during the innervation of skin blood vessels.
- Sweat glands: receive sympathetic innervation, but postganglionic fibers are cholinergic and use acetylcholine as a neurotransmitter.
- Blood vessels: the smooth muscles of the vascular walls have no parasympathetic innervation. They are controlled by postganglionic sympathetic fibers, which are adrenergic and release norepinephrine.
How are skin mechanoreceptors classified by adaptation rate?
Skin mechanoreceptors for which adaptation rates are indicated in sources are divided into rapidly and slowly adapting.
| Receptor type | Adaptation rate |
|---|---|
| Pacinian corpuscles | Fast |
| Meissner corpuscles | Fast |
| Hair follicle apparatus | Fast |
| Golgi corpuscles | Slow |
| Merkel corpuscles | Slow |
What specific links does the microcirculatory bed of the skin consist of?
The structural and functional unit of the microcirculatory bed includes the following links:
- Arteriole.
- Precapillary arteriole (precapillary).
- Capillary; in the skin, capillaries extend from the subpapillary network into the dermal papillae, forming loops there.
- Postcapillary venule (postcapillary).
- Arteriovenous anastomoses; in the skin, they are present in large numbers and serve for thermoregulation.
- Venule.
To easily remember the functions of the deep receptors of the reticular layer, use the mnemonic: "Pacinian Pressures, Ruffini Reaches/Stretches". Pacinian corpuscles perceive pressure, while Ruffini corpuscles respond to tension and skin stretching.
Class and exam questions
How does the venous network of the skin differ from the arterial network?
In general, the venous bed runs parallel to the arterial one, but two consecutive venous plexuses are formed immediately in the subpapillary zone, whereas there is only one arterial network there.
Which receptors provide temperature sensitivity and the sensation of pain?
Free nerve endings in the epidermis, as well as non-free (yet unencapsulated) nerve endings in the papillary layer of the dermis accompanied by lemmocytes, are responsible for the perception of pain and temperature.
What is the mechanism of action of Meissner tactile corpuscles?
Upon light pressure on the skin, the thin capsule of the corpuscle deforms. This mechanical action is transmitted via lemmocytes to the dendrite of the sensory neuron, leading to the generation of a nerve impulse.
What else to study on this topic
- Morphology of arteriovenous anastomoses and their role in thermoregulation
- Histological appearance of Meissner corpuscles stained with hematoxylin and eosin
- Structure of T-dependent zones around postcapillary venules of the dermis
- Ultrastructure of the layered capsule of Vater-Pacini corpuscles
- Interaction of dendrites with lemmocytes in encapsulated nerve endings
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