INTRODUCTION
The skin is the largest organ in the human body and often, the most overlooked. The integumentary system serves as the ultimate canvas for the plastic surgeon, and its surface can only be appropriately camouflaged with an intricate knowledge of its form and function. The skin serves an important protective function as the first line barrier to potential harm such as infection or trauma, and its contiguous structure envelopes and largely defines a considerable majority of the human body.
The skin is perhaps the most important biologic tool that the plastic surgeon has to work with; its healing capacity, malleability, mobility, functional adnexa, and complex cellular matrix make flaps as well as grafts available to the surgeon. Although the dermatologist is, indeed, the skin specialist, it is the plastic surgeon's responsibility to have a comprehensive understanding of this organ for both aesthetic and reconstructive purposes. Whether it be aging face procedures or cancer reconstruction, the end result is directly related to the postoperative status and appearance of the visible skin.
The face, more so than anywhere else on the human body, parades the skin and all its features in a manner that is most noticeable to the observer. For example, severe acne is easily identified in the unmasked face, whereas equal levels of acne on the torso or back may not immediately grasp the observer's attention. Detailed knowledge of all layers of the facial skin and associated adnexal structures, as well as an in-depth understanding of its vascular supply is vital and will be discussed in this chapter. In addition, the physiologic role of temperature regulation and how it contributes to integumentary attributes will be described with a brief description of sensation and metabolic function of the skin.
Ideally, skin flaps and grafts would uniformly follow rules and surgical principles to give an aesthetic postoperative result; this is not always the case and it is the knowledge of the skin anatomy and physiology that helps the surgeon adapt accordingly to different skin types. For example, some patients have scalp skin that is more immobile than others. Based on the knowledge of vascular factors and influences, a relatively immobile flap can be improved with the use of adjunctive surgical techniques such as tissue expansion. Tissue expansion is just one example of a technique that requires an intricate knowledge of the layers of the skin and the microphysiologic changes that lead to improved survival.
On a more basic level, the facial surgeon spends a great deal of time elevating and manipulating soft tissue for reasons that include cancer reconstruction, redraping of lax skin, and coverage of soft tissue defects. Each of these may require elevation in a specific plane or layer; the surgeon may want to have a thinner and more immediate subdermal plane of elevation when redraping cutis laxa, whereas a large soft tissue defect may require a deeper and thicker plane of elevation for increased blood supply as well as tissue bulk. Placing these sorts of specific descriptions into practice is easier to perform when they are well conceptualized. A distinctive part of the surgical complexity of the skin is that every layer is not grossly distinctive to the naked eye, and clinical judgment plays a large role in deciphering the road map that is the integument. This chapter emphasizes the anatomy of the 2facial skin with a comprehensive outline that is broken down into the skin layers, the physiologic role of the skin, and finally a conclusion that ties this large organ system together.
THE ANATOMIC LAYERS OF THE SKIN
In order of superficial to deep, the skin is composed of the epidermis, dermis, and subcutaneous tissue. The layers of the skin are traditionally described, in dermatologic texts, from deep to superficial. This is intuitive in the sense that the development of the cell structures begin at the basal layers and progress superficially. Nonetheless, this chapter will take an alternative approach and describe the anatomic elements from superficial to deep as this is the method in which the surgical practitioner encounters these components. While trying to grasp the concept of skin anatomy, it is best to consider the entire system as a map and each layer as a road that gives you access to varying structures. Figure 1.1 gives an overall picture of the gross structure of the skin from the depth perspective.
Fig. 1.1: The gross structure of the skin from the depth perspective. The structural layers of the epidermis, dermis, and subdermal tissues are depicted.
The Epidermis
The epidermis, the outermost surface, is composed of cell types that include dendritic cells, Langerhans cells, melanocytes, and Merkel cells. None of these aforementioned cell types is as populous as the keratinocytes, which comprise at least 80–85% of epidermal cells.1,2 Keratinocytes result in the formation of keratin, which establishes a stratified squamosal network that can be described as the epidermal coat. The epidermis does not contain its own layer of blood vessels, but it is thin enough that it can be supported entirely by underlying dermal networks. Within the epidermis are layers, termed stratum, and named as individual components; these are described in this chapter, and Figure 1.2A shows a representation of where each is located, whereas Figure 1.2B shows a histologic slide depicting these layers as well as the underlying dermis. The epidermis contains the melanocytes, which are the cells of origin of malignant melanoma.3,4 Melanin, which comes from melanocytes, has a protective function against ultraviolet light and is situated mainly in the stratum basale of the epidermis.5
Figs. 1.2A and B: (A) Representation of the location of each of the individual subcomponent layers of the epidermis and how they correlate with each other. (B) Histologic slide depicting the layers that make up the subcomponents of the epidermis. The solid circle indicates the reticular dermis; the open rectangle marks the papillary dermis. The black arrow points to the stratum basale; green arrow, to the stratum spinosum; red arrow, to the stratum granulosum; orange arrow, to the stratum corneum.
The keratinocyte moves from dermal–epidermal attachments up toward the surface, creating distinct epidermal layers through its progression.6 The thickness of the epidermis varies depending on location with the eyelids and postauricular area being the thinnest (~0.05 mm thick) and the palms and soles being the thickest (~1.5 mm thick).4
The stratum corneum is the outermost layer of the epidermis and is largely lipophilic; although it does contain some water, it is <20% of its total composition. Accordingly, its thickness varies on the basis of its state of hydration from 10 to 20 μm.7 It is the stratum corneum, which plays the most important role as the barrier that helps prevent entry of harmful pathogens.8,9 The slightly acidic pH of the stratum corneum contributes to its pathogenic averting properties. The stratum corneum serves as a conduit for conducting skin sensation, and this is a direct result of its mechanical properties such as elasticity and yield stress.10 There is a network of cells, known as corneocytes, which are completely surrounded in a lipid layer and comprise the majority of the stratum corneum. These corneocytes are actually the terminally differentiated form of the keratinocyte cell. It is these corneocytes that are directly responsible for the mechanical barrier that is created, and its lipophilic properties permit fluid retention.11 One important aspect to bear in mind is that the stratum corneum consists of anucleated or dead cells, making it the final phase of keratinocyte differentiation.
The stratum lucidum will be mentioned briefly for purposes of completion. However, it should be noted that the facial, head, and neck skin does not contain this layer. The stratum lucidum is a layer found exclusively in thickened areas of skin such as the palms of the hands and soles of feet and is composed of dead skin cells.
The stratum granulosum is often referred to as simply the granular layer. This is a thin layer in which reside keratohyalin granules, which promote cross-linking of keratin.12,13 This zone in addition to the overlying corneum helps maintain water and avoid volume loss from the body via its lipophilic nature.
The stratum spinosum is also known as the spinous layer and described as “prickly”. These descriptions are a result of the histologic appearance of their cellular desmosomes or intercellular bridges.14 The majority of this epidermal layer has tonofilaments that can be seen in large numbers, gathered in coarse bundles when studied by electron microscopy. This differs from the stratum granulosum where there are significantly less tonofilaments.15 In addition, on electron microscopy the more complete structure of the desmosome can be appreciated when compared with light microscopy. There is a distinct size difference in the intercellular bridges based on the depth level of the stratum spinosum. Specifically, the desmosomes that connect the basal cells to the spinous cells, or the deeper layers, are smaller than those in the remainder of the spinosum layer.15 The stratum spinosum together with the stratum basale is termed the Malpighian layer.
The stratum basale is the deepest layer of the epidermis and therefore separates it from the underlying dermis. This tier consists of columnar or cuboidal cells, which are in direct contact with the basement membrane. The Malpighian layer contains melanin pigment, mostly in the stratum basale. The stratum basale represents an important anatomical landmark for the facial plastic surgeon; it is the deepest part of the skin that is treated with a superficial skin treatment such as microdermabrasion.16 The microstructure of the stratum basale is illustrated in Figure 1.3.
The Epidermal Adnexa
The adnexa refers to the appendages of the integumentary system. The epidermal adnexa consists of important appendages including pilosebaceous units, sweat glands, and sebaceous glands not associated with hair follicles (such as those encountered in the eyelids).
The physiology of the pilosebaceous unit will be described in conjunction with its anatomy as it is a key dynamic structure of the integumentary system. Figure 1.4A is a depiction of a pilosebaceous unit with its components. 4These units represent an extremely important clinical structure for the plastic surgeon as acne is a manifestation of inflammation of the pilosebaceous units. The pilosebaceous unit is formed by a hair follicle that contains both hair and sebaceous glands. Its three basic components are the hair follicle, the sebaceous gland, and the arrector pili muscle that causes the hair to stand up in response to sympathetic stressors. Figure 1.4B is a histologic slide showing these components. The sebaceous glands secrete lipids and are found in high densities in the face and scalp. It is after puberty that these glands begin to function as a result of the hormonal influence of androgen.17,18 In males, the vellus hair on the face transforms into terminal hairs and the opposite occurs on the scalp.19 The sebaceous glands produce sebum, which produces an individual-specific odor and is thought to be involved in sexual and social attractions. Furthermore, sebum delivers vitamin E and other compounds to the stratum corneum. The sebum has antimicrobial properties and is known to be fungistatic.19 The pilosebaceous unit can present as one of three anatomic forms, the terminal hair follicle, the vellus hair follicle, and the sebaceous follicle. The terminal follicle, such as that found in the distribution of the male beard, consists of thick stiff and long hair. Because its diameter is wide, it occupies most of the canal, thereby preventing debris from depositing and acne from forming. The vellus follicle is smaller than the terminal one and has larger sebaceous glands, and the hairs from these follicles are much smaller and are barely perceptible to the observer. The vellus hairs are commonly referred to as “peach fuzz.” Finally, the sebaceous follicle is the most likely site of acne, because it has a very deep and wide canal, which is easily filled with debris or other irritant. It has a very tiny hair, which is imperceptible in relation to the large canal and sebaceous glands.20,21
Possibly, the most important function of the pilosebaceous unit, in the eyes of the plastic surgeon, is its role as a reservoir for keratinocytes and stem cells, which proliferate and aid in epithelialization and complete wound healing.22,23 To summarize, the very important pilosebaceous unit plays a role in thermal regulation, hair production, sebum production, cell signaling, and wound healing. Additional roles include ultraviolet protection and sensory perception.
The Dermis
The dermis is the next layer of skin, immediately below the epidermis. It serves to provide support and nutrients to the overlying epidermis. Its thickness varies throughout the body, from 0.3 mm on the eyelid to 3.0 mm on the back. Epidermal appendages, as discussed in epidermal adnexa, exist in par within the dermis and exit through it.24 Within the dermis are scattered mast cells and tissue macrophages. The bulk of cells within the dermis, however, are fibroblasts, whose role is to uphold the structural components.
Figs. 1.4A and B: (A) Depiction of a pilosebaceous unit with its components. These units represent an extremely important clinical structure for the plastic surgeon as acne is a manifestation of inflammation of the pilosebaceous units. (B) A histologic image of the pilosebaceous unit. The black arrow indicates a hair follicle; solid rectangle marks a sebaceous gland; solid square marks the arrector pili muscle.
The key structural component of the dermis is collagen. Abundant collagen is responsible for the tensile strength of dermis as well as most of dermal fat free dry weight.24
Several varieties of collagen types are found in the dermis with a distribution of approximately 80% type I collagen, 15% type III, 5% type IV, and type V.24 The 4:1 ratio of type I to type III collagen is also present in scars after wound healing. Elastic fibers are present in dermis and arranged in all directions contributing to skin recoil. Aging and ultraviolent light damages these fibers and causes wrinkles.25
The elastic nature of skin can be exploited for reconstructive purposes. Tissue expanders placed subcutaneously gradually inflate and stretch collagen and elastin in the dermal layer ultimately increasing the surface area of skin. Ground substances in the dermis are displaced with a resultant dermal structure that is thinner while the epidermis thickens.
Dermal architecture is subdivided into two layers: papillary dermis (pars papillaris) and the deeper reticular dermis (pars reticularis). There is no sharp demarcation between the two layers; however, each has its own specialized composition and function.25 Figure 1.5 is representative of the two subcomponents of the dermal structure.
Papillary Dermis
Papillary dermis is the thinner more superficial layer that forms the junction of the epidermis and dermis. Its form consists of wavy, undulating finger-like projections of dermis into the epidermis reminiscent of a mountainous landscape. These projections, called rete pegs, maximize surface area between the layers and expedite oxygen and nutrient transport.24 These rete pegs diminish with age and can cause epidermal gliding and shearing.
Structurally, connective tissue of the papillary dermis is arranged in a more chaotic fashion than that of the reticular dermis. The loose configuration of this tissue is occupied by more ground substance composed of a variety of anionic polysaccharides or glycosaminoglycans. This matrix is governed by fibroblasts and mast cells and has implications in water binding and collagen interaction.24 Its fluid gel-like composition facilitates nutrient and hormone transport through the dermis. Ground substance also provides fullness to the skin and protects against compressive forces directed toward surface of the body.25
Papillary dermis also contains unmyelinated nerve endings that provide pain, itch, and temperature sensations.
Reticular Dermis
Situated just deep to the papillary layer of dermis, the reticular dermis extends to the subcutaneous tissue. It is thicker and contains more collagen and elastic tissue arranged in parallel to the skin, in a more organized fashion than the papillary dermis. In 1861, Langer described the collagen orientation as a “lattice-like network with much extended rhomboidal meshes”.24 The coarse bundles of collagen give this layer strength aiding in surgical skin closure.
Subcutaneous Tissue
Located beneath the dermis, the subcutaneous tissue or hypodermis consists mainly of adipocytes and loosely joins the skin to deeper structures. Medications administered in this layer have rapid uptake because it is replete with vasculature.26
Skin Vasculature
Cutaneous arteries arise, either directly or indirectly, from underlying source arteries particularly from underlying muscles.3 These source arteries penetrate the muscle and fascia and exit toward the surface in an orientation that is perpendicular to the skin. Its distal and superficial branches comprise the dermal and subdermal plexus, which are essentially webs of interconnected vessels. It is these dermal and subdermal plexus that provide the basis of random patterned skin flaps used in reconstructive surgery. A few examples of such flaps include advancement, transposition, bilobed, and rotation flaps.6
Fig. 1.6: A basic distribution of the vascular territories that supply the skin of the face and scalp. These supplying arteries are in relatively constant locations and knowledge of their territories is useful in designing local flaps.
The survival of these flaps is directly related to the vascular supply, which can be increased by adjusting the base to length ratio of the flap. As a general and loose rule, in a random patterned flap, the flap should be no longer than three times its width, and additional increase in vasculature can be obtained by delaying the flap inset.27
Facial vasculature territories have been well delineated by studies performed by Whetzel and Mathes. They described a detailed analysis of 11 vascular territories of the face and head and divide them up into three patterns of vascularization. These include:
- Small and densely populated arteries that supply the anterior face (facial and infraorbital artery perforators)
- Large and more sparsely populated arteries that supply the lateral face (transverse facial, submental, and zygomatico-orbital)
- Small and densely populated arteries that supply the scalp (superficial temporal and posterior auricular).28
Figure 1.6 shows a basic distribution of the vascular territories that supply the skin of the face and scalp. These supplying arteries are in relatively constant locations, and knowledge of their territories is useful in designing local flaps.
PHYSIOLOGIC ROLE OF THE SKIN
The integumentary system serves its role as a major protector from the outside world, but its additional dynamic responsibilities are diverse and include sensation, thermoregulation, and metabolism.26 Each of these is described separately, although their interrelated factors may blur the lines of separation in vivo, particularly with regards to thermoregulation and metabolism.
Sensation
Sensory receptors are located throughout the skin and serve a host of functions. Nerves carrying sensation signals from the skin can be either myelinated or unmyelinated (naked nerve fibers). Those that are unmyelinated nerve fibers are terminally exposed in epidermis and respond to itch, pain, and temperature. These sensations can be particularly noticeable when felt on the face. Myelinated fibers correspond to end organs categorized as mechanoreceptors, thermoreceptors, and nociceptors. Mechanoreceptors respond to stretch, vibration, pressure, and touch. Thermoreceptors detect temperature changes. Nociceptors are specific for pain. Specific examples of mechanoreceptors include Merkel cells in epidermis for touch, Meissner corpuscles in dermal papillae for textural touch, Pacinian corpuscle in hypodermis for vibration, and finally peritrichial nerve endings associated with hair follicles for hair movement.29 The distribution of receptors varies depending on the location of the body. With age and certain diseases, that can cause neuropathies such as diabetes, the function of the receptors becomes less sensitive and can result in traumatic or thermal injury.
Thermoregulation
Temperature control is a major function of skin. External temperature, as detected by skin afferents, enters a feedback loop with the anterior hypothalamus. The hypothalamus correlates this with the body's internal core temperature and serves as a thermostat, altering blood flow to the skin in concert with sweating or shivering to return temperature to its thermoneutral set point. Anatomically, arteriovenous shunts governed by the autonomic system link the subpapillary plexus with the deep plexus of vessels near the junction of the dermis and subcutaneous tissue. Flow through arteriovenous shunts is altered by sympathetic vasoconstrictor nerves acting on α-1 and α-2 arterioles. These arterioles are tonically active at neutral temperatures, 7allowing the body to adjust to minor temperature fluctuations with blood flow manipulation. When thermoneutral, blood flow through the skin is approximately 250 mL/min.30 In cold temperature, flow through these anastomoses decreases and blood flow is diverted to the deeper plexus of vessels. During overheating, blood flow to the superficial skin can increase massively, up to 6–8 L/min, partially because of relaxation of arterioles but mainly because of cutaneous vasodilation.30 As a result, blood shunts to the superficial subpapillary plexus from deeper areas of circulation and its heat can dissipate by convection with the coordinated cooling effects of evaporation of sweat released by eccrine sweat glands.26 Interestingly, the temperature threshold for sweating and blood flow alterations is not the same, with sweating usually occurring at a higher threshold. Cutaneous vasodilation, also controlled by the cholinergic autonomic system, is not understood as well as the vasoconstrictor system. It in part works with the production of nitric oxide and is responsible for most of the increase in cutaneous blood flow during hyperthermia. It has a limited role in cold temperature, where shunt arterioles are stimulated and blood flow is diverted to deeper vessels reducing heat loss at the surface. Aside from reflex neurologic control of temperature, local temperature changes to a region of skin will affect blood flow. Heating skin locally increases blood flow and is thought to be mediated by local neural control via C-fibers. Local cooling can almost stop blood flow to a region of skin by activating arterioles, this time without input from the central nervous system.30 Cutaneous blood supply to some areas of the body, such as the face, can also be affected by emotional state.29
Knowing the mechanisms of blood flow regulation to the skin makes it easy to understand why certain surgical practices are followed. Simple maneuvers such as avoiding ice packs or caffeine can be employed to maximize blood flow to a new reconstruction. The topical application of nitropaste to a reconstruction is another example. Savvy surgeons can implement the fundamentals of thermoregulation to their advantage.
Other Functions of Skin
Aside from providing a waterproof barrier to the outside world, skin has important immunologic and metabolic functions. Antigen presenting cells, such as Langerhans cells, situated in the epidermis constantly sample the environment and activate T lymphocytes when exposed to an antigen.29 Metabolically, skin is essential for the transformation of provitamin-D to previtamin-D during ultraviolet light exposure. In addition, skin is part of sexual signaling. As, the most exposed organ in the body, it is a visible proxy measure for health and youth, attracting the opposite sex.26
CONCLUSION
This chapter illustrates the composition, both structurally and functionally, of the integumentary system in a comprehensible way. The skin has several layers that should be thought of as individual components with unique characteristics and functions. Like peeling away the layers of an onion, each individual depth level or stratum should be visualized in the mind's eye as its own unique structural entity when performing surgery.
The surgeon should understand that the structural epidermis, in its entirety, is composed of cells in a series of phases of the life cycle of the keratinocyte, with the most visible exterior made up of a nuclear keratin. The stratum corneum is the most important of the barrier-providing layers, and the stratum basale separates the epidermis from the underlying dermis.
The epidermal adnexa, which largely originates in the dermis and makes its way into the epidermis, is one of the keys to skin regeneration and wound healing. Its role in providing skin-derived stem cells, with further study, may prove to be of significant clinical value for therapeutic use in other parts of the body.31 It is specifically the follicular stem cell niche or the “bulge” of the appendage that is composed of multipotent cells, making the epidermis and its appendages a unique entity.32
The dermis, structurally maintained by collagen and fibroblasts, is the foundation and substantive nutrient providing groundwork for the epidermis. Its two subcomponents are the papillary and reticular dermis. The papillary dermis is intimately related to the epidermis with its projections and tight adherence, whereas the reticular dermis is a stronger framework-type structure. This is clinically important for the surgeon in suturing techniques. A satisfactory plastic closure often requires a deep dermal element to reduce wound tension as well as eliminate dead space. It is the more structurally sound reticular dermis that has the strength for good apposition with deep dermal sutures. Although it is not easy to tell the exact demarcation point of reticular dermis grossly, the strength on closure lets the surgeon know that they 8are indeed in the stronger, deeper layer. Another very important situation in which the surgeon needs to be aware of their dermal depth is in skin resurfacing. Whether by dermabrasion, laser techniques, or chemical peels, the deep reticular dermis should be avoided to prevent scar formation. This is done by clinical judgment and observation, such as identification of uniform white fibrils and pinpoint bleeding.
The vascular supply of the skin is based on plexuses and arcades of small vessels originating from larger axial vessel. These arcades anastomose liberally and form the basis of the so-called “random patterned” flaps. The skin of the face, scalp, and neck has a disproportionately stronger blood supply when compared with other parts of the body, allowing for improved wound healing and decreased rates of infection. The specific territories of vascular supply to the face have been well established and are extremely useful in the planning and execution of operative procedures. When rhytidectomy procedures are performed with this vascular preservation principle in mind, the postoperative wound-healing course can improve and complications such as distal edge necrosis can be reduced.
Skin physiology can be difficult to separate from skin anatomy as there is a dynamic component to the structural evolution that is constantly occurring, particularly in the epidermis. However, the distinctive sensation, thermoregulation, immunity, wound-healing factors, and metabolic activity attest to both the complexity and the importance of this large organ system.
The ability of the surgeon to work intelligently with the integumentary system provides endless possibilities for his/her procedural armamentarium. It is a lack of understanding of the capabilities and anatomic intricacies of the skin that leads to poor results and unexpected outcomes.
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