Introduction
Skin is one of the visible organs of body and ensheaths all other organs. It plays a major role in defense (immunity), synthesis (vitamin D), homeostasis (temperature regulation, water and electrolyte balance) and esthetics (complexion, wrinkles, baldness). Dermatosurgery and esthetic surgery deals with the surgical aspects of skin and its appendages. However, this subject is now driven by esthetics, the wellness component rather than diseases, the illnesses. Better understanding of basic features gives an advantage with regard to safety and efficacy while performing dermatosurgical procedures. While detailed anatomy can be found in a textbook of dermatology, this chapter briefly presents structural and embryological aspects of skin and its appendages. Applied anatomy of head and neck is considered in detail in the next chapter and detailed anatomy of each region are elaborated in respective chapters on different dermatosurgical procedures.
Skin is made up of 2 layers namely, epidermis and dermis with a basement membrane zone in between (Figure 1-1 and Table 1-1), and a buttress of subcutaneous fat underneath. It is composed of cells (keratinocytes, corneocytes, melanocytes, etc.), fibers (collagen, elastic), ground substance, adnexal structures (eccrine glands, apocrine glands, pilosebaceous units), smooth muscles (arrectores pilorum), nerves and nerve receptors, vessels (vascular, lymphatic) and appendages (hair and nail).4
Embryology
Skin and its appendages are derived from ectoderm, neuroectoderm and mesoderm. Development of skin goes through three phases: Specification (0–2 months), Morphogenesis (2–5 months), Differentiation (5–9 months). It is regulated by various signaling pathways like Notch, Wnt, Sonic Hedgehog (Shh), bone morphogenetic proteins and fibroblast growth factors, many of whom play a vital role in ageing later in life. Skin starts developing by 3 weeks of intrauterine life with formation of a single layer of cells derived from ectoderm lateral to neuroectoderm. At 4 weeks, it is composed of two layers: outer periderm or epitrichial layer and inner, stratum germinativum. Tonofilaments (12–16 weeks) and keratohyaline granules (16–21 weeks) appear later, with completion of cornification by 24 weeks. At 8–11 weeks, middle layer starts forming adding layers to skin. Basal lamina is secreted by basal cells and matures to form dermoepidermal junction in adults by 12 weeks.
Early attempt to form hair follicle is seen at 9 weeks of intrauterine life at eyebrows, upper lip and chin. There is aggregation of mesenchymal cells close to ectoderm leading to elongation of ectodermal cells (Pre-Germ stage). These elongated cells also increase in number and start projecting down into mesenchyme. Simultaneously, the underlying collection of mesenchymal cells increases in number forming rudimentary hair papilla (Hair Germ stage). The short nobbings of ectodermal downgrowths form slanting columns of cells with basal cells reorienting themselves perpendicular to the long axis of growth (Hair-peg stage). The advancing tips of these downgrowths become bulbous and form a cup enclosing the aggregates of mesenchymal cells (Bulbous Hair Peg stage). These cells in the lower layers of hair bulb are Matrix cells, which form outer and inner sheaths and hair. As the matrical cells proliferate and cornify, they are pushed up to form hair. The junction of matrical cells and cornified cells is the Adamsons' fringe. At 16 weeks, 3 bulges are seen on the posterior aspect of these columns. Arrector pili muscle attaches to lower most bulge, middle bulge differentiates into sebaceous glands and upper most bulge forms apocrine gland. These apocrine glands persist in axilla, areola, periumbilical and anogenital areas and regress to form anlage in other sites. Earliest hairs emerge from follicular openings in eyebrow region at 17 weeks. By 20 weeks, entire body except palms, soles, terminal phalanges, glans and labia minora are covered by hairs. Under the influence of maternal hormones, sebum secretion starts by 15th week. Thus hair follicle grows from a close interaction of ectoderm with mesenchyme—an interaction which occurs through life during each hair cycle.5
Eccrine glands also develop as ectodermal downgrowths into dermis. They first appear in palms and soles and start secreting by 6th month of intrauterine life.
Nail development is earliest recognized in first trimester as a quadrangular area on tips of fingers and toes dorsally and is delineated by lateral and proximal shallow grooves. The primordial nail structure is made of three layers; surface, intermediate and germinative layers. Intermediate and germinative layers grow proximally and form nail matrix. Nail plate starts to form by 14 weeks and by 20 weeks, it covers nail bed completely. At the same time granular layer is also lost from the nail matrix. Among all epithelial derivatives, nail plate is the first structure to cornify.
Dermis also starts developing in parallel with formation of nerves by 5 weeks, vessels, both vascular and lymphatic by 12 weeks, delicate collagen by 12 weeks and mature collagen by 16 weeks and elastic fibers by 24 weeks. Subcutis develops from aggregation of mesenchymal cells around blood vessels with accumulation of fat.
Summarizing the development, epidermis, pilo-sebaceous-apocrine units, eccrine glands and nails are derived from ectoderm; melanocytes, nerves and sensory receptors from neuroectoderm and dermis, blood vessels, lymphatic channels, cells like Langerhans cells, fibrocytes, mast cells, macrophages, subcutis and muscles are derived from mesoderm.
Epidermis (Figure 1-2)
Epidermis is the outermost layer of the skin and is composed of stratified squamous epithelium. From below upwards, it consists of five layers: stratum basale, stratum spinosum, stratum granulosum, stratum lucidum (present only in palms and soles) and stratum corneum. Stratum basale is the lower most layer and mitotically active region. It is usually one cell thick and cells divide every 18-19 days. There are stem cells in interfollicular region and follicular bulge area. These cells divide to form transient amplifying cells which on further division form terminally differentiated cells, the keratinocytes. Basal cells are small cuboidal or columnar with large nucleus and dense cytoplasm. Stratum spinosum cells bond with each other through desmosomes which on histological sections look like spines and due to shrinkage of cell imparts polygonal shape. There are intracytoplasmic membrane bound Odland bodies or Lamellar granules which contain free sterols, glycosylceramide, cathepsin, corneodesmosin and enzymes like lipases, acid phosphatases and glycosidases. The content of these granule are discharged into intercellular space to form impermeable membrane which is essential for skin barrier function. Stratum granulosum is 1-3 layers thick and derives its name due to the presence of keratohyaline granules in its cells. Stratum lucidum is a thin, homogenous hyalinized layer present between stratum granulosum and stratum corneum. It is present over glabrous skin of palms and soles. Stratum corneum is the outer most layer of epidermis and is made up of corneocytes. Corneocytes are flat, anuclear eosinophilic cells. Single corneocyte covers about 25 basal cells. This layer gives toughness and resilience to skin. Cells lack desmosomes but are held together by a cementing substance, cholesterol sulphate. Shedding of corneocytes occurs by breaking down of cholesterol sulphate to cholesterol by cholesterol sulphatase enzyme.
Figure 1-2: Structure of epidermis Uppermost layer—Stratum corneum, Stratum granulosum, Stratum spinosum, Stratum basalis
Cornified cellular envelope is present along the inner surface of cornified cells, parallel to skin surface. Intracellular proteins like involucrin, cornulins, elafin, envoplakin, periplakin, keratolinin aggregate to form a protein envelope due to the action of transglutaminase enzymes. This gets covalently bound to lipid envelope containing ceramides, cholesterol sulphate, integrins. Cytoskeleton of cells is composed of elaborate network of structural components consisting of microfilaments (6 nm), intermediate filaments (10 nm) and microtubules (25 nm).
Total renewal time for epidermis is 39 days; 13 days for proliferation, 12 days for migration and 14 days for shedding. Knowledge of cell turnover helps understand the principle of several procedures such as chemical peels, and laser resurfacing.
6Skin thickness: Skin is thickest on the palms and soles of the feet (1.5 mm thick), while the thinnest skin is present on the eyelids and in the postauricular region (0.05 mm). Dermal thickness too varies in different parts of face, eyelid (5–6 mm), cheek (1.0–2.0 mm), forehead (2.0 mm), nose (2.0 mm). Male skin is generally thicker than female skin in all anatomic locations. Children have relatively thin skin, and likewise elderly people too have thin skin. This thinning with ageing is primarily a dermal change, with the loss of elastic fibers, epithelial appendages and ground substance, though epidermis too undergoes some thinning.
Applied Anatomy
Knowledge of the barrier in skin is important for application of topical preparations. Cell adhesion and renewal plays an important role in several treatments such as resurfacing, and post-resurfacing procedures. Knowledge of thickness of skin in different areas is vital in ensuring optimum choice of treatments and outcomes after such treatments. Levels of peels are determined by thickness and penetration of different ablative lasers too varies in depth.
Melanocytes: These are found in the basal layer of the epidermis as well as in hair follicles. Melanocytes, derived from neural crest cells, primarily function to produce a pigment, melanin. Melanin accumulates in organelles called melanosomes that are incorporated into dendrites anchoring the melanosomes, which are transferred into the surrounding keratinocytes. The ratio of melanocytes to keratinocytes is approximately 1:30, though different it may vary in regions, in hair and exposed skin. Absolute numbers of melanocytes are the same among the sexes and various races. Differing pigmentation among individuals is related to melanosome size, amount of melanin and its aggregation pattern, rather than cell number.
Skin Types
Depending on tanning and buring responses, six types of skin have been recognized by Fitzpatrick and Parrish scale (Table 1-2). They also correlate roughly with skin color and have been used widely in assessing responses to resurfacing techniques. Indian and Asian skin varies greatly in color, and presents in different shades, and do not always conform accurately to this classification. Most Indians and Asians belong to type 3–5 of Fitzpatrick scale.
Applied Anatomy
Excessive stimulation of melanocytes in the process of wound healing can lead to increased melanin synthesis resulting in hyperpigmentation—a common complication of all resurfacing procedures in brown skin. Hence, it is important to prime the skin with sunprotection and depigmenting creams before initiation of such procedures.
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Dermis
The dermis is composed of two layers: (1) the superficial papillary dermis and (2) the deeper reticular dermis. The papillary dermis is thinner, consisting of loose connective tissue containing capillaries. The reticular dermis consists of thicker collagen fibers, elastic fibers, larger blood vessels and other appendages. There is a netwok of closely interlaced elastic fibers, and coarse bundles of collagen fibers arranged in layers parallel to the surface. The reticular layer also contains resident cells such as fibroblasts, nerve endings, lymphatics and epidermal appendages.
Another important component of the dermis is the ground substance, an amorphous material that fills spaces between the fibrillar and cellular components of the dermis, imparting turgidity and resilience. The dermal matrix consists of glycosaminoglycans (mainly hyaluronic acid and dermatan sulfate) and glycoproteins. With ageing, there is a decrease in these substances and the synthetic variant of hyaluronic acid is widely used as a filler in rejuvenation treatments.
The fibroblast is the major cell type of the dermis. These cells produce and secrete procollagen and elastic fibers. The collagen fibers provide tensile strength and are important in wound healing. Collagen constitutes 70% of the weight of the dermis. Elastic fibers constitute less than one percent of the weight of the dermis, but play an important functional role in defining elasticity of skin. Both tensile strength and elasticity are important concepts in excisions and wound healing.
Fibronectin, the major filamentous glycoprotein component of the dermal matrix, is also produced by the fibroblasts. Fibronectin ensheaths collagen and elastin bundles and also helps in attachment of keratinocytes to 7the basal lamina, a role which is important in wound healing.
Epidermal Appendages
Epidermal appendages are derived from ectoderm as epidermal downgrowths during embryonic stage. They include eccrine and apocrine glands with their ducts and pilosebaceous unit comprising of sebaceous gland and hair; they all act as reservoir of epidermis. The keratinocytes from the adnexa migrate to the skin surface and cause reepithelialization after an injury. Injury over the face heals faster when compared to back, owing to the presence of more adnexal structures on the face—a principle which explains the beneficial effects of full face resurfacing by ablative laser therapy and dermabrasion for acne scars.
Hair Follicle (Figure 1-3)
They are distributed all over the body surface in varying density being more numerous on the scalp and face, and absent on the palms, soles, dorsa of the terminal phalanges, labia minora, clitoris and glans penis.
Types of hairs
- Lanugo hair (Latin: Lana—wool): Seen in fetus, they are soft, fine and lightly pigmented.
- Vellus hair (Latin: vellus—fleece): The fine hairs that cover most of the body of youngsters and adults.
- Terminal hair: Long coarse pigmented hairs with larger diameters, distributed over the eyebrows, eyelashes, scalp, beard, axillae and pubis. Terminal hairs are the only hairs to possess a medulla.
Human hair growth is cyclical and shedding is asynchronous. Hair cycle has three phases and is affected by various endogenous and exogenous factors (Table 1-3).
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At any given point of time, follicles are found in all three phases of hair growth. This is extremely important for laser hair removal, because follicles in the anagen phase, which contain melanin and melanocytes, are susceptible to destruction, whereas resting follicles are more resistant. This explains why multiple treatments of an area may be necessary to ensure adequate hair removal. The intervals at which laser has to be performed depends on the length of the anagen phase which varies from area to area.
Hair follicles show considerable morphological and functional heterogeneity. Hairs differ from site to site in shaft length, thickness, curl, color and androgen sensitivity. Hair grows on an average 0.4 mm per day. Terminal hairs grow longer and diameter exceeds 0.03 mm, whereas vellus hairs diameter is less than 0.003 mm. In androgenetic alopecia there is miniaturization of hair with the formation of vellus-like hair which are depigmented, small and measure less than 0.003 mm in diameter. The scalp hair of white people is round whereas in black it is oval and has curvature just above the bulb which causes hair to curl. Caucasian hair follicles are oriented obliquely to the skin surface, whereas the hair follicles of black skin are oriented almost parallel to the skin surface. Asians have vertically oriented follicles that produce straight hairs. These anatomic variations are an important consideration in avoiding scarring alopecia when making incisions in the scalp. Hair color depends on the degree of melanization and distribution of melanosomes within the hair shaft. Melanosomes are large in blacks and small and aggregated in whites. Red hair is characterized by spherical melanosomes. Melanocytes are decreased in gray hair.
Androgens are responsible for the follicular heterogeneity. Follicles over the body are either very sensitive 8(axilla and inguinal hair), mildly sensitive (beard hair), or insensitive to androgens (eyebrow).
Hair bulge is now known to be an important site for biotechnologist; stem cells are now produced from hair bulb and have great importance in future in organ transplant and artificial skin production.
Hair naturally grows in one to four hair follicular groupings called follicular units. This arrangement has become the basis of follicular unit transplantation, which is the redistribution of naturally occurring follicular groupings from the occipital donor region into the bald region of the recipient sites.
Sebaceous gland (Figure 1-4): It is a lipid producing gland that arises from the hair follicle at the junction of infundibulum and the isthmus. Sebaceous glands are associated with hair follicles all over the body—a relationship which defines the concept of pilosebaceous unit. The sebaceous glands exude lipids by disintegration of entire cells, a process known as holocrine secretion. Sebaceous glands of various sizes are distributed over the entire surface of the body with the exception of palms, soles and the dorsa of the feet. The glands may also be found in certain non-hairy sites, including the eyelids (meibomian glands), the nipples (Montgomery's glands), buccal mucosa and vermillion border of the lip (Fordyce spots) and around the genitals (Tyson's glands).
Although sebaceous glands are well developed at birth, they regress soon and mature at puberty under the influence of androgens. They harbor different microorganisms such as propionibacterium acne and demodex folliculorum. Acne and acneiform eruptions are common during this age.
Sweat glands: There are over 2.5 million sweat glands on the skin all over the body. They are classified into two different types: eccrine and apocrine.
- ▪ Eccrine sweat glands: These are the only true sweat glands. Eccrine sweat glands are composed of a intradermal coiled secretory portion, an intradermal duct and an upper intraepidermal part, called acrosyringium. The acrosyringium has a unique symmetrical and helicoidal course. Eccrine glands are found all over the skin especially on the palms, soles, axillae and forehead with exception of oral lips, clitoris, labia minora, and external auditory canals. Eccrine glands play a major role in thermoregulation and electrolyte balance. They are under psychological and thermal control. Sympathetic (cholinergic) nerve fibers innervate eccrine glands. A well-acclimatized person can perspire as much as several liters per hour and 10 L/day. In addition to secreting water and electrolytes, the sweat gland excretes heavy metals, drugs and organic compounds, and macromolecules.
- ▪ Apocrine glands: Apocrine glands are larger, the ducts of which empty into the hair follicles. They are present in the axillae, anogenital region, areolae, periumbilical region, external auditory canals (ceruminous glands) and on the eyelids (Moll's glands). They become active at puberty, producing an odorless protein-rich secretion which when acted upon by skin bacteria gives out a characteristic odor. These glands are under the control of sympathetic (adrenergic) nerve fibers.
Blood and Lymphatic Vessels (Figure 1-5)
The dermis receives a rich blood supply through two intercommunicating plexuses. The cutaneous arteries originate from deeper arteries in the deep fascia located near intramuscular septa and travel upwards towards the skin, where they form horizontally placed deep subdermal and superficial dermal plexuses. There are vertical interconnecting channels between the two plexuses and thus form a three-dimensional scaffolding of arteries and veins. Each deeper large vessel supplies a three dimensional vascular territory from bone to skin termed an angiosome—a concept of relevance in flap surgery.9
Branches from superficial arterial plexus, located at the junction of papillary and reticular dermis form capillary loops in the papillae of the dermis, each with a single loop of capillary vessels, one arterial and one venous. The veins drain into mid-dermal and subcutaneous venous networks. Dilatation or constriction of these capillary loops plays a direct role in thermoregulation of the skin. Lymphatic drainage of the skin occurs through abundant lymphatic meshes that originate in the papillae and feed into larger lymphatic vessels that drain into regional lymph nodes.
Nerve Supply (Figure 1-5)
The skin has a rich innervations with the hands, face and genitalia having the highest density of nerves. Pain, itch and temperature sensation is detected by free sensory nerve ending. Pressure and vibration sensation is mediated by Pacinian corpuscles and touch sensation is by Meissner's corpuscles present in the dermis. The autonomic nervous system supplies the motor innervation of the skin: adrenergic nerves innervate blood vessels, hair erector muscles and apocrine glands while cholinergic nerves innervate eccrine sweat glands. Cutaneous nerves follow the route of blood vessels to the skin. The area supplied by a single spinal nerve, or single segment of the spinal cord, is termed as dermatome. Adjacent dermatomes may overlap considerably, and of importance to note when performing field blocks with local anesthesia.
Conclusion
Sound knowledge of surgical anatomy of skin is vital for the dermatosurgeon. Important anatomical variations, landmarks, anatomical relations should be taken into consideration while planning any cutaneous surgery to obtain satisfactory results and minimize complications.
Recommended Reading
- Bergstresser PR, Costner MI. Anatomy and physiology. In: Bolognia JL, Jorizzo JL, Rapini RP (Eds). Dermatology (2nd edn). Mosby Elsevier; New Delhi: 2008;25-36.
- Chu DH. Structure and development of skin. In: Wolff K, Goldsmith LA, Katz SI, Gilcrest BA, Paller AS, Leffell DJ (Eds). Fitzpatrick's Dermatology in general medicine (7th edn). Mc Graw Hill; New York: 2007;57-72.
- James WD, Berger TG, Elston DM. Skin: basic structure and function. WB Saunders Company; Canada: 2006;1-13.
- McGrath JA, Uitto J. Anatomy and organization of human skin. In: Burns T, Breathnach S, Cox N, Griffiths C (Eds). Rook's Textbook of Dermatology (8th edn). Blackwell Publishing; Oxford: 2010;3.1-3.53.
- Robinson JK. Anatomy for procedural Dermatology. In: Robinson JK, Hanke CW, Siegel DM, Fratila A (Eds). Surgery of the Skin: Procedural Dermatology (2nd edn). Mosby Elsevier; Edinburg: 2010;3-27.
- Savant SS, Gore D. Applied anatomy for dermatosurgeons. In: Savant SS, Atal-Shah R, Gore D (Eds). Textbook and Atlas of Dermatosurgery and Cosmetology (1st edn). ASCAD; Mumbai: 1998;3-11.