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Basic Biology of the Skin 3

© Jones and Bartlett Publishers, LLC. NOT FOR SALE OR DISTRIBUTION

CHAPTER Basic of the 3

The skin is often underestimated for its impor- Layers of the skin: tance in and disease. As a consequence, it’s frequently understudied by chiropractic students 1. —the outer most layer of the skin (and perhaps, under-taught by chiropractic that is divided into the following fi ve layers school faculty). It is not our intention to present a from top to bottom. These layers can be mi- comprehensive review of and physiol- croscopically identifi ed: ogy of the skin, but rather a review of the basic —also known as the biology of the skin as a prerequisite to the study horny layer, consisting mainly of kera- of pathophysiology of skin disease and the study tinocytes (fl at squamous cells) containing of diagnosis and treatment of skin disorders and a known as . The thick layer diseases. The following material is presented in prevents water loss and prevents the entry an easy-to-read point format, which, though brief of . The thickness can vary region- in content, is suffi cient to provide a refresher ally. For example, the stratum corneum of course to mid-level or upper-level chiropractic the and feet are thick as they are students and chiropractors. more prone to . This layer is continu- Please refer to Figure 3-1, a cross-sectional ously shed but is replaced by new cells from drawing of the skin. This represents a typical the (basal cell layer). The cross-section of skin and features most of stratum corneum constitutes about 15 to the major components in such a typical section of 20 layers. skin. Most skin disease can be characterized as ei- —this layer is present in ther epidermal, or dermal, or epidermal and the thick skin of palms and soles and con- eroding, or spreading, into the dermal layer. As sists of a transparent layer of dead cells. It will be seen in Chapter 4, Pathophysiology of the functions as a barrier and also has water Skin, an understanding of normal anatomy and proof properties. is essential to understanding patho- —also known as the physiology and serves as a basis to de-mystify granular layer, consisting mainly of strati- many skin conditions. fi ed squamous cells arranged in 1 to 3 rows containing lamellar granules and tonofi - Embryology of the skin: brils. It is important to note that besides the palms and soles, skin lacks a well defi ned The skin is mainly mesodermal in its embryonic stratum lucidum and stratum granulosum. derivation. Specialized skin cells and structures —also known as the are formed from 3-6 months of gestation. spinous layer consisting mainly of a cuboi- dal cell arranged in multiple layers and Types of skin: synthesizes that function to sup- port structures. The cells are adherent by Non-hairy (glabrous)—a skin type on the specialized cells known as . palms and soles, it has thicker epidermis Stratum basale or Stratum germinava- and lacks follicles. tum—also known as the basal cell layer, is Hairy—a type of skin having hair follicles the deepest layer of the epidermis. The layer and sebaceous . consists of tall columnar cells that are

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Cross section of the skin

Hair shaft

Epidermis

Sebaceous

Hair erector muscle

Blood vessels

Pacinian corpuscle

Subcutaneous Adipose

Figure 3-1 Cross-section drawing of the skin showing the epidermis, dermis and subcutaneous layers.

Stratum corneum

Stratum lucidum

Stratum granulosum

Stratum spinosum

Stratum basale

Figure 3-2 Division of the epidermal layers of the skin.

constantly undergoing and the cell layer is attached to a basement help form new (keratiniza- membrane which serves as a demarcation tion) that will replace the lost ones from or a boundary between the epidermis and stratum corneum. This process takes about dermis. The layer also contains melano- 27 days. Further down the stratum basale, cytes containing , Langerhans cells

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which recognize antigens and present them sweat containing and water and play a to the and Merkel discs role in regulation. To facilitate which detect on skin. heat loss in hot , the 2. Dermo-epidermal junction—a well demar- vessels in the skin dilate and sweat glands cated junction that lies between the epidermis become active. Alternatively, in cold tem- and dermis. peratures skin blood vessels constrict to 3. Dermis—lies between the epidermis and sub- conserve heat and the body stored cutaneous layer. This middle layer of skin con- in the . The burning of brown tains in the form of fat under sympathetic stimulation is com- in bulk and in minimal quantities, with mon in infants. In colder temperatures, the a rich intertwining blood supply. The types of sweat glands become inactive and the erec- cells located in the dermis are fi broblasts, mast tor pili muscles become functional to pro- cells and histocytes. Hair follicles, , lym- mote trapping of air for insulation of skin. ␣ phatic vessels and sweat glands also reside in The adrenergic receptors like 1, innervat- the dermal layer of the skin. ing the skin blood vessels are responsible 4. —also know as the sub- for under sympathetic or hypodermis is the lower most layer stimulation. comprising mainly of fat (adipose) which pro- Lubrication of the skin is provided by se- vides protection from injury, produces heat baceous glands, which produce an oily sub- and serves as a cushion for the body. stance known as sebum. Occlusion and in- fection of these glands can lead to conditions such as . Innervation of the skin: —Langerhans cells in the skin Parasympathetic nerves—consisting of are dendritic cells that take up microbial cholin ergic that release acetylcho- antigens in the skin to transform into anti- line to the sweat glands. gen presenting cells and provide immunity Sympathetic—consisting of adrenergic by interacting with T cells. The name Lang- neu rons that release norepinephrine to the erhans comes from the German physician sweat glands, arteriolar and and anatomist that discovered these cells in erector pili muscle. the skin when he was a medical student. Storage —the skin is an which stores Functions of the Skin: to provide insulation. This is mainly in the subcutaneous layer. Protection and repair which is provided Sensation—sensation occurs through spe- mainly by keratinocytes while UV protec- cialized structures known as mechanore- tion is offered by melanocytes. The subcu- ceptors: taneous layer protects the deeper body or- • —vibration. gans. Gentle stroking of the skin with a • Meissner’s corpuscle—tapping and fl icker, blunt object can result in white line re- point discrimination. sponse caused mainly by constric- • Ruffi ni’s corpuscle— movements tion. A deeper using a tongue blade and stretch. will lead to the triple cell response, result- • Hair follicle receptor—speed and direc- ing in a red line, fl are and wheal. The wheal tion of movement. is caused by the release of histamine that • Merkel’s discs—vertical dimpling of the acts as a vasodilator in local response to in- non hairy skin. jury. The eliciting of the red wheal is known • Tactile discs—vertical dimpling of the as dermographism that is more pro- hairy skin. nounced in patients who suffer from —detection of . (urticaria). D synthesis —skin is a rich source Skin color is given by melanocytes that of 7-dehydrocholesterol and under the ef- contain melanin. fect of UV light is converted into Vitamin Temperature regulation and of D (cholecalciferol) that is ingested mainly waste products—sweat glands produce from diet such as milk and dairy products.

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Cholecalciferol is converted into 25-hy- droxycholecalciferol (25-OH) in the liver Nail root and fi nally to activated 1, 25 hydroxychole- calciferol (1,25 OH) in the kidneys. The activated 1, 25 hydroxycholecalciferol plays a vital role in from the Nail intestine and kidneys. Aesthetic—skin can be seen as a mode of communication or attraction. Hyponichium Absorption—the skin has the ability to ab- sorb and water. Certain such as topical steroids that are applied topically Phalanx could be absorbed through skin surface. Figure 3-3 Cross section of a typical fi ngernail.

Skin Types: Skin can be classifi ed based on its reaction to ul- Lunula—the half moon shaped structure traviolet radiation: at the base of the nail. —the beneath the Type Definition Description nail bed. I Always burns but never Pale skin, red hair, Cuticle—the skin that overlaps the nail tans plate. II Usually burns, Fair Skin sometimes tans References III May , usually tans Darker Skin Burns DA, Breathnach SM, Cox N, Griffi ths CE. Rook’s IV Rarely burns, always Mediterranean Textbook of , 7th Ed. Oxford UK. tans Blackwell Publishing 2004 V Moderate Latin American, Fitzpatrick JE, Morelli JG. Dermatology Secrets. 3rd Ed. constitutional Middle Eastern Philadelphia, PA. Mosby. 2006 pigmentation Mackie RM. Clinical Dermatology 4th Ed. New York, VI Marked constitutional Black New York. Oxford Press 2004 pigmentation Habif TP Skin Disease Diagnosis and Treatment. 2nd Ed. Philadelphia, PA. Elsevier Mosby 2005 Fitzpatrick TB, Wolff K, Johnson RA, Suurmond R. Essential skin facts: Fitzpatrick’s Color Atlas & Synopsis of Clinical Der- Largest organ of the body. matology. 5th Ed. New York, New York. Mc Graw- Accounts for 15% body weight. Hill 2005 The organ of the body that is most exposed Standring S. Gray’s Anatomy. Elsevier 39th Ed. Phila- to bacteria, UV light, , and other delphia, PA. Elsevier Mosby 2006 Costanzo LS. Physiology. 3rd Ed. Philadelphia, PA environmental stressors. Elsevier. 2006 Every 24 hours the surface of skin sheds Richard W, Hunter J, Savin J, Dahl M. Clinical Derma- dead layer of cells, and on average 40 kg of tology. 4th Ed Sussex UK. Blackwell Publishers skin is shed during lifetime. 2008 Dead skin cells can become a component Eady RA. McGrath JA. Recent advances in the molecu- of household dust. lar basis of inherited skin diseases. Adv. Genet. 2001;43: 1–32 Structure of the Nail: Nail bed—the hard surface of the nail that can be visualized.

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