© Jones and Bartlett Publishers, LLC. NOT FOR SALE OR DISTRIBUTION CHAPTER Basic Biology of the Skin 3 The skin is often underestimated for its impor- Layers of the skin: tance in health and disease. As a consequence, it’s frequently understudied by chiropractic students 1. Epidermis—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 anatomy and physiol- croscopically identifi ed: ogy of the skin, but rather a review of the basic Stratum corneum—also known as the biology of the skin as a prerequisite to the study horny cell 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 protein known as keratin. 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 bacteria. 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 hands and feet are thick as they are students and chiropractors. more prone to injury. 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 stratum basale (basal cell layer). The cross-section of human 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- Stratum lucidum—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. physiology is essential to understanding patho- Stratum granulosum—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 Stratum spinosum—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 keratins that function to sup- port structures. The cells are adherent by Non-hairy (glabrous)—a skin type on the specialized cells known as desmosomes. palms and soles, it has thicker epidermis Stratum basale or Stratum germinava- and lacks hair 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 glands. consists of tall columnar cells that are 29 03ch_pg029-032_Wiles.indd 29 10/7/09 1:36:52 PM 30 CHAPTER 3 Basic© Jones Biology and of Bartlett the Skin Publishers, LLC. NOT FOR SALE OR DISTRIBUTION Cross section of the skin Hair shaft Epidermis Melanocytes Sebaceous gland Hair erector muscle Hair follicle Dermis Sweat gland Blood vessels Pacinian corpuscle Subcutaneous Adipose tissue 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 cell division and the cell layer is attached to a basement help form new keratinocytes (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 melanin, Langerhans cells 03ch_pg029-032_Wiles.indd 30 10/7/09 1:37:02 PM © Jones and Bartlett Publishers, LLC. NOT FOR SALECHAPTER OR DISTRIBUTION 3 Basic Biology of the Skin 31 which recognize antigens and present them sweat containing urea and water and play a to the immune system and Merkel discs role in temperature regulation. To facilitate which detect pressure on skin. heat loss in hot temperatures, the blood 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 burns fat stored cutaneous layer. This middle layer of skin con- in the adipose tissue. The burning of brown tains connective tissue in the form of collagen fat under sympathetic stimulation is com- in bulk and elastin 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, nerves, 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. Subcutaneous tissue—also know as the sub- for vasoconstriction under sympathetic cutis 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 acne. Innervation of the skin: Immunity—Langerhans cells in the skin Parasympathetic nerves—consisting of are dendritic cells that take up microbial cho linergic neurons 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 smooth muscle and and anatomist that discovered these cells in erector pili muscle. the skin when he was a medical student. Storage —the skin is an organ which stores Functions of the Skin: fats 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- • Pacinian corpuscle—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 capillary constric- • Ruffi ni’s corpuscle—joint movements tion. A deeper stroke 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 hives • Nociceptors—detection of pain. (urticaria). Vitamin 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 excretion of D (cholecalciferol) that is ingested mainly waste products—sweat glands produce from diet such as milk and dairy products. 03ch_pg029-032_Wiles.indd 31 10/7/09 1:37:03 PM 32 CHAPTER 3 Basic© Jones Biology and of Bartlett the Skin Publishers, LLC. NOT FOR SALE OR DISTRIBUTION Cholecalciferol is converted into 25-hy- Nail cuticle droxycholecalciferol (25-OH) in the liver Nail root Lunula 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 calcium absorption from the Nail bed 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 oxygen and water. Certain drugs 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. Hyponychium—the soft tissue beneath the Type Definition Description nail bed. I Always burns but never Pale skin, red hair, Cuticle—the skin that overlaps the nail tans freckles plate. II Usually burns, Fair Skin sometimes tans References III May burn, usually tans Darker Skin Burns DA, Breathnach SM, Cox N, Griffi ths CE. Rook’s IV Rarely burns, always Mediterranean Textbook of Dermatology, 7th Ed.