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The Skin: Layers, Structures, Functions Layers of the Skin Layers of the Skin • The skin is composed of 3 layers:1,2 • • A thin outer layer called the epidermis • A thicker middle layer called the dermis • The hypodermis or subcutaneous tissue, which sits below the dermis, and is abundant in fat (adipose tissue) The skin also contains blood vessels (arteries, veins, and capillaries), sensory nerve fibres, and accessory structures such as hair, nails, sebaceous glands, and sweat glands. References 1.Weller R, Hunter J, Savin J, et al. The function and structure of the skin. In: Sugden M, Blundell R, eds. Clinical Dermatology. 4th ed. Malden, MA: Blackwell Publishing, Inc.; 2008:10-33. 2.VenesD, BidermanA, Fenton B, et al, eds. Taber's Cyclopedic Medical Dictionary. 21st ed. Philadelphia, PA: F.A. Davis Company; 2009. Epidermis • The epidermis is the topmost layer of the skin. • It is between 0.1 and 1.0 mm thick, and is made up of many layers of closely packed cells. • It can also become a portal through which foreign substances may enter the body, and thus houses cells that participate in the immune response to protect the body from these invaders.1,2 References 1.Weller R, Hunter J, Savin J, et al. The function and structure of the skin. In: Sugden M, Blundell R, eds. Clinical Dermatology. 4th ed. Malden, MA: Blackwell Publishing, Inc.; 2008:10-33. 2.Norris DA. Structure and function of the skin. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 24th ed. Philadelphia, PA: Elsevier Saunders; 2012:2498-2503 Cells of the Epidermis Cells of the Epidermis Cells of the Epidermis • • The epidermis is primarily made up of keratinocytes, which produce a protein called keratin that gives the skin its strength. Keratinocytes account for approximately 85% of all epidermal cells are also an important component of hair and nails.1,2 References 1.Weller R, Hunter J, Savin J, et al. The function and structure of the skin. In: Sugden M, Blundell R, eds. Clinical Dermatology. 4th ed. Malden, MA: Blackwell Publishing, Inc.; 2008:10-33. 2.Norris DA. Structure and function of the skin. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 24th ed. Philadelphia, PA: Elsevier Saunders; 2012:2498-2503 Cells of the Epidermis Cells of the Epidermis Langerhans cells can be thought of as highly specialised dendritic cells. • They are produced in the bone marrow, then migrate to their target tissue: the middle of the epidermis. • They are an important subset of antigenpresenting cells (APC) that process antigens and present them to T cells in the skin or lymph nodes. • They become more abundant at sites of inflammatory skin disorders.1,2 References 1.Weller R, Hunter J, Savin J, et al. The function and structure of the skin. In: Sugden M, Blundell R, eds. Clinical Dermatology. 4th ed. Malden, MA: Blackwell Publishing, Inc.; 2008:10-33. 2.Norris DA. Structure and function of the skin. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 24th ed. Philadelphia, PA: Elsevier Saunders; 2012:2498-2503 Cells of the Epidermis Cells of the Epidermis Melanocytes are the only cells that can synthesise melanin, a pigment that helps protect the skin from damaging ultraviolet radiation1,2 References 1.Weller R, Hunter J, Savin J, et al. The function and structure of the skin. In: Sugden M, Blundell R, eds. Clinical Dermatology. 4th ed. Malden, MA: Blackwell Publishing, Inc.; 2008:10-33. 2.Norris DA. Structure and function of the skin. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 24th ed. Philadelphia, PA: Elsevier Saunders; 2012:2498-2503 Cells of the Epidermis Cells of the Epidermis Merkel cells are concentrated near hair follicles and enable the sensation of fine touch.1 References 1.Weller R, Hunter J, Savin J, et al. The function and structure of the skin. In: Sugden M, Blundell R, eds. Clinical Dermatology. 4th ed. Malden, MA: Blackwell Publishing, Inc.; 2008:10-33. Epidermal Layers In normal skin, the thickness of the epidermis is kept constant by the balance between shedding of the superficial layer of cells and cell division at the deepest layer.1 References 1.Weller R, Hunter J, Savin J, et al. The function and structure of the skin. In: Sugden M, Blundell R, eds. Clinical Dermatology. 4th ed. Malden, MA: Blackwell Publishing, Inc.; 2008:10-33. Epidermis Layers Epidermis Layers The horny layer is the outermost layer of the epidermis. It is comprised of dead, flattened keratinocytes that provide a barrier for water loss, infectious agents, and toxic chemicals 1 References 1.Weller R, Hunter J, Savin J, et al. The function and structure of the skin. In: Sugden M, Blundell R, eds. Clinical Dermatology. 4th ed. Malden, MA: Blackwell Publishing, Inc.; 2008:10-33. Epidermis Layers Epidermis Layers The granular layer is composed of 2 to 3 layers of keratinocytes undergoing differentiation and programmed cellular death (apoptosis).1,3 Keratinocytes in this layer secrete the lipids needed to prevent the loss of water from the body.1 References 1.Weller R, Hunter J, Savin J, et al. The function and structure of the skin. In: Sugden M, Blundell R, eds. Clinical Dermatology. 4th ed. Malden, MA: Blackwell Publishing, Inc.; 2008:1033. Epidermis Layers Epidermis Layers The spiny layer is composed of keratinocytes undergoing differentiation. They are attached to each other by projections that resemble prickles or spines.1 References 1.Weller R, Hunter J, Savin J, et al. The function and structure of the skin. In: Sugden M, Blundell R, eds. Clinical Dermatology. 4th ed. Malden, MA: Blackwell Publishing, Inc.; 2008:10-33. Epidermis Layers Epidermis Layers The basal layer is the deepest layer of the epidermis. It is composed of a single layer of columnar cells and is responsible for skin cell proliferation.1 References 1.Weller R, Hunter J, Savin J, et al. The function and structure of the skin. In: Sugden M, Blundell R, eds. Clinical Dermatology. 4th ed. Malden, MA: Blackwell Publishing, Inc.; 2008:10-33. Dermis The dermis provides structural and nutritional support to the epidermis Dermis Similar to the epidermis, the dermis also contains Langerhans cells. In addition, the dermis contains dermal dendritic cells that play a role in immune response together with Langerhans cells.1 References 1.Weller R, Hunter J, Savin J, et al. The function and structure of the skin. In: Sugden M, Blundell R, eds. Clinical Dermatology. 4th ed. Malden, MA: Blackwell Publishing, Inc.; 2008:10-33. Dermis Fibroblasts are the primary cell of the dermis and the connective tissues they produce give the skin its elasticity, tensile strength, and resilience to tears.1 References 1.Weller R, Hunter J, Savin J, et al. The function and structure of the skin. In: Sugden M, Blundell R, eds. Clinical Dermatology. 4th ed. Malden, MA: Blackwell Publishing, Inc.; 2008:10-33. Dermis Downward growths of the epidermis and upward projections of the dermis create ridges that increase the bonding area between the epidermis and dermis. These ridges are most readily observed on the fingertips as fingerprints.1 References 1.Weller R, Hunter J, Savin J, et al. The function and structure of the skin. In: Sugden M, Blundell R, eds. Clinical Dermatology. 4th ed. Malden, MA: Blackwell Publishing, Inc.; 2008:10-33. Cells of the Dermis Cells of the Dermis Mast cells can trigger a local inflammatory response to antigen by releasing substances that act on blood vessels. They are particularly important in allergic responses.1 References 1. Murphy K. Basic concepts in immunology. In: Janeway'sImmunobiology. 8th ed. New York, NY: Garland Science; 2012:1-36 Cells of the Dermis Langerhans cells are highly specialised dendritic cells. They are an important subset of antigen-presenting cells (APC) that process antigens and present them to T cells in the skin or lymph nodes. Langerhans cells become more abundant at sites of inflammatory skin disorders.1,2 References 1.Weller R, Hunter J, Savin J, et al. The function and structure of the skin. In: Sugden M, Blundell R, eds. Clinical Dermatology. 4th ed. Malden, MA: Blackwell Publishing, Inc.; 2008:10-33. 2.Norris DA. Structure and function of the skin. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 24th ed. Philadelphia, PA: Elsevier Saunders; 2012:2498-2503. Cells of the Dermis Lymphocytes possess a powerful ability to recognize and mount a targeted attack against pathogenic microorganisms. However, they require the participation of the innate immune system to initiate this attack.1 References 1. Murphy K. Basic concepts in immunology. In: Janeway'sImmunobiology. 8th ed. New York, NY: Garland Science; 2012:1-36. Cells of the Dermis Fibroblasts are the primary cell type of the dermis. They are capable of producing collagen, elastin, and reticular protein fibres, from which the connective tissues develop.1,2 References 1. Weller R, Hunter J, Savin J, et al. The function and structure of the skin. In: Sugden M, Blundell R, eds. Clinical Dermatology. 4th ed. Malden, MA: Blackwell Publishing, Inc.; 2008:10-33. 2. VenesD, BidermanA, Fenton B, et al, eds. Taber's Cyclopedic Medical Dictionary. 21st ed. Philadelphia, PA: F.A. Davis Company; 2009. Cells of the Dermis Macrophages are always present in the dermis. They are considered to be a front-line component of the dermal innate immune response.1 References: 1.Murphy K. Basic concepts in immunology. In: Janeway'sImmunobiology. 8th ed. New York, NY: Garland Science; 2012:1-36. Components of the Dermis The dermis is a fibrous and elastic matrix of connective tissue that provides a structural scaffold for various cells, vessels, nerves, and epidermal appendages, such as hair, sweat glands, and sebaceous glands.1,2 Although the skin consumes little oxygen, it has an abundant blood supply that is found in the dermis. In addition to playing an important role in temperature regulation, the blood vessels in the dermis supply the sweat glands and hair follicles.1,2 References 1. Weller R, Hunter J, Savin J, et al. The function and structure of the skin. In: Sugden M, Blundell R, eds. Clinical Dermatology. 4th ed. Malden, MA: Blackwell Publishing, Inc.; 2008:10-33. 2. Norris DA. Structure and function of the skin. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 24th ed. Philadelphia, PA: Elsevier Saunders; 2012:2498-2503. Types of Fibres in the Dermis Collagen fibre:1 • Constitutes 70% to 80% of the dermis • Prevents skin from tearing Reticular fibre:1 • Fine collagen fibres that surround blood vessels and accessory structures Elastic fibre:1 • Accounts for approximately 2% of the dermis • Returns skin to its unstretched state References Weller R, Hunter J, Savin J, et al. The function and structure of the skin. In: Sugden M, Blundell R, eds. Clinical Dermatology. 4th ed. Malden, MA: Blackwell Publishing, Inc.; 2008:10-33. Hypodermis The hypodermis serves to insulate the body from temperature irregularities, absorb shock, and provide a calorie reserve.1 Consisting primarily of adipose tissue, the hypodermis or subcutaneous tissue lies beneath the dermis, separating it from underlying muscles.2 References 1. Weller R, Hunter J, Savin J, et al. The function and structure of the skin. In: Sugden M, Blundell R, eds. Clinical Dermatology. 4th ed. Malden, MA: Blackwell Publishing, Inc.; 2008:10-33. 2. Norris DA. Structure and function of the skin. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 24th ed. Philadelphia, PA: Elsevier Saunders; 2012:2498-2503 Structures of the Skin Structures of the Skin • Other key structures in the skin include: • hair, • nails, • sweat glands, and • sebaceous glands.1 References 1. Weller R, Hunter J, Savin J, et al. The function and structure of the skin. In: Sugden M, Blundell R, eds. Clinical Dermatology. 4th ed. Malden, MA: Blackwell Publishing, Inc.; 2008:10-33. Hair Follicles • Each hair develops from a group of epidermal cells found at the base of the follicle in the region of cell division. This process is nourished by blood circulation through the hair papilla, a projection of connective tissues in the deep end of the follicle.1 • As epidermal cells are pushed toward the surface, they become keratinized and die, forming hair shafts composed of fused plates of keratin that project from the surface of the epidermis.1 • The hair shaft can act as a channel for discharge of products from apocrine sweat glands and sebaceous glands.1 References 1. Norris DA. Structure and function of the skin. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 24th ed. Philadelphia, PA: Elsevier Saunders; 2012:2498-2503. Hair Follicles • • Hairs do not grow continuously, but pass through a life cycle:1 1. Growth: about 3 years 2. Cessation 3. Rest: about 3 months After this, a new hair develops from the hair follicle and the existing hair falls out.1 References 1. Norris DA. Structure and function of the skin. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 24th ed. Philadelphia, PA: Elsevier Saunders; 2012:2498-2503. Types of Hair Follicles There are 3 types of hair follicles:1 References 1. Norris DA. Structure and function of the skin. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 24th ed. Philadelphia, PA: Elsevier Saunders; 2012:2498-2503. Types of Hair Follicles There are 3 types of hair follicles:1 • Vellus: located over most of the body References 1. Norris DA. Structure and function of the skin. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 24th ed. Philadelphia, PA: Elsevier Saunders; 2012:2498-2503. Types of Hair Follicles There are 3 types of hair follicles:1 • Vellus: located over most of the body • Terminal: located on the scalp, beard area, axilla, groin, and other hairy areas References 1. Norris DA. Structure and function of the skin. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 24th ed. Philadelphia, PA: Elsevier Saunders; 2012:2498-2503. Types of Hair Follicles There are 3 types of hair follicles:1 • Vellus: located over most of the body • Terminal: located on the scalp, beard area, axilla, groin, and other hairy areas • Sebaceous: located on the scalp, face, beard, chest, back, axilla, and groin; have a minimal hair shaft but hypertrophied sebaceous glands References 1. Norris DA. Structure and function of the skin. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 24th ed. Philadelphia, PA: Elsevier Saunders; 2012:2498-2503. Sweat Glands There are 2 types of sweat glands:1 1. 2. Apocrine • Scent glands that secrete products into hair follicles • Develop following onset of puberty • Secretion increased by heightened tension, such as fear or sexual excitement • Present in axilla, external genitalia, areolar skin around nipples, and perianal area Eccrine • Secrete sweat directly onto skin’s surface in response to heat • Found throughout the skin, but concentrated in the palms, soles, and head References 1. Norris DA. Structure and function of the skin. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 24th ed. Philadelphia, PA: Elsevier Saunders; 2012:2498-2503. Sebaceous Sweat Glands Sebaceous glands produce a type of oil called sebum. These glands:1 • Are located throughout dermis, except for palms of hands and soles of feet • Become activated by onset of puberty • Secrete fatty acids, triglycerides, and other lipids into hair follicles References 1. Norris DA. Structure and function of the skin. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 24th ed. Philadelphia, PA: Elsevier Saunders; 2012:2498-2503. Apocrine Sweat Glands Apocrine sweat glands are anchored in the dermis, and the composition of their discharge is not completely understood.1 They secrete strong-smelling substances and are responsible for body odour and pheromones.1,2 Locations:1 1. Axilla 2. External genitalia 3. Skin around nipples 4. Perianal area Development:1 • Following onset of puberty References 1. Norris DA. Structure and function of the skin. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 24th ed. Philadelphia, PA: Elsevier Saunders; 2012:2498-2503. 2. Weller R, Hunter J, Savin J, et al. The function and structure of the skin. In: Sugden M, Blundell R, eds. Clinical Dermatology. 4th ed. Malden, MA: Blackwell Publishing, Inc.; 2008:10-33. Apocrine Sweat Glands Apocrine sweat glands are anchored in the dermis, and the composition of their discharge is not completely understood.1 They secrete strong-smelling substances and are responsible for 1,2 localisation of apocrine sweat glands • Since the anatomical body odour and pheromones. closely corresponds to the typical distribution of HS lesions, Locations:1apocrine sweat glands were historically thought to be 1. Axilla implicated in the pathogenesis of the disease. • However, 2. External genitalia this theory has since been discarded in favour of a primary role for follicular occlusion in the development of 3. Skin around nipples 3,4 the disease. 4. Perianal area Development:1 • Following onset of puberty References 1. Norris DA. Structure and function of the skin. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 24th ed. Philadelphia, PA: Elsevier Saunders; 2012:2498-2503. 2. Weller R, Hunter J, Savin J, et al. The function and structure of the skin. In: Sugden M, Blundell R, eds. Clinical Dermatology. 4th ed. Malden, MA: Blackwell Publishing, Inc.; 2008:10-33. 3. Layton A. Pathology of HidradenitisSuppurativa. In: JemecGBE, RevuzJ, Leyden JL, eds. HidradenitisSuppurativa. Germany: Springer; 2006:25-33. 4. PoliF, JemecGBE, RevuzJ. Clinical Presentation. In: JemecGBE, RevuzJ, Leyden JL, eds. HidradenitisSuppurativa. Germany: Springer; 2006:11-24. Functions of the Skin Functions of the Skin The skin performs multiple protective and regulatory functions.1 1. Protection 2. Immunologic Response 3. Regulation of Body Temperature 4. Sensory Input 5. Metabolic Functions 6. Barrier to Water References 1. Weller R, Hunter J, Savin J, et al. The function and structure of the skin. In: Sugden M, Blundell R, eds. Clinical Dermatology. 4th ed. Malden, MA: Blackwell Publishing, Inc.; 2008:10-33. Functions of the Skin The skin performs multiple protective and regulatory functions.1 1. Protection • The epidermis resists friction and tangential stress. Melanin pigment and antioxidant enzymes in the epidermis protect the skin against radiation. • The horny layer of the epidermis acts as a barrier to external substances; it is thickened on the palms of the hand and soles of the feet to provide extra padding and protection. • The reticular dermis is a barrier to injury and trauma as well as a cushion for underlying structures.2 References 1. Weller R, Hunter J, Savin J, et al. The function and structure of the skin. In: Sugden M, Blundell R, eds. Clinical Dermatology. 4th ed. Malden, MA: Blackwell Publishing, Inc.; 2008:10-33. 2. Norris DA. Structure and function of the skin. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 24th ed. Philadelphia, PA: Elsevier Saunders; 2012:2498-2503. Functions of the Skin The skin performs multiple protective and regulatory functions.1 2. Immunologic Response • The skin is the outermost arm of the immune response and is designed to defend against infection, abnormal cells, and toxins through innate and adaptive immune responses.2,3 References 1. Weller R, Hunter J, Savin J, et al. The function and structure of the skin. In: Sugden M, Blundell R, eds. Clinical Dermatology. 4th ed. Malden, MA: Blackwell Publishing, Inc.; 2008:10-33. 2. Norris DA. Structure and function of the skin. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 24th ed. Philadelphia, PA: Elsevier Saunders; 2012:2498-2503. 3. Murphy K. Basic concepts in immunology. In: Janeway'sImmunobiology. 8th ed. New York, NY: Garland Science; 2012:1-36. Functions of the Skin The skin performs multiple protective and regulatory functions.1 3. Regulation of Body Temperature • Sweat and blood vessels play an important role in body temperature regulation2 • Evaporation from eccrine sweat glands is critical for thermoregulation. • Dilation or constriction of blood vessels regulates heat exchange in the skin. • Up to 4% to 5% of total blood volume in the body may be stored in skin vessels References 1. Weller R, Hunter J, Savin J, et al. The function and structure of the skin. In: Sugden M, Blundell R, eds. Clinical Dermatology. 4th ed. Malden, MA: Blackwell Publishing, Inc.; 2008:10-33. 2. Norris DA. Structure and function of the skin. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 24th ed. Philadelphia, PA: Elsevier Saunders; 2012:2498-2503. Functions of the Skin The skin performs multiple protective and regulatory functions.1 4. Sensory Input • The skin is the largest sensory organ of the body.1,2 • The skin and mucous membranes are the principal sites of both pleasant and unpleasant sensations.1,2 • Specialised sensory structures register pressure applied to the skin, and free nerve endings detect heat and pain.1,2 References 1. Weller R, Hunter J, Savin J, et al. The function and structure of the skin. In: Sugden M, Blundell R, eds. Clinical Dermatology. 4th ed. Malden, MA: Blackwell Publishing, Inc.; 2008:10-33. 2. Norris DA. Structure and function of the skin. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 24th ed. Philadelphia, PA: Elsevier Saunders; 2012:2498-2503. Functions of the Skin The skin performs multiple protective and regulatory functions.1 5. Metabolic Functions • One of the metabolic functions of the skin is the synthesis of vitamin D by keratinocytes. 1,2 • Sunlight converts molecules in the epidermis to a precursor of vitamin D, which is transported to the kidneys to be converted into active vitamin D.1,2 • Active vitamin D facilitates gastrointestinal absorption of calcium.1,2 References 1. Weller R, Hunter J, Savin J, et al. The function and structure of the skin. In: Sugden M, Blundell R, eds. Clinical Dermatology. 4th ed. Malden, MA: Blackwell Publishing, Inc.; 2008:10-33. 2. Horst RL, Reinhardt TA, Reddy GS. Vitamin D metabolism. In: Feldman D, Pike JW, GlorieuxFH, eds. Vitamin D. 2nd ed. Burlington, MA: Elsevier Inc.; 2005:15-36. Functions of the Skin The skin performs multiple protective and regulatory functions.1 5. Barrier to Water • Sebum is secreted into hair follicles and helps keep the hairs and the skin soft and relatively waterproof.1 • The horny layer of the epidermis prevents water loss by evaporation.1,2 References 1. Weller R, Hunter J, Savin J, et al. The function and structure of the skin. In: Sugden M, Blundell R, eds. Clinical Dermatology. 4th ed. Malden, MA: Blackwell Publishing, Inc.; 2008:10-33. 2. Norris DA. Structure and function of the skin. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 24th ed. Philadelphia, PA: Elsevier Saunders; 2012:2498-2503.