Basic Science of the Skin PDF

Basic Science of the Skin 10/22/2022 Saja Hamed, Ph.D  The skin is the largest organ of the human body (16% of total body wt.) The skin is composed of: - Epidermis - Dermis - Subcutaneous tissue 10/22/2022 Saja Hamed, Ph.D The Epidermis The epidermal barrier: - L...

Basic Science of the Skin 10/22/2022 Saja Hamed, Ph.D  The skin is the largest organ of the human body (16% of total body wt.) The skin is composed of: - Epidermis - Dermis - Subcutaneous tissue 10/22/2022 Saja Hamed, Ph.D The Epidermis The epidermal barrier: - Limit passive water loss from the body - Reduce the absorption of chemicals from the environment - Prevent microbial infection 10/22/2022 Saja Hamed, Ph.D  Transepidermal Water Loss The water produced during respiration is normally lost through the skin. It moves through the epidermis as water vapor, hence called transepidermal water loss (TEWL). TEWL varies with body site, with normal values of ~6 grams/m2/hour. 10/22/2022 Saja Hamed, Ph.D  Epidermal layers and cells 10/22/2022 Saja Hamed, Ph.D  Layers of the Epidermis English Latin Alternate Basal cell layer Stratum basale S Germinativium Malpighian layer Prickle layer S. Spinosum Malpighian layer Granular Layer S. Granulosum Malpighian layer Clear Layer Stratum Lucidum Horny Layer Stratum Corneum 10/22/2022 Saja Hamed, Ph.D Epidermis: The top layer of the skin is itself divided into layers. The main barrier to water loss and entry of compounds in the skin is the SC. Understanding the SC is key to understanding the behavior of topical products The predominant cell type of the epidermis is keratinocytes They make keratins; the major structural proteins of the SC There are 2 other cell types in the epidermis: 1. Melanocytes 2. Langerhans cells 10/22/2022 Saja Hamed, Ph.D  Courtesy of SB Hoath, MD 10/22/2022 Saja Hamed, Ph.D Epidermis Melanocytes: - the pigment producing cells of the skin and hair - Found in the basal layer and make pigment granules called melanosomes containing melanin - Melanosomes are transferred to the epidermal keratinocytes - They impart some protection to the cell nucleus from UV light and give the skin its color 10/22/2022 Saja Hamed, Ph.D  Melanocytes: The melanocyte is a dendritic cell that sits in the basal layer of the epidermis and transfers pigment to keratinocytes in the form of pigment granules called melanosomes. 10/22/2022 Saja Hamed, Ph.D  The Langerhans’ Cell - Dendritic immune cells - Antigen-presenting cell of the skin - Important to the immune function of the epidermis - Participate in contact allergy 10/22/2022 Saja Hamed, Ph.D  Contact sensitization Small molecules (Haptens) penetrate and bind to proteins. Hapten-protein complex recognized by Langerhans cells Move to lymph nodes and stimulate clonal proliferation of sensitized t-cells T-cells return to epidermis to mount reaction 10/22/2022 Saja Hamed, Ph.D  Contact sensitization 10/22/2022 Saja Hamed, Ph.D  The Finn chamber Photo from Color text of Contact Dermatitis, Larsen, Adams and Maibach, W. B. Saunders Co. Philadelphia 1992 10/22/2022 Saja Hamed, Ph.D  Patch testing for contact allergy Photo from Color text of Contact Dermatitis, Larsen, Adams and Maibach, W. B. Saunders Co. Philadelphia 1992 10/22/2022 Saja Hamed, Ph.D  A positive reaction Photo from Color text of Contact Dermatitis, Larsen, Adams and Maibach, W. B. Saunders Co. Philadelphia 1992 10/22/2022 Saja Hamed, Ph.D  Most reactions to cosmetic products are to preservatives or fragrances Photo from Color text of Contact Dermatitis, Larsen, Adams and Maibach, W. B. Saunders Co. Philadelphia 1992 10/22/2022 Saja Hamed, Ph.D  Benadryl Photo from Color text of Contact Dermatitis, Larsen, Adams and Maibach, W. B. Saunders Co. Philadelphia 1992 10/22/2022 Saja Hamed, Ph.D  Cinnamal Photo from Color text of Contact Dermatitis, Larsen, Adams and Maibach, W. B. Saunders Co. Philadelphia 1992 10/22/2022 Saja Hamed, Ph.D  Nickel 10/22/2022 Saja Hamed, Ph.D  Keratinocytes Keratinocytes are the cells that make up the majority of the epidermis produced by stem cells which reside at the base of the epidermis. When the stem cells divide they create “daughter cells” which slowly move to the top of the epidermis The process of daughter cells maturation and moving to the top is called keratinization 10/22/2022 Saja Hamed, Ph.D  Timing is everything in SC formation Keratinization process is also referred to as the “cell cycle” Normal cell cycle of the epidermis is 26 -42 days Desquamation process normally occurs invisibly with shedding of individual cells Disturbances of this process may result in accumulation of partially detached keratinocytes 10/22/2022 Saja Hamed, Ph.D 10/22/2022 Saja Hamed, Ph.D  Psoriasis 10/22/2022 Saja Hamed, Ph.D  Psoriasis A hyperproliferative hyperkeratosis In psoriasis the skin turns over much too fast. Nucleated cells can be found in psoriatic plaques. These cells are called parakeratotic and should not exist in the SC 10/22/2022 Saja Hamed, Ph.D  Epidermal layers and cells As daughter cells move through the epidermis and mature, they develop different characteristics The layers of the epidermis are named for these characteristics 10/22/2022 Saja Hamed, Ph.D Basal layer: located at the base of the epidermis, cells are cuboidal in shape, responsible for maintaining the epidermis by continually renewing the cell population 10/22/2022 Saja Hamed, Ph.D Spinous layer: the cells have spiny attachments called desmosomes 10/22/2022 Saja Hamed, Ph.D Granular layer: Is named for the granules that appears in the cells: 1. Keratohyalin granules: full of protein (filaggrin, Loricrin) 2. Lamellar bodies: contain lipids, proteolytic enzymes involved in desmosomes hydrolysis, defensins At the granular layer keratinocytes are transformed to corneocytes also called squames to form the SC: 1. The nucleus is digested 2. The cytoplasm disappears 3. The lipids are released into the intercellular space 4. The cell membrane is replaced by a cell envelope (made of cross-linked protein with lipids covalently attached to its surface) 10/22/2022 Saja Hamed, Ph.D Granular layer: 10/22/2022 Saja Hamed, Ph.D  The cornified cell envelope (after Steinert) 15 nm thick 66% Loricrin Involucrin is on the outer surface Keratin fibers attach to inner surface Lipids are attached to the outer surface 10/22/2022 Saja Hamed, Ph.D Transglutaminase: A membrane bound enzyme that forms the cornified cell envelope by cross-linking proteins with isopeptide bonds. 10/22/2022 Saja Hamed, Ph.D  SEM of Corneocytes The corneocytes that results from this transformation is a flat cell (a hexagon or pentagon) with a surface area of 1000µm2 and a thickness of 0.5 to 1µm 10/22/2022 Saja Hamed, Ph.D  Keratohyalin Proteins Filaggrin: - Contains a high level of positively charged amino acids - Filament-aggregating proteins - The role of filaggrin is to help form the keratin microfibrils - After filaggrin forms the microfibrils it is modified to come off the microfibrils - It is then digested by enzymes to produce important components of the natural moisturizing factor (NMF) of the SC 10/22/2022 Saja Hamed, Ph.D  Formation of the coiled- coil Two keratin chains interact to form a coiled-coil. 10/22/2022 Saja Hamed, Ph.D  The Coiled Coil 10/22/2022 Saja Hamed, Ph.D  The microfibril In the stratum granulosum filaggrin interacts with keratin coiled-coils causing condensation to a 32 chain structure called the microfibril 10/22/2022 Saja Hamed, Ph.D  At the SG/SC interface keratinocytes are transformed to corneocytes Keratinocytes are normal cells with the normal constituents of cells i.e cell membrane, nucleus, cytoplasm Corneocytes or squames are flat cells with no cytoplasm, full of condensed keratin fibers and covered by a very tough structure called the cell envelope 10/22/2022 Saja Hamed, Ph.D Stratum Corneum On most body sites it is 12 to 16 cell layers thick – Flattened cells called corneocytes or squames with resistant cell envelope and covalently attached lipids – Content is keratin filaments – Squames joined by desmosomes (protein glue spots) Intercellular lipids (lipids outside the squames) – Multiple layers of polar but relatively hydrophobic lipids between cells – Ceramides, cholesterol and long chain fatty acids 10/22/2022 Saja Hamed, Ph.D  SC thickness and TEWL with Body Site BODY SITE SC LAYERS TEWL Face n = 84 9 2 No Data Forehead n = 8 9 1 No data Eyelid n = 16 82 15 Cheek n = 43 10  2 11 Upper Arm n = 2 14 4 Back n = 3 13  3 5 Abdomen n = 44 144 4 Thigh 16±4 4 Forearm n = 4 164 4-6* Data from Ya-Xian et al Arch Dermatol Res 291:555-559(1999) 10/22/2022 Saja Hamed, Ph.D  The barrier to water loss through the skin (TEWL) is the stratum corneum and removing the SC by tape stripping leads to dramatically increased TEWL From MS thesis of Scott Coapman UC 1988 10/22/2022 Saja Hamed, Ph.D Stratum Corneum The SC is modeled as a brick wall Bricks = The corneocytes with their resistant cell envelopes and keratin microfibrils The bricks are linked by desmosomes Mortar = layers of lipids found between cells The lipid is the main barrier to water passing out through the SC 10/22/2022 Saja Hamed, Ph.D The SC is composed of protein-rich corneocytes embedded in a bilayer lipid matrix arranged in a “brick and mortar” fashion 10/22/2022 Saja Hamed, Ph.D  In the SC the bricks are joined together by protein glue spots called desmosomes 10/22/2022 Saja Hamed, Ph.D  Desmosomes in the SC Desmosomes are digested to allow desquamation Upper SC Digested desmosome Lower SC Intact desmosome Rawlings et al (JSCC 45, 203, 1994) 10/22/2022 Saja Hamed, Ph.D Stratum Corneum The lipids organize into multiple layers between the SC cells The SC contains no phospolipids Phospholipids are broken down by phosholipases in the lower SC This produces fatty acids which may play a role in producing the acid pH of the SC (SC surface pH 4-5.5) The SC acidic pH (acid mantle) may play a role in protecting against colonization of skin surface by harmful bacteria 10/22/2022 Saja Hamed, Ph.D 10/22/2022 Saja Hamed, Ph.D  The barrier lipids of the SC There are multiple lipid layers between the cells Micrograph courtesy of Ronald Warner 10/22/2022 Saja Hamed, Ph.D 10/22/2022 Saja Hamed, Ph.D  Lipid distributions in the epidermis Note that there are no phospholipids in the SC 10/22/2022 Saja Hamed, Ph.D Stratum Corneum The Horny layer function as a protective layer It defends us against: - Dehydraton (i.e. prevent transepidermal water loss (TEWL)) - External toxins - Bacterial infection - Protect the more fragile keratinocytes below from mechanical disruption Byproducts formed from the breakdown of filaggrin (i.e. amino acids and their metabolites) are called Natural Moisturizing Factor (NMF) Extracellular lipids and intracellular NMF play an important role in skin hydration 10/22/2022 Saja Hamed, Ph.D  Natural Moisturizing Factor Inside the cells of the SC NMF consists of lactate, amino acids, pyrollidone carboxylic acid (PCA) Gives the SC its humectant (water-binding) qualities It is made of very water soluble chemicals so that it can absorb large amounts of water even when humidity levels are low This enables the SC to retain a high water content even in a dry environment It also provides an important aqueous environment for enzymes 10/22/2022 Saja Hamed, Ph.D  Natural Moisturizing Factor Ichthyosis vulgaris patients  sever dryness and scaling of the skin Normal skin washed with soap has lower levels of NMF compared to skin not washed with soap Levels of NMF decline with age  contribute to the increased incidence of dry skin in the elderly population 10/22/2022 Saja Hamed, Ph.D 10/22/2022 Saja Hamed, Ph.D  Ichthyosis vulgaris results from a defect in profilaggrin synthesis. 10/22/2022 Saja Hamed, Ph.D Dermis It is responsible for the thickness of skin: - Varies over different parts of the body - Double between the ages of 3 and 7 years and at puberty - Decrease with aging Consists mostly of collagen Primary cell type is fibroblasts They produce collagen, elastin, other matrix proteins, and enzymes (e.g. collagenase) Nerves, blood vessels, sweat glands, and immune cells (e.g. mast cells, lymphocytes, and macrophages) 10/22/2022 Saja Hamed, Ph.D  Dermis: The dermis is composed of an amorphous intercellular substance. Within this amorphic substance are: - Cells of the dermis - Collagen and elastin fibers - Blood vessels - Nerves and sensory organ - sebaceous glands - Hairs - Sweat glands Its upper level has projections that extend into depressions in the epidermis 10/22/2022 Saja Hamed, Ph.D  Papillary dermis: - uppermost portion - Contain fine , delicate type III collagen fibers Reticular dermis: - lower portion - It is a net of thick, dense connective tissue composed primarily of thick type I collagen fibers - Type I collagen provides support and bulk to the skin 10/22/2022 Saja Hamed, Ph.D 10/22/2022 Saja Hamed, Ph.D  The junction between the epidermis and dermis is known as the dermal-epidermal junction (DEJ) 10/22/2022 Saja Hamed, Ph.D Collagen Give the skin its strength Comprises 70-80% of dermis dry weight A complex family of 18 proteins (11 are present in the dermis) Is synthesized in the fibroblasts as procollagen: - Proline (Prolyl hydroxylase) hydroxyproline - Lysine (lysyl hydroxylase)  hydroxylysine - Both reactions require the presence of Fe++, ascorbic acid, and - ketoglutarate 10/22/2022 Saja Hamed, Ph.D 10/22/2022 Saja Hamed, Ph.D Elastin They are thinner than collagen fibers Responsible for the skin’s elasticity Forms a delicate, freely branching fibers which can be stretched by 100% or more but return to their original length when the stress is removed Elastin degrades with significant levels of sun exposure = Elastosis Collagen and elastic fibers are embedded in a water-binding, gelatinous like ground substance that contains glycosaminoglycans 10/22/2022 Saja Hamed, Ph.D Ground substance Proteoglycans Role: - Support for other dermal components - Mediate attachment of fibroblasts and growth factors involved in dermal repair 10/22/2022 Saja Hamed, Ph.D Glycosaminoglycans (GAGs) Polysaccharide chains composed of repeating disaccharide units that are linked to a core protein They bind water  may contribute to the maintenance of salt and water balance GACs content decrease with aging The most abundant GACs in the dermis is Hyaluronic acid (HA) and dermatan sulfate HA is a popular ingredients in cosmetic products because it acts as a humectant 10/22/2022 Saja Hamed, Ph.D  What is the Net Charge of the proteins found in the skin’s upper layer? Isoelecetric Point= 3.5-4.5 PH=4.5-5.5 Conditioning Hand sanitizers 10/22/2022 Saja Hamed, Ph.D