Module 3.1 Skin N. Swailes, Ph.D. Required reading Mescher AL, Junqueira’s Basic Histology Text and Atlas, 12th Edition, Chapter 18: pp316-331 Learning objectives 1) Use your knowledge of the basic tissues to describe the histological organization of skin. 2) Identify the epidermis and discuss its embryological origin, organization and functions. 3) Identify the dermis and hypodermis and discuss their embryological origins, organization and functions. 4) Examine a variety of skin adnexa and determine their function. 5) Think about the histological organization of skin the next time you are making an incision during Gross Anatomy. A: General organization Skin is divided into 3 main regions, each providing a distinct role in the overall function of the skin. 1. Epidermis - is the most superficial layer of the skin - is derived from embryonic ectoderm and is a specialized stratified squamous epithelium - forms first physical and immunological barrier to invasion of foreign substances into the body. 2. Dermis - is connective tissue derived from embryonic mesoderm - provides mechanical support and elasticity of the skin - contains immune cells involved in defense against foreign invaders passing through the epidermis 3. Hypodermis - is connective tissue derived from embryonic mesoderm - also known as subcutaneous connective tissue - recognized as the superficial fascia in gross anatomy - stores adipose tissue for cushioning, insulation and energy. - 1 | Page: Skin Swailes PART B: Epidermis The epidermis consists of four main cell type: the main cell is the stratified squamous epithelial cells called (A) Keratinocytes, other cells are less abundant and include: (B) Melanocytes (pigment producing), (C) Langerhans cells (antigen presenting) and (D) Merkel cells (tactile cells). A. Keratinocytes From the dermis outwards the epidermis consists of four layers of keratinocytes (five in thick skin): 1. Stratum basale - layer of columnar ‘palisades’ on the basement membrane at dermal-epidermal junction - attached to the basal lamina by hemidesmosomes and to each other by desmosomes - undergo intense mitosis and are responsible for 5 1 the constant production of epidermal cells 2. Stratum spinosum 4* - the thickest part of the epidermis 3 - consists of slightly flattened cells that actively 1 synthesize keratin filaments - the filaments radiate outward and converge on the desmosomes that anchor the cells to each other 3. Stratum granulosum - 3-5 layers of flattened cells undergoing terminal keratinization 2 1 - granular appearance due to: i. Keratohyalin granules Contain the protein filaggrin that binds together keratin filaments in the cytoplasm. ii. Lamellar (Odland) bodies Contain lipid that when released by exocytosis surround the keratinocytes and 1 prevent water loss and serve a waterproofing 1 function. 4. Stratum lucidum* - an additional layer of pale staining keratinocytes only seen in thick skin (not illustrated here) 5. Stratum corneum - 15-20 stacked layers of flattened non-nucleated keratinized cells filled with filamentous keratin. Melanocyte Keratinocytes Merkel cell Langerhans cell - continuously shed at the surface of the corneum 2 | Page: Skin Swailes B. Melanocytes - neural crest derived cells that have migrated into the stratum basale of the epidermis - have characteristically long, irregular processes that branch into the epidermis between the keratinocytes of the stratum basale and spinosum - one melanocyte serves 5-6 basal keratinocytes forming a epidermal-melanin unit Pigmentation Melanin is: - primarily responsible for skin color - synthesized by melanocytes and accumulates in vesicles called melanosomes - transported within melansomes to the end of melanocyte processes. - phagocytosed when keratinocytes phagocytose the melansome containing processes - is released from melansomes around the keratinocyte nuclei. One melanocyte plus the keratinocytes that ingest its melanin make up a epidermal-melanin unit. - absorbs and scatters sunlight to protect nuclear DNA from UV radiation. Clinical Correlations Skin color Melanocytes of people with ancestral origins near the equator where the need for protection against UV light is greatest produce more melanosomes more rapidly and accumulate them in keratinocytes more abundantly resulting in dark skin. Tanning Tanning is the result of a darkening of pre-existing melanin and an increase in melanin synthesis and its transfer to surrounding keratinocytes in response to exposure to solar radiation C. Merkel cells Merkel cells are mechano-receptors derived from neural crest cells and located in the stratum basale. They are present in greatest numbers in areas of high tactile sensitivity. Their basolateral surfaces contact sensory nerve fibers that penetrate the basal lamina. D. Langerhans cells Langerhans cells are antigen-presenting cells. They are found in the stratum spinosum but send out long processes between the keratinocytes of all layers. Due to this extensive coverage of the skin, they can can present any antigens that may invade the epidermis to surrounding T-lymphocytes triggering an immune response. 3 | Page: Skin Swailes PART C: Dermis and Hypodermis A. Dermis The dermis is the connective tissue component of the skin. It supports the epidermis and binds it to the hypodermis and contains the numerous blood vessels that supply the avascular epidermis with nutrients and are responsible for thermoregulation. If you examine the dermis carefully you will observe a number of features: 1. Dermal papillae - a series of dermal projections into the epidermis - more pronounced in regions of skin that regularly receive pressure (soles, hands etc). 2. Papillary dermis - a thin layer of loose irregular connective tissue below the basement membrane - it forms the dermal papillase - anchoring fibrils (Type VII collagen) in this layer insert into the basal lamina to hold the epidermis and dermis together. 3. Reticular dermis - a thick layer of dense irregular connective tissue immediately below the papillary dermis. - contains Type I collagen bundles which gives skin its strength against tensile forces - contains elastin fibers (seen only with special stains) that provide skin with its elasticity. - contains glands and hair follicles (see adnexa). 4. Vascular network - a deep vascular plexus lies between the reticular dermis and hypodermis - deep plexus supplies a superficial plexus that lies between papillary and reticular dermis - capillary loops extend from the superficial plexus into the dermal papillae - the plexuses, particularly the superficial plexus, vasoconstrict and vasodilate under cold and hot environment conditions to assist in thermoregulation 5. Meissner’s (tactile) corpuscles - elliptical structures located in the dermal papillae of the fingertips, palms and soles - they are encapsulated mechanoreceptors for light touch B. Hypodermis The hypodermis or subcutaneous tissue layer is composed of loose irregular connective tissue and adipocytes (fat cells). It is highly vascular and is therefore a prime site for administration of drugs (like insulin). If you examine the hypodermis carefully you may observe the following feature: 1. Pacinian (lamellated) corpuscles - large, oval structures deep in the reticular dermis or hypodermis - have concentric lamellae of flat Schwaan-type cells that surround an axon - are speicalized in the sensation of coarse touch, pressure (sustained touch) and vibrations 4 | Page: Skin Swailes PART D: Adnexa A. Glands 1. Sebaceous glands Location - widely distributed except palms and soles of feet - often associated with hair follcles Structure - acinar Secretion - sebaceous glands are holocrine glands - sebum is a lipid secretion released by decomposition of secretory cells Function - sebum helps maintain the stratum corneum and hair, it also has Sebaceous gands are antibacterial and antifungal properties branched acinar glands - a disturbance in the flow of sebum from these glands is one of the reasons for developing acne - a chronic inflammation of the sebaceous gland that obstructs sebum flow 2. Eccrine (merocrine) sweat glands Location - widely distributed in the skin - most abundant on the soles of the feet - ~3 million sweat glands in the body - produce around 1.8 liters of sweat per hour Structure - coiled, tubular gland - pale staining secretory cells located in hypodermis - dark staining duct portion difficult to locate but opens at skin surface Secretion - sweat is produced by secretory cells that form the deep portion of the gland in the hypodermis. - myoepithelial cells contract in response to cholinergic stimulation by Eccrine sweat glands are sympathetic nerves to increase flow of secretion into the duct. coiled tubular glands. Function - sweat is an effective means of temperature regulation, removing excess heat from the body by evaporation. 5 | Page: Skin Swailes 3. Apocrine sweat glands Location - axillae, genital region, external auditory meatus, eyelids and areola of the nipple. Structure - coiled tubular glands - wider lumen than eccrine sweat glands - empty sweat into hair follicles not onto the skin surface - myoepithelial cells contract when stimulated by adrenergic sympathetic nerves to increase flow of secretion Secretion - despite the gland name, this is a holocrine secretion - sweat produced is an odorless, milky secretion - when broken down by bacteria it causes body odor Function - function in humans is unknown - in other mammals they function as scent glands Apocrine glands are coiled tubular glands. B. Nails