Skin Structure and Function PDF

Skin Function Protective Thermoregulatory Metabolic Sensory Thick and Thin Skin  Types of Skin cells Keratinocytes: form the keratinized stratified squamous epithelium. arranged in five layers responsible for keratin formation. Melanocytes:. melanin-‐producing cells (stratum basale...

Skin Function Protective Thermoregulatory Metabolic Sensory Thick and Thin Skin  Types of Skin cells Keratinocytes: form the keratinized stratified squamous epithelium. arranged in five layers responsible for keratin formation. Melanocytes:. melanin-‐producing cells (stratum basale and hair follicle). Derived from neural crest then migrate to epidermis has irregular cytoplasmic process and rich in mitochondria and well-‐ developed Golgi apparatus (site of melanin synthesis) and short cisternae of RER. Langerhans cells :APC. IMMUNE SYSTEM(stratum spinosum) Merkel’s cells: mechanoreceptor for light touch sensation(stratum basale)  Epidermal layers Stratum basale : single layer of basophilic cuboidal or columnar cells( desmosomes) on the B.M ( hemidesmosomes) at dermal-epidermal junction. it has high mitotic activity. Renewal of the epidermis 15-30 days. Keratin filament.( Melanocytes, Merkel cells). During differentiation the cells move upward the amount and type of keratin filament increase. Stratum spinosum: (thickest layer).consist of polyhedral cells having central nuclei with nucleoli and cytoplasm. Stratum germinative( basale and basal layer of spinosum)keratin filament assemble to into microscopic visible bundles called Tonofibril. Cytoplasmic extension (spines).( langerhans cells). Stratum granulosum: consist of 3 to 5 layers of flatted cells undergoes terminal process of keratinization. Keratohyaline granules( filaggrin and keratin)non membrane bound basophilic masses. membranous Golgi derived Lamellar granules( layers of lipid, impermeable layer around the the cells act as a barrier against the water loss). Stratum lucidum: found only in Thick skin. Flattened eosinophilic keratniocytes held by desmosomes. Nuclei and all organelles are lost. Cytoplasm consist of packed keratin in electron dense matrix. Stratum corneum: 15 to 20 layers of squamous keratinized cells filled with keratin filament. By the end of keratinization this cells have only amorphus fibrillar protein( cornified layer) continues shed at epidermal surface. Figure 18-2 Copyright © McGraw-Hill Companies Copyright © McGraw-Hill Companies Stratum spinosum Copyright © McGraw-Hill Companies Stratum granulosum and lucidum Copyright © McGraw-Hill Companies In Thin skin. Stratum corneum, granulosum and spinosum are thinner than those of thick skin. And no stratum lucidum.  Melanocytes Copyright © McGraw-Hill Companies Copyright © McGraw-Hill Companies  Melanin Synthesis Tyrosinase enzyme in Golgi derived vesicles. Tyrosine DOPA DOPA quinine Melanin Melanin deposited in the vesicles which are called melanosomes. Melanosomes transported to the tip of cytoplasmic extension then to keratinocytes cytoplasm( phagocytosis) of stratum basale and spinosum ( supranuclear cap absorb and scatter the sunlight, DNA). Keratinocytes are the melanin depot. Epidermal melanin unit: one melanocyte and keratinocytes into which it transfer the melanosomes. Euomelanin brown or black pigment. pheomelanin red pigment. Melanin and carotene (skin color) Langerhans cells APC, stratum spinosum Copyright © McGraw-Hill Companies Merkel cells epithelial tactile cells Copyright © McGraw-Hill Companies  Dermis Connective tissue rich in elastic fibers, blood and lymph supply). It has epidermal derivative such as hair follicles, sweat and sebaceous glands. Composed of papillary and reticular layer. Papillary layer has dermal papillae, loose connective tissue, connective tissue cells. Collagen I and III and elastic fibers , blood and lymph, Tactile corpuscles. Anchoring fibril.(VII). Copyright © McGraw-Hill Companies Reticular layer contain dense irregular connective tissue collage type I and elastic fibers, blood vessels, nerve and sensory receptor and ducts of sweat glands, hair follicle. Blood and lymphatic supply Subpapillary plexus located between papillary and reticular layer. Cutaneous plexus lie between the dermis and hypodermis. The skin has three venous plexus , subcpapillary, cutaneous, and in the middle of dermis. A-V anastomosis in dermis play a major role in control of heat loss. Lymphatic vessels start at dermal papillae and accompany arterial cutaneous and subpapillary plexuses. Sensory receptors Unencapsulated receptors : Merkel cells: associated with expanded nerve ending. Light touch receptor. Free nerve ending: in papillary dermis. Extend to lower epidermal layers. Response for high and low temperature, pain, itching. Function as a tactile receptors. Root hair plexus. Sensory fibers around the the base of hair follicles in reticular dermis. Detect the hair movement. Copyright © McGraw-Hill Companies  Encapsulated receptors All of them are mechanorecptors Meissner corpuscles: in dermal papillae. Abundant in fingertips, palm, and soles. Krause end bulb: present in dermis Pacinian corpuscles : found in reticular dermis and hypodermis. Ruffini corpuscles: present I hypodermis. Autonomic innervation to sweat gland is also present  Subcutaneous tissue Consist of loose connective tissue that bind the skin to the subjacent organs. Also called hypodermis or superficial fascia. Contain adipocytes that vary in number in different body region. Copyright © McGraw-Hill Companies  HAIR Elongated keratinized structure that form within epidermal invagination, The hair follicles. The color, size, shape and texture of hairs vary according to age, genetic background and the region of the body. All the skin has minimal hair except skin of palms and soles, lips, glans penis, clitoris, and labia minora. Hair grows discontinuously with period of growth followed by period of rest.  Structure of hair Hair follicle has a terminal dilation called a hair bulb. Dermal papilla insert into the base of hair bulb and contain a capillary network. Keratinocytes continues with those of stratum basale cover the dermal papilla. These cells form the matrix of hair root. The part of hair extend beyond the skin surface is the hair shaft. Copyright © McGraw-Hill Companies The cells in hair root matrix differentiate with variable amount and types of keratin. The keratin of hair is harder and more compact. The moderate keratinized cells form the central Medulla of hair root. Heavily keratinized densely packed cells make up the cortex. The most peripheral cells of hair root comprise the cuticle. Thin layer of heavily keratinized squamous cells covering the cortex. Copyright © McGraw-Hill Companies The outermost layer of cells , peripheral epithelial layer differentiate into the internal and external root sheath. Epithelial root sheath consists of two layers. Internal root sheath completely surround he initial part of the hair root but degenerate above the level of attached sebaceous gland. External root sheath covers the internal sheath and extend all the way to epidermis. Continues with basal and spinouts layers. Figure 18-14 Copyright © McGraw-Hill Companies Melanocytes are located between the papilla and epithelial cells of hair root. Hair follicles separated from the dermis by noncellula haylaine layer called glassy membrane, formed by the thickening of basal lamina. The surround dermis form a connective tissue sheath. The arrector pili muscle small bundle of smooth muscle extend from midpoint of fibrous sheath to the dermal papillary layer.pull the hair shaft to more erect position.  Hair growth Anagen period of mitotic activity and growth. Catagen short period arrest of growth and regression of the hair bulb. Telogen final period of inactivity during which the hair may shed.  Nail Process of keratinization also produces, the nails. Hard plates of keratin on dorsal surface of each distal phalanx. Proximal part of nail is the nail root. Is covered by a fold of skin from which the stratum corneum extend as the cuticle or eponychium. Nail plate is bound to a bed of epidermis, Nail bed; contain only basal and spinous epidermal layers. The nail root forms from nail matrix. Cells divide and move distally and become keratinized. The nail root mature and harden as the nail plate. Continues growth in the matrix pushes the nail plate forward over nail bed. Distal end of plate becomes free of nail bed at epidermal fold called Hyponychium.  Sebaceous gland Present in dermis over most of the body except in soles and palms. Branched acinar glands with several acini converging at a short duct. Open into upper portion of a hair follicle. Hair follicle and its associated sebaceous glands make up a pilosebaceous unit. In hairless region. Sebaceous gland open onto the epidermal surface. Acini of sebaceous glands are holocrine. They have basal layer of flattened epithelial cells on B.M., which proliferate and are displaced centrally and differentiate as a large lipid produced sebocytes filled with small fat droplet. Sebocytes undergo autophagy and release the lipid, Sebum. Cover the surface of epidermis and hair shafts. weak antibacterial and antifungal. Sebocytes undergo autophagy and release the lipid, Sebum. Cover the surface of epidermis and hair shafts. weak antibacterial and antifungal. Sebum mixture of lipids (cholesterol, Triglyceride, ester) Hydrolyzed by bacterial enzymes. Secretion of S.G increase at puberty (testesterone, ovarian and adrenal androgen).  Sweat gland Eccrine sweat gland: simple coiled tubular. Widely distributed in the skin most numerous on foot soles. There are 3 million eccrine sweat glands. Secretion rate exceeding that of other exocrine glands. Sweating is physiological response to increased body temperature during exercise. Or thermal stress. Secretory and ducts of the gland are coiled and have small lumen and open at skin surface. Secretory part (acini) has stratified cuboidal epithelium with three cell types: Clear cells: pale staining produce sweat has abundant mitochondria and microvilli provide large surface area. Interstitial fluid from the capillary rich dermis transported through the clear cells to lumen of the gland.(ion-transport cells). Dark cells: pyramidal in shape. Fill with eosinophilic granules. Do not contact the basal lamina. Secrete mixture of glycoprotein with bactericidal activity. Myoepithelial cells: on the basal lamina contract to to move the watery secretion into the duct. The ducts: have two layers of acidophilic cuboidal epithelium cells rich in mitochondria and Na, K ATPase to reabsorb the Na and prevent the excessive loss of electrolytes. Sweat flow continues in spiral channel through the five layers to an excretory Sweat pore  Apocrine sweat glands: Confined to skin of axillary and perineal regions. Their development depends on sex hormones. Function after puberty. The secretory lumen of apocrine glands much larger lumen and consist of simple cuboidal, eosinophilic cells undergo exocrine secretion. (protein rich sweat) Ducts open to hair at epidermis follicle. Apocrine sweat gland secretes odorless secretion. May acquire odor as a result of bacterial activity.