Skin Anatomy PDF

SKIN  2 AGENDA Epidermis Dermis Subcutaneous tissue Sensory receptors Hair Nails Skin glands  3 SKIN Consists mainly of a superficial stratified squamous epithelium, the epidermis, and a thicker layer of connective tis...

SKIN  2 AGENDA Epidermis Dermis Subcutaneous tissue Sensory receptors Hair Nails Skin glands  3 SKIN Consists mainly of a superficial stratified squamous epithelium, the epidermis, and a thicker layer of connective tissue, the dermis, which overlies a subcutaneous hypodermis. Click to add picture EPIDERMIS 5 Consists of keratinocytes that undergo a terminal differentiation process called keratinization in a series of steps that form distinct epidermal strata or layers. The stratum basale is one layer of mitotically active cuboidal cells attached by hemidesmosomes and integrins to the basement membrane and to each other by desmosomes. The stratum spinosum has several layers of polyhedral cells attached to each other by desmosomes at the tips of short projections containing bundled keratin, or tonofibrils. The stratum granulosum is a thinner layer of keratinocytes, now flattened and filled densely with keratohyaline granules containing filaggrin and other proteins binding the tonofibrils. EPIDERMIS 6 The superficial stratum corneum protects against water loss, friction, and microbial invasion, and consists of flattened, terminally differentiated cells, or squames, which are slowly lost. The epidermis-dermis interface is enlarged and strengthened by interdigitating epidermal ridges or pegs and dermal papillae in which microvasculature also supplies nutrients and O2 for the epidermis. Melanocytes in the basal epidermis synthesize dark melanin pigment in melanosomes and transport these to adjacent keratinocytes, which accumulate them to protect nuclear DNA from UV damage. Antigen-presenting cells called Langerhans cells form a network through the epidermis, intercepting and sampling microbial invaders before moving to lymph nodes in an adaptive immune response. Click to add picture Click to add picture Click to add picture Click to add picture In keratinocytes moving upward from the stratum spinosum (S), differentiation proceeds with the cells becoming filled with numerous large, amorphous masses of protein called keratohyaline granules. Cells with these basophilic granules make up the stratum granulosum (G), where keratin filaments are cross-linked with filaggrin and other proteins from these granules to produce tight bundles filling the cytoplasm and flattening the cells. Smaller organelles called lamellar granules undergo exocytosis in this layer, secreting a lipid- rich layer around the cells which makes the epidermis impermeable to water. The lipid envelope and the keratin-filled cells determine most of the physical properties of the epidermis. Click to add picture The cells leaving the stratum granulosum, still bound together by desmosomes, undergo terminal differentiation and in thick skin appear as a dense, thin layer called the stratum lucidum (L). Here proteins are dispersed through the tonofibril bundles, giving this layer a regular, “clear” appearance. In the most superficial stratum corneum (C), the cells have lost nuclei and cytoplasm, consisting only of flattened, keratinized structures called squames bound by hydrophobic, lipid-rich intercellular cement. At the surface, they are worn away (thick skin) or flake off (thin skin) Click to add picture Click to add picture Click to add picture Click to add picture Melanocytes are located in the epidermal basal layer and synthesize melanin granules and transfer them into neighboring keratinocytes. Transfer occurs through many long, branching melanocyte processes that extend into the spinous layer and are not seen in routine microscopy. In light microscopy melanocytes (M) typically appear as rounded, pale-staining or clear cells just above the dermis (D). Melanocytes are difficult to distinguish from Merkel cells by routine microscopy. Langerhans cells are also rounded, poorly stained cells but are typically located more superficially than melanocytes, in the stratum spinosum. Melanocyte - with irregular cytoplasmic processes between neighboring keratinocytes for transfer of melanin to those cells. A melanocyte is located on the basal lamina (BL) and has well-developed Golgi complexes (G) producing the vesicles in which melanin is synthesized. As they fill, these vesicles become melanin granules, which accumulate at the tips of the dendritic cytoplasmic extensions before transfer to keratinocytes. Click to add picture Click to add picture Click to add picture  Click to add picture DERMIS Two major layers: a superficial papillary layer or loose connective tissue with a microvascular plexus a thicker dense irregular reticular layer containing larger blood vessels  Click to add picture DERMIS The layer of connective tissue that supports the epidermis and binds it to the subcutaneous tissue (hypodermis). The thickness varies with the region of the body and reaches its maximum of 4 mm on the back. The surface of the dermis is very irregular and has many projections (dermal papillae) that interdigitate with projections (epidermal pegs or ridges) of the epidermis, especially in skin subject to frequent pressure, where they reinforce the dermal-epidermal junction. 23  24 SENSORY RECEPTORS: UNENCAPSULATED RECEPTORS Tactile (or Merkel cells), each associated with a disc or expanded axonal ending, which function as tonic receptors for sustained light touch and for sensing an object’s texture. Free nerve endings in the papillary dermis and extending into lower epidermal layers, which respond primarily to high and low temperatures (thermoreception), pain (nociception), and itching (pruritus), but also function as important tactile receptors. Root hair plexuses, a web of sensory fibers surrounding the bases of hair follicles in the reticular dermis that detects movements of the hairs.  25 SENSORY RECEPTORS: ENCAPSULATED RECEPTORS Meissner corpuscles elliptical structures consisting of sensory axons winding among flattened Schwann cells arranged perpendicular to the epidermis in the dermal papillae. Initiate impulses when light-touch or low-frequency stimuli against skin temporarily deform their shape. Numerous in the fingertips, palms, and soles but decline slowly in number during aging after puberty. Lamellated (pacinian) corpuscles large oval structures found deep in the reticular dermis and hypodermis, with an outer capsule thin, concentric lamellae of flattened Schwann cells and collagen surrounding a highly branched, unmyelinated axon. Sense coarse touch, pressure (sustained touch), and vibrations, Also found in the connective tissue of organs located deep in the body, including the wall of the rectum and urinary bladder  26 SENSORY RECEPTORS: ENCAPSULATED RECEPTORS Krause end bulbs Simpler encapsulated, ovoid structures, with extremely thin, collagenous capsules penetrated by a sensory fiber. In the skin of the penis and clitoris where they sense low- frequency vibrations. Ruffini corpuscles have collagenous, fusiform capsules anchored firmly to the surrounding connective tissue, with sensory axons stimulated by stretch (tension) or twisting (torque) in the skin. 27  28 SUBCUTANEOUS TISSUE The subcutaneous layer consists of loose connective tissue that binds the skin loosely to the subjacent organs, making it possible for the skin to slide over them. Also called the hypodermis or superficial fascia Contains adipocytes that vary in number in different body regions and vary in size according to nutritional state. The extensive vascular supply at the subcutaneous layer promotes rapid uptake of insulin or drugs injected into this tissue.  29 EPIDERMAL APPENDAGES: HAIR Hairs form in hair follicles, in which keratinocytes comprising the matrix of the deep hair bulb proliferate rapidly and undergo keratinization to form the medulla, cortex, and cuticle of a hair root. A large dermal hair papilla penetrates the base of the hair bulb, and its vasculature supplies nutrients and O2 for proliferating and differentiating cells. The growing hair root is surrounded by internal and external root sheaths continuous with the epidermis, a glassy membrane formed in part by the basal lamina, and a connective tissue sheath. 30 31  32 EPIDERMAL APPENDAGES: NAILS Formed in a manner similar to hairs: keratinocytes proliferate in the matrix of the nail root and differentiate with the formation of hard keratin as a growing nail plate with edges covered by skin folds. NAILS - hard, keratinized derivatives formed in a process similar 33 to that of the stratum corneum and hair. (a) Surface view of a finger (b) A diagrammatic sagittal shows the nail’s major parts, section includes major internal including the crescent-shaped details of the growing nail and the white area called the lunula, hyponychium where the free end which derives its color from the of the nail plate is bound to opaque nail matrix and epidermis. immature nail plate below it. NAILS 34 A sagittal section from a finger shows the proximal nail fold (PNF) and its epidermal extension, the eponychium (E) or cuticle. The nail root (NR), the most proximal region of the nail plate (NP), is formed like the hair root by a matrix of proliferating, differentiating keratinocytes. These cells make up the dorsal nail matrix (DNM) and ventral nail matrix (VNM), which contribute keratinized cells to the nail root. The mature nail plate remains attached to the nail bed (NB), which consists of basal and spinous epidermal layers over dermis (D), but is pushed forward on this bed by continuous growth in the nail matrix.  35 EPIDERMAL APPENDAGES: GLANDS Sebaceous glands produce sebum by terminal differentiation of sebocytes, the classic example of holocrine secretion, secreting this oily substance onto hair in the follicles or pilosebaceous units. Eccrine sweat glands in the dermis produce sweat that is mostly water onto the skin surface, where its evaporation provides an important mechanism for cooling the body. Apocrine sweat glands are restricted to skin of the axillae and perineum, have much wider lumens than eccrine glands, develop after puberty, and secrete protein-rich sweat onto the hair of hair follicles.  GLANDS OF SKIN 36 1. Sebaceous glands - embedded in the dermis except in the thick, glabrous skin of the palms and soles. Sebum is a complex mixture of lipids that includes wax esters, squalene, cholesterol, and triglycerides that are hydrolyzed by bacterial enzymes after secretion. 2. Sweat glands - The ducts of eccrine sweat glands have two layers of more acidophilic cells filled with mitochondria and having cell membranes rich in Na+, K+- ATPase. Eccrine sweat glands - widely distributed in the skin and are most numerous on the foot soles Apocrine sweat glands - development depends on sex hormones and is not complete and functional until after puberty 37 38 APOCRINE SWEAT GLANDS 39 The secretory portions (S) of apocrine sweat glands have lumens that are much larger than those of eccrine sweat glands. The ducts (D) of apocrine glands also differ from those of eccrine glands in opening into hair follicles (H) rather than to the epidermal surface. Eosinophilic cells show apical blebs suggestive of the pinching-off secretory process as well as features typical of apical granule exocytosis. Involves both merocrine and apocrine secretion. THANK YOU 