Skin Histology PDF

Histology Skin Histology Skin The skin The largest single organ, typically accounting for 15–20% of total body weight. The skin is composed of the : Epidermis : an epithelial layer of ectodermal origin Dermis : mesodermal connective tissue The junction of dermis and epidermis is irregular due to int...

Histology Skin Histology Skin The skin The largest single organ, typically accounting for 15–20% of total body weight. The skin is composed of the : Epidermis : an epithelial layer of ectodermal origin Dermis : mesodermal connective tissue The junction of dermis and epidermis is irregular due to interdigitation of dermal papillae with epidermal ridges. Beneath the dermis lies the subcutaneous tissue or hypodermis Skin The skin The skin differ according to the site mainly depend on epidermal thickness. Thick skin : 400 to 1400 micro m ( stratum lucidum ) in palm and sole. Thin skin : 75 to 150 micro m; Skin elsewhere on the body The dermis is thickest on the back, where it is 30–40 times as thick as the overlying epidermis. The amount of subcutaneous fat is generous on the abdomen and buttocks compared with the nose and sternum, where it is meager. Skin The skin The total skin thickness ( epidermal plus dermal ) also varies according to the site. Skin on the back is about 4 mm thick, whereas that of the scalp is about 1.5 mm thick Skin The epidermis Consists mainly of a stratified squamous keratinized epithelium composed of cells called keratinocytes Three less abundant epidermal cell types are also present: Pigment-producing cells ( melanocytes ) Antigen-presenting cell (Langerhans ) Tactile epithelial cells ( Merkel cells ) Skin The epidermis Consist of 5 layers(stratum = Layer) Stratum Basale (Germinativum) Stratum spinosum Stratum granulosum Stratum lucidum (only thick skin) Stratum corneum Cells of The epidermis Keratinocytes Melanocytes Langerhans cells Merkel cells Skin The epidermis Skin Keratinocytes Epithelial cells that form the stratified squamous keratinized epithelium of Epidermis The most numerous cells found in the Epidermis. Have the specialized function of producing keratin Slow-cycling stem cells provide a reservoir for regeneration of the epidermis( The basal layer divided to replace the superficial dead cells) Keratinocytes play an active role in the immune function of the skin Skin Keratinocytes Skin Melanocytes Melanocytes are derived from the neural crest and by the eighth week of development can be found within the fetal epidermis. In normal, sun-protected trunk epidermis, melanocytes reside in the basal layer at a frequency of about 1 in every 10 basal keratinocytes. Areas such as the face, shins, and genitalia have a greater density of melanocytes and in heavily sun-damaged facial skin, Mart-1 immunostaining can demonstrate ratios of melanocytes to basal keratinocytes Skin Melanocytes They give the skin its color They present in stratum Basale and Stratum Spinosum. Melanin granules → migrate through the cytoplasmic extensions → enter Keratinocytes Contain pigments : Eumelanin (dark hair) Pheomelanin(red hair) The number of melanocyte is constant per area in human regardless of sex and race. Skin color is results of amount of melanin granules and melanocytes activity (race) Skin How skin varies with ethnicity ? Racial differences in skin color are not caused by differences in the number of melanocytes. It is the number, size, and distribution of the melanosomes or pigment granules within keratinocytes that determine differences in skin color. Pale skin has fewer melanosomes, and these are smaller and packaged within membrane bound complexes. Dark skin has more melanosomes, and these tend to be larger and singly dispersed. Skin Langerhans cells Star shaped cells, mainly in stratum spinosum. Derived from macrophages They are antigen –presenting cells (APC) Skin Merkel cells Generally found in thick skin (Merkel cell, can be found in the basal layer of the palms and soles, oral and genital mucosa, nail bed, and follicular infundibula ) Similar to Epidermal epithelial cells but show dense granules They are mechano-receptors. Skin Dermis Connective tissue that supports the epidermis and binds it to the subcutaneous tissue (hypodermis) The dermis contains two layers Papillary layer : Outermost & Thin layer loose connective tissue Has less fibers & more cells Reticular layer Deeper & Thicker Dense connective tissue Has more fibers & fewer cells Skin Dermis Connective tissue composed from mainly bundles of type I collagen and a network of elastic fibers is also present , providing elasticity to the skin. Spaces between the collagen and elastic fibers are filled with proteoglycans rich in dermatan sulfate. Cells of dermis Fibroblast ….. Collagen synthesis Mast cell ….. Histamine Macrophages and leukocytes ……phagocytosis Skin Basement membrane BM is always found between the stratum Basale and the papillary layer of the dermis and follows the contour of the interdigitations between these layers Blood vessels of skin Papillary plexus … in dermal papillae Reticular plexus … in dermis and hypodermis Innervation of skin A variety of sensory receptors are present in skin unencapsulated receptors Tactile discs for light touch. Free nerve endings respond primarily to high and low temperatures, pain, and itching, but also function as tactile receptors. Root hair plexuses detects movements of the hairs. Skin Encapsulated receptors ( tactile mechanoreceptors ) Tactile corpuscles (Meissner corpuscles) in fingertips, palms and soles. They detect light touch. Lamellated (Pacinian) corpuscles specialized for sensing coarse touch, pressure, and vibrations. Krause corpuscles & Ruffini corpuscles are other encapsulated, pressure-sensing mechanoreceptors Skin Skin aging Skin ageing occurs as two distinct phenomena : Intrinsic chronological ageing in which changes are attributable to the passage of time Photoaging that is the superposition of structural and functional changes caused by chronic UVR exposure The main clinical features of photoaging are fine and course wrinkling, dryness, coarseness, telangiectasia , yellowness and irregular patchy pigmentation & Skin also loses its mechanical properties Skin Effect of UV exposure on the skin : Change in color Darkening of the skin (tanning) …..( freckles, melasma) Change in texture and laxity Malignant melanoma Non melanocytic skin cancers Sun burns Photodermatosis ( skin diseases caused and aggravated by sun light) A major component of aesthetic disharmony in the aging face is the loss or redistribution of subcutaneous fat Skin laxity occurs as a result of a decreased underlying framework A thinned and damaged dermis, and a breakdown in elastic and collagen fibers.