Skin as an organizing model

1. Skin as an organizing model What is it? (Anatomy) What does it do? (Physiology) What is it made of? (Histology) Where does it come from? (Embryology) What can go wrong? (Pathology) Frolich, Human Anatomy, Skin

2. What is it (anatomy)? • Outer covering of body • Keratinized • Hair—head, pubic regions • Nails—fingers, toes • Divided into segments called dermatomes (“Zebra-man”) • CE (clinical example): Shingles Frolich, Human Anatomy, Skin

3. Dermatomes and segmentation in basic body plan Frolich, Human Anatomy, Skin

4. Dermatomes and spinal nerves Frolich, Human Anatomy, Skin

5. Hair and nails—modified structures of epidermis • Nails • Scale-like epidermal structure • Cells bind together and have “hard” keratin • Grows out from root of nail • Hair • Each shaft has three layers of keratinzed cells filled with “hard” keratin • Flat, ribbon-like shaft produces kinky hair; oval shaft makes wavy hair; round shaft makes coarse hair • Hair color due to amount of melanins of different colors made my melanocytes at base of hair follicle; red hair also has iron-containing pigment; gray/white hair due to decreased melanin production • Hair follicle • Fold of epidermal surface into dermis • Hair grows from here • Has nerve plexus to give touch/tickle sensation • Connective tissue sheath derived from dermis • Hair length due to relationship between active and inactive phases of follicle (e.g., eyebrow follicles active only three to four months; head follicles can be continuosly active for years) Frolich, Human Anatomy, Skin

6. Nails—scale-like epidermis Frolich, Human Anatomy, Skin

7. Hairs and hair follicles Frolich, Human Anatomy, Skin

8. What is it made of?(histology—study of tissues) • Skin is organ made up different tissue types • Layers of skin are two fundamental types of tissue organization • Epidermis = epithelium • Dermis = connective tissue Frolich, Human Anatomy, Skin

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10. Epidermis—a stratified epithelium Frolich, Human Anatomy, Skin

11. Layers of epidermis • Stratum basale/germinativum (“basal or “forming” layer) • One layer thick mitotic cells • 10-25% melanocytes with processes into next layer • Merkel cells with sensory neurons • Stratum spinosum (“prickly” layer) • Cells appear spiny due to numerous desmosomes • Many Langerhans cells • Stratum granulosum (“grainy” layer • Cells flatten • Organelles/nuclei begin to disintegrate • Keratin precursor granules begin to form • Lamellated granules with water-proof lipid form and will be spewed out between cells • Stratum corneum (“horny” layer) • Cells are dead—too far from underlying capillaries to live • 20-30 cells thick up to ¾ of dermal thickness • Keratin, thickened membranes and glycolipids between cells provide “overcoat” for body to protect against water loss and other possible “assaults” on body Frolich, Human Anatomy, Skin

12. Cells in epidermis • Keratinocytes—epidermal cells that make keratin • Merkel cells—associated with touch sensory neurons • Langerhans cells—macrophages (from dermis) migrate in to form spider-like immune barrier • Melanocytes (at border with dermis) make pigment to give skin color Frolich, Human Anatomy, Skin

13. Dermis—connective tissue substrate Frolich, Human Anatomy, Skin

14. Dermal layers • Papillary layer (layer with “nipples” or tiny projections) • Thin superficial layer of areolar connective tissue • Papillae house capillary loops, nerve endings, sweat glands • In hands and feet papillae on dermal ridges forming fingerprint patterns • Reticular layer • 80% of thickness • Dense irregular collagen fibers, mostly parallel to skin surface • Predominant direction of fibers forms cleavage or tension lines in skin (important for incisions) • Flexure lines in skin at joints where reticular layer is bound to underlying connective tissues to provide “give” when joint flexes Frolich, Human Anatomy, Skin

15. Glands and keratinized appendages: Is it dermis…or epidermis? • Sebaceous glands • Clumps of epithelial tissue distributed within dermis • Secrete “sebum”—oily, fat-based substance that is also anti-bacterial • Located all over body • Sweat glands • Microscopic clumps of epithelial tissue distributed within dermis, duct extends out through dermis to pore (not “pores” of face complexion which are hair follicles) • More than 2.5 million glands per person • Eccrine sweat glands, concentrated on hands and soles of feet and forehead, secrete sweat to cool body, also “cold sweat” of fear, emotion. • Apocrine glands, concentrated in armpits and groin, analogous with sexual scent glands of other animals, odor comes from bacteria that concentrate here. • Ceruminous glands: modified sweat glands in ear canal produce ear wax • Mammary glands: modified sweat glands in female breast produce mother’s milk Frolich, Human Anatomy, Skin

16. Skin Glands Frolich, Human Anatomy, Skin

17. What does it do (physiology)? • Physiology is often common sense • [brainstorm] Frolich, Human Anatomy, Skin

18. What does it do? • PROTECTION (mostly epidermis) • Air/Water movement in and out: keratin, lipids from sebaceous glands • Sun/U-V: Melanin from melanocytes and DNA in keratinzed cells prevents burning from sun • Cold/heat = frostbite/heat stroke/thermoregulation: surface blood vessels • Chemical barriers—”acid mantle” from low “pH of skin secretions retards bacterial growth • Immune cells in dermis and epidermis fight infection and present antigens to other immune cells • Evaporation of sweat cools body • Closing of dermal capillaries prevents heat loss • Supplement liver in “disarming” cancer-causing chemicals that penetrate epidermis • STRUCTURAL • Integrity of body: irregular collagen fiber layers • Muscle attachment: sub-dermal layer where muscle tendons insert • SENSATION—nerve endings in Merkel cells • METABOLIC FUNCTION—vitamin D synthesis in epidermal cells Frolich, Human Anatomy, Skin

19. Embryology • Remember basic body plan—tube within a tube • Skin is part of outer tube • Epidermis is formed from ectoderm • Dermis from underlying mesoderm • Skin reflects basic segmented organization of human body seen in vertebrae, ribs, nerves. Shingles makes skin segments—dermatomes—visible Fill in the table! Frolich, Human Anatomy, Skin

20. What can go wrong? (Pathology) • Skin cancer (what are the cells involved?) • Basal cell carcinoma (a) • Common and benign, 30% of white people get, slow-growing and easily incised surgically • Squamous cell carcinoma (b) • Less common, faster-growing but still easily removed surgically • Melanoma (c) • Highly metastatic (spreads rapidly) • Related to U-V exposure • Early detection key—can start in retina (importance of ophthalmologic exams) Frolich, Human Anatomy, Skin

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22. Burns Epithelial epidermal cells can regenerate—where are they? Frolich, Human Anatomy, Skin

23. Interrelationship with other systems • Systems are not isolated, but just one way of analyzing how body works • Skin is tightly linked to other systems of body • Skeletal: Skin forms “exoskeleton”—stiff attachment point for muscles; also synthesizes Vitamin D necessary for calcium deposit process • Muscular: Many muscles attach on the skin; muscles lose heat through skin • Nervous: Sensory cells (Merkel cells) transmit touch sensation to neurons; blood vessel, sweat, arrector pili muscles controlled by nervous system • Cardiovascular: Skin as blood reservoir, heat exchange organ • Immune: Very important role of skin as “first barrier” with Langerhans cells and macrophages as cellular immune response; edema or swelling common in infected skin regions • Reproduction: skin responds to erotic stimuli; mammary glands are highly modified sweat glands in skin; stretching of skin to accomondate fetus during pregnancy Frolich, Human Anatomy, Skin

24. Distribution of Skin Color • Skin color around world Frolich, Human Anatomy, Skin

25. Genetics of skin color From Genetics Chapter On-Line Biology Book Frolich, Human Anatomy, Skin