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Lect. 12 integumentary system



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  • 1. Lecture 12
  • 2. SKIN The largest organ in the body. about 16% of total body weight (adult) 1.2 - 2.3 m2 of surface area to the external environment.  Body surfaces Variation of skin structure at different sites classified as thick or thin skin depending on thickness of epidermis  Thickest – skin of the upper back (~ 5 mm)  Thinnest - upper and lower eyelids (< 1 mm)
  • 3. Functions of the skin Protection physical, biological, against UV light, from dehydration  Sensation Diff. receptors for touch, pressure, pain, temp.  the skin collects infos. about the external environment.  Thermoregulation Thick hair (in most mammals) – for heat conservation Adipose and sweat glands (humans)
  • 4. Metabolic function Synthesis of Vit.D3 with UV absorption – for Ca metabolism. Adipose (subcutaneous) – major storage of triglycerides for energy. Sexual attractant texture and appearance of skin, hair and nails
  • 5. 3 Layers Epidermis Dermis Hypodermis or Subcutaneous L. (subcutis)
  • 6. Epidermis E keratin K Dermis D Subcutaneous / subcutis SC The dermis and subcutis contain an assortment of skin appendages, i.e. hair follicles, sebaceous glands, eccrine glands EG and ducts ED and, in some areas, apocrine glands.
  • 7. Skin appendages Specialized structures: Hair follicles Nail Sweat glands – apocrine and eccrine Sebaceous glands
  • 8. Epidermis Surface layer in contact with the exterior stratified squamous keratinizied epithelial tissue Thickest on the palms and soles. Epidermal cells are derived from the basal layer of keratinocyte stem cells that undergo continuous cell division underneath. Newly formed cells push to the surface; they flatten and harden as they accumulate keratin.  Eventually, the keratinized cells die and are sloughed off.
  • 9. Epidermal cell types • Keratinocytes (structure) • Melanocytes (pigment) • Merkel cells (sensation) • Langerhans’ cells (phagocytes - immune defense)
  • 10. KeratinocytesMost numerous epidermal cell – found in all layers of the epidermis. Chief function is the production of keratin – a tough fibrous protein that gives strength and confers a lot of protective ability. Tightly connected to one another by desmosomes cells. • Provides continuity, strength, and protection. • Is the reason skin flakes off in sheets rather than as individual
  • 11. Melanocytes • Spider-shaped epithelial cells that synthesize the protein pigment melanin. • Melanin is made and then packaged into membrane- bound granules called melanosomes. • Granules are transferred to the keratinocytes in the 2 deepest layers of the epidermis.
  • 12. Melanin granules accumulate on the “apical part” of the nucleus of the keratinocytes. Melanin granules protect the DNA within the nucleus from being damaged by the UV radiation from the sun.
  • 13. People of the same size have approximately the same number of melanocytes but may differ in their level of activity: Dark skinned - the melanocytes continuously produce large amounts of melanin. Light skinned - the melanocytes produce less melanin.
  • 14. Langerhans cell Intraepidermal antigen- presenting cells (APC) recognize, phagocytose, and process foreign antigens, and then present them to T lymphocytes for an immune response. potent stimulators of cell- mediated immunological responses in the skin. active and present in increased numbers in epidermis and upper dermis in many inflammatory skin diseases.
  • 15. chemical carcinogens, immunosuppressive agents and excessive UV light have all been shown to reduce the number and effectiveness of Langerhans cells these are all factors which predispose to the dev’t. of epidermal tumours. Langerhans cell (L)
  • 16. Merkel cells intra-epidermal touch receptors. rare in thin skin in the stratum basale contain small dense granules may function as sensory mechanoreceptors or as neuroendocrine cells
  • 17. Skin types 1. Thick Skin  Found on soles of feet and palms of hands and corresponding parts of fingers and toes.  Contains 5 epidermal layers or strata (“sheets”):  Stratum basale  Stratum spinosum  Stratum granulosum  Stratum lucidum  Stratum corneum 2. Thin skin  Found everywhere else on the body.  Contains only 4 layers. (lacks a stratum lucidum).  The remaining 4 layers are thinner than those of thick skin.
  • 18. Cell layers of epidermis  S. corneum  S. lucidum  S. granulosum  S. spinosum  S. basale / germinativum  Epidermal ridges
  • 19. Stratum basale – basal layer • “stratum germinativum” • single layer of columnar or cuboidal keratinocyte stem cells, which are mitotically active. • deepest layer • Responsible for constant regeneration of other layers
  • 20. Stratum spinosum - prickle cell layer 2nd deepest layer. Consists of 8- 10 layers of cells. Cells of the lower layers can still be mitotic. As cells get pushed upward, they begin to flatten and begin to make the precursors of keratin. A.k.a. the “prickly layer,” because in tissue sections, they shrink and pull back. This makes their exposed desmosomes connecting adjacent cells appear to resemble spikes or spines.          Stratum          Stratum
  • 21. Stratum granulosum – granular layer  3 to 5 layers of flattened polygonal cells  cells accumulate keratohyalin granules with phosphorylated proteins, “granulosum”  contain lamellar granules which are lipid and protein rich  are discharged extracellularly to produce a “cement” that seals the skin to foreign objects & water  most superficial layer in which nuclei are present, but no cell division occurs
  • 22. Stratum lucidum a translucent thin layer of extremely flattened eosinophilic cells nuclei and organelles not present filaments and desmosomes retained cells contain eleidin, a transformation product of keratohyalin
  • 23. 3-5 layers of flat, dead keratinocytes. Appears clear in the light microscope because it lacks nuclei and organelles which typically stain well. Stratum lucidum
  • 24. Stratum corneum - keratin layer Normally composed of flat flakes and sheets of keratin, coated with an anti-wetting agent synthesised by the cells of the granular layer. Protects against mechanical abrasion – cells can absorb impacts simply flake off if necessary Prevents pathogen entry Prevents desiccation (drying out)
  • 25. Notice the 4 layers of thin skin in both the cartoon and the photomicrograph. Thin skin
  • 26. Thick Skin
  • 27. Dermis • Strong, flexible fibrous connective tissue. • Divided into papillary dermis and reticular dermis.
  • 28. DERMIS Papillary dermis is the upper 1/5 of the dermis and consists of loose CT very fine interlacing collagen and elastic fibers. Projects upward (as dermal papillae) to interdigitate and form a strong connection with the epidermis. Red arrow indicates the papillary dermis
  • 29. Papillary dermis • Provides an arena for immune cells to fight invaders. • Heavily invested with blood vessels • Arterioles, capillary loops and venules, lymphatics • they constrict in cold weather and dilate in warm weather. • Also contains multiple fine sensory receptors. • Meissner’s corpucles – touch receptors • Free nerve endings - pain and itch • associated w/ Merkel cells in the epidermis.
  • 30. DermisReticular dermis is lower 4/5 and consists of dense irreg. CT. Fibers are much larger than in papillary dermis Collagen – skin’s strength and resiliency. Elastin – skin’s ability to stretch and recoil. Blood vessels, nerves and majority of the appendages of the skin. Lymphocytes, mast cells, macrophages  (↑in number - skin disease) Blue arrow indicates the reticular dermis
  • 31. Skin appendages Specialized structures: Hair follicles Nail Sweat glands – apocrine and eccrine Sebaceous glands
  • 32. Notice the hair shaft, hair follicle, papilla, and the multiple sebaceous glands.
  • 33. Hair follicle Surrounded by fibrous CT sheath A tubular structure consisting of 5 concentric layers of epithelial cells From outer – inner layer: Hair root sheath External root sheath (1) Internal root sheath (2) Hair Shaft Cuticle (3) Cortex (4) Medulla (5) 1 2 3 4 5
  • 34. Hair bulb – bulbous expansion at the base of a hair follicle.
  • 35. Hair bulb Hair papilla / dermal papilla stromal core of CT Small bld. vessels – nourishment Nerve endings – for sensation Germinative cell (GC) layer / Matrix - basal cells in the hair bulb; mitotically active Melanocytes – interspersed with germinative layer level of melanin synthesis determine hair pigment. Root sheath region – cells produced from GC layer / matrix being pushed upward. indistinguishable yet as internal and external root sheaths
  • 36. As cells in the matrix are pushed toward the skin surface, the inner 3 layers undergo keratinization. Medulla – moderately keratinized Cortex – highly keratinized; forms the bulk of the hair Cuticle cells – also keratinizing, forming a hard, thin cuticle (overlapping keratin plates) on surface of hair Vellus hair - fine and soft hair (body hair) In infancy, child, females Terminal hair – coarser hair of scalp, pubic*, axillary* *Due to male hormone secretion at puberty Males – vellus hair replaced by terminal hair
  • 37. Hair and Hair Follicles  Hair root plexus - network of free nerve endings  Wrapped around the bulb of the follicle  sensory function.  Arrector pili muscle - a bundle of smooth muscle attached to each hair.  In times of fright or cold, these muscles contract and cause the hair to stand on end – and produces goose bumps.  Increases airflow in mammals with significant hair (i.e., not humans) and increases the apparent size of an animal with significant hair. Vestigial in humans.
  • 38. … still about hairs Cross sectional shape of hairs varies bet. races Straight hair – round (Mongols) Wavy hair – oval (Europeans) Curly hair – kidney shaped (Africans) Structure of hair follicles depends on the type of hair being produced. Scalp and other terminal hair – long and straight Body hair (fine) / vellus – short and plump
  • 39. The arrow indicates an arrector pili muscle. Identify the shaft, root, follicle, hair papilla, and sebaceous gland.
  • 40. Sebaceous gland Situated at a point about 1/3 length of a hair follicle from the surface. Lies within fibrous CT sheath that surround follicle Glandular epith. – outgrowth of external root sheath Basal cells – generate the secretory cells Acinar glands with several sacs
  • 41. Sebaceous glands • Most have short ducts that empty into neck of hair follicle, or onto the skin directly (eyelids, lips, glans penis and glans clitoridis) • Sebum is a complex mix of triglycerides, waxes, cholesterol and esters, with mild anti-bacterial and anti-fungal activity • activity controlled by sex-hormones
  • 42. Sweat gland Eccrine sweat glands Secrete watery sweat For thermoregulation Frequent on palms, soles, foreheads and axillae. Sweating – a minor route of excretion Apocrine sweat glands Produce a viscid, milky secretion Start to be functional at puberty Confined to the areolae of breasts, axillae and genital regions Sweat composiiton: • Na and Cl ions, urea and small mol. Wt. metabolites, other ions
  • 43. Coiled secretory portion  coiled eccrine duct  straight ascending duct * duct may become coiled again as it pass through epidermis (acrosyringium) - particularly apparent in the skin of the soles.
  • 44. Nail • located on dorsal distal phalanx of each finger and toe • growth due to cells in nail matrix at nail root • nail bed epidermis has only strata basale and spinosum • the stratum corneum of the epidermis that overlies the nail root forms the eponychium (cuticle) • hyponychium or nail plate consists of the stratum corneum of the underlying nail bed, and so is a keratinized epithelial layer • nail plate composed of hard keratin lying on nail bed
  • 45. Afferent nervous system Meissner’s corpuscle • a specialized structured nerve ending • touch receptor • confined to dermal papillae • most numerous on hands and feet
  • 46. Meissner’s corpuscle
  • 47. Pacinian corpuscle • an encapsulated nerve ending • pressure and vibration receptor • found in deep dermis or hypodermis • In palms and soles
  • 48. Free nerve endings detect pain and temperature innervation by sympathetic nervous system controls blood flow and hair Ruffini corpuscles Small dermal mechanoreceptors, particularly common in soles.
  • 49. Light-skinned people are more susceptible to the effects of ultraviolet (UV) rays, which may trigger mutations in living epidermal cells.
  • 50. Squamous cell carcinoma and basal cell carcinoma - are the most common forms of skin cancer. The lesions are visible as changes in the normal appearance of the skin, and a biopsy is used to confirm the diagnosis. These lesions usually do not metastasize rapidly, and can be completely removed using simple procedures
  • 51. Malignant melanoma -a more serious form of skin cancer, which begins in melanocytes. * Any change in a pigmented spot or mole (nevus) should prompt a person to see a doctor. Melanoma is serious not because of its growth in the skin, but because it may metastasize very rapidly to the lungs, liver, or other vital organ. Researchers are testing individualized vaccines for people who have had melanoma.
  • 52. Sunscreens Sunscreens contain chemicals such as PABA (para- amino benzoic acid) that block UV rays and prevent them from damaging the epidermis. An SPF (sun protection factor) of 15 or higher is considered good protection. Use of a sunscreen on exposed skin not only helps prevent skin cancer but also prevents sunburn and its painful effects. It is especially important to prevent children from getting severely sunburned, because such burns have been linked to the development of skin cancer years later.