Skin and Derivatives of Skin

1. Skin and Derivatives of Skin

2. Main Layers of Skin • Epidermis (outer layer) • Dermis (inner layer)

4. Epidermis • Functions • Protection against physical, chemical, and radiation injury. • Protection against microorganisms (by barrier, by bacterial binding to cells which will be released and by some phagocytosis) • Reduction of fluid loss

5. Sub-layers of Epidermis • Basal lamina – separates epidermis from dermis • Stratum basale • Deep layer contains living cells that divide to produce new cells • Stratum spinosum • Stratum granulosum • Stratum corneum • Surface layer of dead cells

6. Stages of Epidermal Cell Development • Synthetic Stage • Most cells found in lower part of spinosum layer • Synthesis of specific proteins and lipids (protein filaments, a polar lipid material that will be released from the cell, and keratohylain granules which will form the protein keratin)

7. Stages (Continued) • Transformation Stage • Occurs in layers below Stratum corneum • Intracellular enzymes are released that attack and lyse all organelles • Keratohyalin material fills cell

8. Stages (Continued) • Terminal Stage • Excessive loss of fluid from cells • The narrow intracellular space becomes filled with lipid material • Cells at the end of this stage are found in upper Stratum corneum layer

9. Dermis of Skin • Thick layer of: • Fibrous connective tissue (anchoring fibers, collagen fibers, elastic fibers) • Muscle tissue • Adipose tissue • Blood vessels • Nerves including sensory receptors

10. Dermal Structures of Epidermal Origin • Sebaceous Glands • usually associated with hair follicles – serve primarily to lubricate hair • These glands are important in waterproofing the pelage • Secretions are mainly cellular debris and lipids (oils) from cells that breakdown • Androgens stimulated these glands

11. Structures (Continued) • Sweat Glands • Hypotonic secretions that contain a lot of water • Apocrine Type – open into a hair follicle • Eccrine Type – open onto the surface of the skin independent of a hair follicle • Responsible for most of the sweat that is produced

12. Structures (Continued) • Sweat functions in • Evaporative Cooling • Improving tactile sensitivity • No sweat glands in – Cetaceans, Sirenians, Elephants, Pangolins, Echidnas, Moles, Rodents

13. Structures (Continued) • Only apocrine sweat glands in – Bats, some primitive primate species • More eccrine than apocrine sweat glands in – higher primates and humans

14. Structures (Continued) • Scent glands • May be modified sebaceous glands or modified sweat glands • Used for defense, territory marking, orientation, social interactions

15. Structures (Continued) • Hair • Hair is a keratinized derivative of the epidermis which is found in all mammals at some point in life cycle • Structure is • Central medulla (continuous or with air spaces) • Cortex which constitutes bulk of hair follicle • Cuticle (layer of outer scales that vary in shape)

16. Types of Hair • Vibrissae – Long, stiff hairs well well innervated bases, tactile receptors, definitive growth • Guard Hairs – Serve primarily for protection • Spines – enlarged, stiff guard hairs, ex. quills of porcupines, definitive growth, shed and replaced periodically

17. Types of Hair (Continued) • Guard Hairs (Continued) • Bristles – long, firm hairs with angora growth, ex. horse and lion manes • Awns – firm, expanded distal portion with smaller base, definitive growth

18. Types of Hair (Continued) • Underhair – primarily for insulation • Wool – long, angora growth • Fur – short, definitive growth • Velli – down or fuzz, first hair on an animal

19. Color of Hair • Pigment molecules • Eumelanin – provides shades of black and brown • Pheomelanin – produces shades of red and yellow

20. Pigment in Cells • Contained in melanosomes within melanocytes • Melanosomes are transferred from melanocytes to epithelial cells that become keratinocytes • Pigment in melanocytes and keratinocytes shield stratum basale from UV radiation • Pigment also acts as free radical scavenger, preventing cellular damage

21. Pigment in Hair • Melanocytes transfer melanosomes to epithelial cells at base of hair shaft • Melanosomes are phagocytized by cells that will form the cortex of the hair shaft

22. Head Ornaments • Occur only in Orders Artiodactyla and Perissodactyla

23. True Horns • Found only in Family Bovidae of O. Artiodactyla • An inner core of bone from the frontal bone and a sheath of keratinized epidermis • Grow continuously throughout life of animal • Unbranched • Never shed • Usually in males but can be in females too

24. Pronghorns • Found only in Pronghorn antelope (F. Antilocapridae, O. Artiodactyla) • Similar in structure to true horns except • Sheath is shed annually • Sheath is branched but bone is not • Occur in males and some females, horn on female may be smaller than on male and could be unbranched

25. Giraffes Horns • F. Giraffidae of O. Artiodactyla • Derive from separate bony ossifications that fuse to the skull near the junction of the parietal and frontal bones (are not projections from frontal bone!) • Permanently covered by skin and hair • Occur in both sexes

26. Rhinoceros Horns • O. Perissodactyla • Horn is composed of large mass of elongated, dermal papillae that fuse to form the horn • No core of bone • Horn is really keratinized skin

27. Antlers • F. Cervidae of O. Artiodactyla • Made entirely of bone • Branched • Outgrowths of frontal bone • Shed annually • Covered by “velvet” while growing • Size cannot be used to directly age and animal, size depends upon nutritional state