Skin Physiology 2023 PDF

Structural & Functional Biology Skin & Integumentary System Sarah Hooper, DVM, MS, PhD E-mail: [email protected] 1 Learning Objectives:  List the functions of the skin in mammals  Explain the mechanisms that help the skin protect the animal  Explain the components of the skin  Cells layers...

Structural & Functional Biology Skin & Integumentary System Sarah Hooper, DVM, MS, PhD E-mail: [email protected] 1 Learning Objectives:  List the functions of the skin in mammals  Explain the mechanisms that help the skin protect the animal  Explain the components of the skin  Cells layers of the epidermis and relevant cells types  Keratin synthesis  Function and synthesis of melanin  Nerve endings in the skin  Hairs and feathers  Thermoregulation through the skin  Glands of the skin  The hoof 3 The Skin and the Integumentary System Skin: Largest sensory organ in the body Covers the body thereby protecting cells and tissues Very important for communication in animals Piloerection in localized parts of the skin functions as visual signals From: Sjaastad, Sand, Hove „Physiology of Domestic Animals“ 4 The Skin and the Integumentary System The skin helps animals recognize the structure of surfaces, their composition and temperature. In humans, the skin is an essential component of our affective behavior. 5 Functions of Skin  Protection of the body against mechanical and chemical injury by 3 mechanisms  Physical structure of skin  Hair and keratinized surface  Secretory products  Protect against loss of water, electrolytes, and other constituents Turnover (shedding) of superficial cells and hair helps regulate number of microorganisms and debris on skin surface Antibacterial and antifungal properties  Normal bacterial flora (aka skin microbiome)  Prevents invasion by pathogenic bacteria by occupying microbial niches and producing compounds that inhibit the growth of other microorganisms 6 Functions of Skin  Protection of the body against mechanical and chemical injury  Limitation of loss of water from the body  Immunologic barrier  Thermoregulation  Sensory information provision (pressure, temperature, pain)  Transmission of emotional and chemical signals  Fat storage  Synthesis of 7-dehydrocholesterol, which is converted to vitamin D3 by ultraviolet light 7 Skin structure: Use the weblink on the previous slide and the figure on the right to help you fill in the blanks: Three main components of the skin: The __________, the outermost layer of skin, epidermis a multilayered epithelium The ________, an underlying layer of dermis vascularized connective tissue subcutis or hypodermis also known as the The __________________, subcutaneous layer. ------------------------------------------------------------------ Accessory structures, such as hair, feathers, and glands 9 From: Sjaastad, Sand, Hove „Physiology of Domestic Animals“ The Skin and the Integumentary System The epidermis The epidermis is made up of four cell layers: corneum (layers of keratinized, dead cells still connected by desmosomes) Stratum ____________ granulosum (flattening of cells, high keratin content) Stratum ________________ Stratum spinosum (cells joined by _______________ Desmosomes providing mechanical strength) Desmosomes = prominent cell-to-cell junctions ____________ Stratum basale (stem cells) https://ohiostate.pressbooks.pub/vethisto/chapter/7-structure-of-the-epidermis/ 10 The Skin and the Integumentary System The four cells of the epidermis are……. (Hint: https://www.ncbi.nlm.nih.gov/books/NBK470464/)  Keratinocytes (the most predominant ones)  Melanocytes  Langerhans’ cells  Merkel’s cells 1) Keratinocytes: Produce keratin, the major structural protein in the epidermis Contribute to the formation of the epidermal water barrier Makes up majority of the structure of the skin, hair, and nails 14 The Skin and the Integumentary System Keratin synthesis and the formation of the epidermal water barrier Cell division occurs in the stratum basale (basal layer). These cells synthesize tonofilaments (intermediate keratin filaments). Cells are pushed into the stratum spinosum As it enters tonofilaments synthesis continues and the cell begins to produce: keratohyalin granules that contain proteins (aid in aggregation of keratin filaments), helps convert granular cells to cornified cells (keratinization) and glycolipids-containing lamellar bodies Physiologic Variants https://www.ncbi.nlm.nih.gov/books/NBK470464/) From: Ross & Pawlina „Histology: A Text and Atlas“ 15 The Skin and the Integumentary System Keratin synthesis and the formation of the epidermal water barrier Cells are pushed into the stratum granulosum and continue to stratum corneum. Lamellar bodies are discharged by exocytosis into the spaces between stratum granulosum and stratum corneum Lamellar bodies + lipid envelope -> epidermal water barrier The epidermal water barrier prevents entry of fluids and microorganisms as well as fluid loss from the body From: Ross & Pawlina „Histology: A Text and Atlas“ 16 The Skin and the Integumentary System 2) Melanocytes: Melanocyte precursor cells originate in the neural crest Melanocytes are in close association with a number of keratinocytes in the epidermis Normally located in stratum basale are easily recognized by the presence of melanin granules inside “dendritic” processes From: Sjaastad, Sand, Hove „Physiology of Domestic Animals“ 17 The Skin and the Integumentary System 2) Melanocytes:  Melanin protects the skin against nonionizing UV irradiation  Synthesis starts in premelanosomes from the amino acid tyrosine  As more melanin is produced, melanosomes become more visible at the tip of the dendritic process being then transferred to neighboring keratinocytes (pigment donation) From: Ross & Pawlina „Histology: A Text and Atlas“ Ando, H., Niki, Y., Ito, M., Akiyama, K., Matsui, M. S., Yarosh, D. B., & Ichihashi, M. (2012). Melanosomes are transferred from melanocytes to keratinocytes through the processes of packaging, release, uptake, and dispersion. Journal of Investigative Dermatology, 132(4), 1222-1229. 18 The Skin and the Integumentary System Melanocytes: Light skin vs dark skin melanin degradation Lysosomal activity degradation of melanin is faster in individuals with light skin than in individuals with dark skin and melanosomes are distributed only in stratum basale) Two forms of melanin pigments (genetically determined): Eumelanin -> brownish black Pheomelanin -> reddish yellow Exposure to UV light accelerates the rate of melanin production as a way to protect the skin From: Sjaastad, Sand, Hove „Physiology of Domestic Animals“ 19 The Skin and the Integumentary System 3) Langerhans’ cell: 4) Merkel’s cells: Can be seen in the stratum spinosum First line defenders, play a role in antigen presentation Aka dendritic cells are antigen-presenting cells Modified epidermal cells found in stratum basale Most abundant in skin where sensor perception is acute, such as fingertips Merkel’s cells are associated with a terminal disk of an afferent myelinated nerve fiber (Merkel’s corpuscle -> mechanoreceptor) From: Ross & Pawlina „Histology: A Text and Atlas“ 20 The Skin and the Integumentary System The dermis Strong and flexible connective tissue composed by cells and collagen fibers Cells of the dermis: ____________ (primary cells) Fibroblasts Macrophages Leukocytes Mast cells Tissue derived from the epidermis extend into the dermis and gives rise to sweat glands, sebaceous glands, and the papillae that form hair and feathers https://www.ncbi.nlm.nih.gov/books/NBK535346/ Contributed by Wikimedia Commons, USGOV (Public Domain) 23 The Skin and the Integumentary System Other nerve endings in the skin Hairless skin Haired skin Dermis Meissner Merkel Pacini Hair folicle sensor Ruffini Hypodermis From: Schmidt, Lang, Heckmann „Physiology of Humans“ In hairless skin only; touchreceptors Only in haired skin; mechanoreceptors (tactile hairs) Deep pressure Respond to receptors for mechanical mechanical and displacement of vibratory pressure adjacent collagen fibers Tactile hairs in the whiskers of a cat From: Sjaastad, Sand, Hove „Physiology of Domestic Animals“ 25 The Skin and the Integumentary System Hairs Hair follicles are formed by invaginations of the epidermis into the dermis After a skin injury, cells in stratum basale are capable of forming new hair follicles At the deep end, each follicle is expanded into a hair root or hair bulb that surrounds a portion of dermis, the hair papilla From: Sjaastad, Sand, Hove „Physiology of Domestic Animals“ The matrix region of the hair bulb produces new daughter cells that are pushed towards the surface 26 https://www.proteinatlas.org/learn/dictionary/normal/skin The Skin and the Integumentary System The hair shaft consists of keratinized epithelial cells that are pushed from the hair root outwards the root canal Cuticle: single layer of dead, scale-like keratinocytes Cortex: several layers of dead keratinocytes containing hard keratin Medulla: dead loosely keratinized cells Piloerector muscle (aka arrector pili m.): a smooth muscle associated to hair bulb and anchored to the outer layer of the dermis (sympathetic innervated) Hair follicules have cycles of activity ________(growth)________(regression)_________(resting)__________(shedding) From: Ross & Pawlina „Histology: A Text and Atlas“ 27 The Skin and the Integumentary System Feather Types Flight feathers Cover feathers (down feathers) The vane is the flat part of the feather It consists of barbs that are at angles of about 45 ° The hooklets, which are present only on the barbules on one side of the barb, hook onto the smooth barbule from the adjacent barb The barbules of down feathers lack hooklets From: Sjaastad, Sand, Hove „Physiology of Domestic Animals“ The barbules of down feathers lack hooklets Feathers from the same feather follicle can have different pigmentation RUSVM student: Joe Richichi 28 The Skin and the Integumentary System Thermoregulation Heat transport to the skin. Heat passes through layers with good insulation properties, the subcutaneous adipose tissue and fur or plumage. Mechanisms of heat exchange -> From: Sjaastad, Sand, Hove „Physiology of Domestic Animals“ 29 The Skin and the Integumentary System Glands of the skin Sebaceous glands Sweat glands Mammary glands Virtual histology slides of skin and glands: http://medcell.med.yale.edu/histology/skin_lab.php#vm-slides From: Sjaastad, Sand, Hove „Physiology of Domestic Animals“ 30 The Skin and the Integumentary System Sebaceous glands Duct opens into A hair follicle (most common) Pore on the skin surface Composition: detached, degraded epithelial cells as well as lipids and proteins Functions: lubrication of the skin, provides water impermeability, and inhibition of bacterial growth Stimulated by testosterone There are no sebaceous glands associated with feathers. Uropygial glands in waterfowl produces a secretion that impregnate the feathers of the bird From: Sjaastad, Sand, Hove „Physiology of Domestic Animals“ 31 The Skin and the Integumentary System Sweat glands Eccrine glands: o Very common in primates o Open in pores at the skin surface o Ionic composition is similar to the plasma. In the duct, Na+ and Cl- are reabsorbed o Degree of reabsorption depends on the secretion rate Apocrine glands: o Very common in domestic animals o Open into hair follicle o Odoriferous secretion composed of fatty acids and proteins From: Sjaastad, Sand, Hove „Physiology of Domestic Animals“ 32 The Skin and the Integumentary System Mammary gland Glands + teats = The udder From: Sjaastad, Sand, Hove „Physiology of Domestic Animals“ Alveoli (acini): milk-producing units Lobuli: clusters of alveoli Lobi: clusters of lobuli Nutrients and water required for milk production diffuse from blood capillaries and enter into the epithelial cells Myoepithelial cells are involved in milk ejection 33 The Skin and the Integumentary System Cornified epidermal structures Hooves, claws, and horns in mammals Claws in reptiles and birds Beaks in turtles and birds Shells and scales in reptiles Scales in fish Hooves Horny hoof wall is formed by keratinization of the cells in the coronary band. Recently produced horn is pushed downward as new horn is being formed At the coronary band the dermis of the skin is continuous with the dermis of the hoof. From: Sjaastad, Sand, Hove „Physiology of Domestic Animals“ 34