Skin 2023_Student(1).pptx

SKI N ID-767 Fundamental Histology & Cell Biology 9/26/2023 Casey Boothe, PhD Learning objectives 1. Describe the basic functions of the integument. 2. Discuss the basic structural organization and composition of the skin. 3. Identify the 5 distinct layers of the epidermis 1. Explain the distinct...

SKI N ID-767 Fundamental Histology & Cell Biology 9/26/2023 Casey Boothe, PhD Learning objectives 1. Describe the basic functions of the integument. 2. Discuss the basic structural organization and composition of the skin. 3. Identify the 5 distinct layers of the epidermis 1. Explain the distinct characteristics and functions of each epidermal layer. 2. Distinguish between thick and thin skin epidermis. 4. Identify the structure and functions of the accessory cells of the epidermis (Langerhans cells, Merkel cells, and melanocytes). 5. Identify the two distinct regions of the dermis. 1. Identify accessory structures of the dermis (sensory receptors, hair, nails, glands). 2. Identify distinct functions of the accessory structures of the dermis. Skin • Covers entire surface of the body • Largest organ of the body by weight & volume • Composed of epithelium, connective tissue, muscles, nerves, blood vessels, & accessory structures • Accessory structures: hair; nails; sebaceous, eccrine sweat, & apocrine glands Functions of skin Protection • Serves as a barrier between internal tissues & the environment • Prevents damage by physical trauma, toxic chemicals, radiation, & sunlight Somatosensory function • Sensory receptors allow skin to constantly monitor the environment • Sensations of touch, pressure, pain, warmth, cold, vibration Prevention of dehydration • Forms a waterproof barrier • Prevents loss of body fluids Thermoregulation • A constant body temperature is normally easily maintained thanks to insulating components (i.e., fatty layer & hair on head) and mechanisms for heat loss (i.e., sweat production + dense superficial microvasculature) Sexual signaling Metabolic • Many features of skin (like pigmentation & hair) are visual indicators of health involved in attraction • Sex pheromones produced by apocrine sweat glands • Cells of skin synthesize vitamin D3, needed in calcium metabolism and proper bone formation Layers of the skin Thick Skin vs. Thin Skin Referr ed to as glabro us skin We distinguish between thick skin & thin skin based on the thickness of the epidermis EPIDERMI S THICK SKIN THIN SKIN Epidermis: stratum basale • Deepest layer of epidermis • Composed of: • A single layer of cuboidal cells • Many of these are stem cells that are actively dividing to give rise to cells in the other 4 layers • Melanocytes (top 2 images) • Produce melanin • Merkel cells (bottom 2 images) • Sensory receptor cells that respond to light touch stimuli and for sensing an object's texture Epidermis: stratum spinosum • Typically the thickest layer • Cells are interconnected by numerous desmosomes • Langerhans cells (modified macrophages) are often found in this layer • Participate in the cutaneous immune response by capturing antigens & presenting them to lymphocytes Epidermis: stratum granulosum • Composed of flattened cells with keratohyalin granules in their cytoplasm • This layer is more prominent in thick skin Epidermis: stratum lucidum • Very thin layer of flattened & tightly packed keratinocytes that are filled with keratin filaments • Nuclei of these cells are beginning to be eliminated • Only found in thick skin Epidermis: stratum corneum • Most superficial layer • Contains numerous flattened cells that are filled with keratin • Cells lack organelles and are anucleate • Cells in this layer are continually being sloughed off & replaced • Form a barrier to prevent water loss and entry of pathogens • Much thicker in thick skin than thin skin Dermis • Located deep to the epidermis • Composed of a superficial papillary layer & a deep reticular layer • Papillary layer: loose connective tissue • Reticular layer: dense irregular connective tissue (much thicker) • Contains blood vessels, nerves, & sensory receptors • Sensory receptors: free nerve endings & Meissner corpuscles • Free nerve endings detect pain, temperature, and itching • Meissner corpuscles are responsible for discriminative touch Dermis continued • Epidermal pegs/ridges: portion of epidermis that projects into dermis • Dermal papilla: portion of dermis that projects into the epidermis • These are significant because they prevent the epidermis from detaching from the dermis Hypodermis • Located deep to the dermis • Transition layer between the skin & underlying muscle • Contains loose connective tissue, adipose tissue, nerves, arteries, veins • Pacinian corpuscles – sensory receptors that respond to vibration (look like onions!) Sebaceous glands • Found in thin skin & usually associated with hair follicles • Classified as simple branched acinar glands • Typically secrete sebum into a hair follicle • Sebum lubricates the skin & coats & protects hair shafts from becoming brittle • Release their products by holocrine secretion Arrector pili muscles • Arrector pili muscles are bundles of smooth muscle cells that span between hair follicles & papillary layer of the dermis • Sympathetically innervated • Contract to stand the hair up in response to cold or fear Eccrine sweat glands • Regulate body temperature & aid in excretion of metabolic wastes • Found in both thick & thin skin • Produce sweat which consists of mostly water as well as some ions, waste, & metabolic products • Myoepithelial cells are contractile cells that help push secretory products into & along the lumen Apocrine sweat glands • Also called sexual scent glands • Possibly involved in thermoregulation • Found in specific regions of thin skin such as the axilla, areola (nipple), perianal & genital areas • Lumens much larger than those of eccrine sweat glands • Release their products by apocrine secretion • Product is thick & viscous & consists of protein, ammonia, lipids, & carbohydrates • Myoepithelial cells are found with apocrine sweat glands as well Hair • Found in thin skin • Hair follicles produce hair & maintain hair growth • Hair follicles extend from epidermis to dermis or hypodermis • Hair growth is discontinuous & controlled by various hormones Hair growth • Anagen phase • Active growth stage • Lasts 2-6 years • Catagen phase • Regression phase • Lasts about 3 weeks • Telogen phase • Resting stage • Lasts about 3 months Hair • Hair bulb = balloon-shaped structure formed at the basal region of the hair follicle • Composed of hair root & dermal papilla • Hair root = contains melanocytes & epithelial cells called the matrix (or germinal matrix) • Melanocytes produce melanin granules in the hair bulb; these granules give color to hair • Cells in the matrix are capable of cell division & give rise to the inner root sheath & the hair • Dermal papilla (hair papilla) contains capillaries & nerve fibers that supply the hair follicle • Epithelial cells of the matrix form a cap around the dermal papilla Structures of the hair follicle containing a hair shaft: • Hair medulla = thin core of the hair shaft • Hair cortex = keratinized cells surrounding the medulla • Hair cuticle = outermost layer of the hair shaft • Inner root sheath = cellular sheath that extends from the hair bulb & surrounds & grows along with the hair • Outer root sheath = a cellular sheath which is a continuation of the epidermis • Connective tissue sheath (dermal root sheath) Nails • Translucent, hard, keratinized sheet resting on the tip of each digit • Composed of: • Nail plate = nail itself; hard keratin • Nail root (also called nail matrix or germinal matrix) = contains many layers of epithelial cells that produce the nail plate; seen as the lunula in the living state • Nail bed = a layer of epidermis beneath the nail plate; extends from nail matrix to the hyponychium • Eponychium (cuticle) = junction zone between skin of the finger & nail plate; forms a protective seal • Perionychium (nail wall) = skin surrounding the edge of the nail • Hyponychium = junction seal between nail plate & skin of the fingertip Questions?