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Chapter 6 Skin and Its Appendages. Structure of the Skin. Structure. Picture page 163 ( 197) Integumentary system = skin Skin is a thin relatively flat organ Classified as a membrane: cutaneous membrane Cutaneous Membrane is divided into epidermis and dermis. Epidermis.
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Structure • Picture page 163 ( 197) • Integumentary system = skin • Skin is a thin relatively flat organ • Classified as a membrane: cutaneous membrane • Cutaneous Membrane is divided into epidermis and dermis
Epidermis • Outer, thinner epithelial layer. • Develops from ectodermal germ layer, usually by the 17th week of gestation the baby’s has all the essential characteristics of the adult’s • Avascular
Dermis • Inner, thicker connective layer • Derived from the mesoderm • Vascular
Dermal-epidermal junction • Specialized area where cells of epidermis meet connective tissue of dermis • characteristics of the adult structure by the 9th week of gestation
Subcutaneous Layer • Also called hypodermis • Lies beneath the dermis • Rich in fat and aerolar tissue • Irregular connective tissue
Epidermis Dermis Subcutaneous layer (hypodermis)
Epidermis Page 200 in home books
Cell Types • Epidermis is composed of several types of epithelial cells • Keratinocytes • Most important cell in the epidermis • Comprise over 90% of epidermal cells • Filled with tough, fibrous protein called keratin • principle structural element of outer skin
Cell Types • Melanocytes • Contribute color to skin • Protect from UV light • Can be completely absent from skin in some non-lethal conditions
Cell Types • Langerhans Cells • Dendritic cells (immune cells) • Play a role in immune reactions that effect the skin • Cells originate in the bone marrow but migrate to deep cell layers of the epidermis early in life
Cell Layers of EpidermisStrata Page 200 in home books
Stratum basale • Base layer • Single layer of columnar cells • Only cells in this deepest layer of epithelium undergoes mitosis • Cells migrate from this layer to other layers until they reach the surface and are shed off
Stratum Spinosum • Also called spiny layer • Stratum germinativum is used to describe the stratum basale and Stratum Spinosum together • Formed from 8-10 layers of irregularly shaped cells with very prominent intercellular bridge or desmosomes • Desmosomes appear to pull points of the plasma membranes of adjoining cells toward one another. Gives spiny appearance • Cells are rich in RNA making them well equipped to start protein synthesis needed for the production of keratin
Stratum Granulosum • Granular layer • Process of surface keratin formation begins • Sheet 2-4 layer deep filled with intensely staining granules called keratohylin (required for surface keratin formation) • Cells start to degenerate • High levels of lysosomal enzymes are present in cytoplasm and nuclei are in the process of breaking down • In thin skin this layer may not be visible
Stratum Lucidum • Clear layer • Keratinocytes are very flat, closely packed and clear • Nuclei are usually absent • Dying cells are filled with eleidin which is eventually transformed to keratin • Absent in thin skin
Stratum Corneum • Horny layer • Most superficial layer of epidermis • Composed of thin squamous cells • At surface cells are dead and continuously being shed • Desmosomes holding together Keratinocytes strengthen this layer • Keratinization: process in which cells from deeper layers migrate, fill with keratin and move to surface
Stratum Corneum • Sometimes called barrier area of skin • Protects from water loss and environmental threats • Glycophospholipids cement keratin into water proof barrier. • Glycophospholipids can be washed away by excessive soaking. The keratin can then absorb water appearing puffy and wrinkled • Diseases can cause layer to thicken • Hyperkeratosis: Thick, dry, scaly skin that is inelastic and subject to painful fissures
Thin and Thick Skin • There are up to 5 layers of stratum or cell layers. (stratum corneum, lucidum, granulosum, spinosum, basale) • Epidermal tissue can be categorized thin or thick skin • Most of the body surface is covered by thin skin • Hairless skin covering palms, fingertips, soles of feet or other areas associated with friction has a covering of thick skin
Thick skin • Each of 5 strata of epidermis are present and each stratum are generally several layers thick • Hair not found in thick skin • Thick skin underlying dermal papillae are raised in curving parallel friction ridges forming fingerprints or footprints • Ridges allow us to pick up and manipulate small items and supplies slip resistance to the feet.
Thin skin • Number of cell layers in the epidermal stratum are less than in thick • One or more strata may be entirely absent • Friction ridges are not present
Epidermal Growth and Repair • Turnover or regeneration time describes period required for a cell population to mature and reproduce • To maintain constant thickness, new cells must be formed at the same rate that old keratinized cells flake off from stratum corneum. • Current research suggest regeneration time is about 35 days • Abrasion can accelerate skin regeneration time. The result is an intense stimulation of mitotic activity in the stratum basale and shortened turnover period • Continued abrasion can result in an abnormally thick stratum corneum, producing calluses at the point of abrasion.
Epidermal Growth and Repair • Callus formation is normal but there are several skin diseases by abnormally high mitotic activity in the epidermis resulting in scales and lesions • 10%-12% of cells in the stratum basale enter mitosis each day • Cells migrating to surface proceed upward in vertical columns from groups of 8-10 of these basal cells undergoing mitosis • Each group of active basal cells, with its vertical column of migrating keratinocytes is called epidermal proliferating unit (EPU)
Review • Identify two main layers of skin • Terms thick and thin refer to which primary layer of skin • How do thin and thick skin differ • Identify two main cell types found in the epidermis • List 5 layers of epidermis (strata)
Dermis Page 200 in home books
Dermal-Epidermal Junction • Specialized area where cells of epidermis meet connective tissue of dermis • characteristics of the adult structure by the 9th week of gestation • lies between papillary layer and the stratum basale • Combines basement membrane and includes specialized fibrous elements and a polysaccharide gel • cement the superficial epidermis to the dermis (glue the two layers together) • Provides mechanical support for epidermis
Dermis • Also called the corium or “true skin” • Vascular • Composed of thin papillary layer and thicker reticular layer • Much thicker than the epidermis • Serves a protection function against mechanical injury
Dermis • Specialized network of nerves and nerve endings called somatic sensory receptors process sensory information • At various levels the dermis may contain muscle fibers, hair follicles, sweat and sebaceous glands and many blood vessels • It is rich vascularity that plays an important role in temperature regulation (discussed later)
Papillary Layer • Thin superficial layer of the dermis • Forms bumps called dermal papillae on its surface • Dermal-Epidermal Junction lies between papillary layer and the stratum basale • Composed of loose connective tissue elements along with thin collagenous and elastic fibers • Has characteristic ridges on surface due to conforming tightly to dermal papillae • Fingerprints and footprints allow us to grip so we can grasp small objects and walk upright on slippery surfaces (Friction Ridges)
Reticular Layer • Thicker layer of dermis • More of the dense reticulum or network of fibers (most are collagenous) than in papillary layer • Dense collagenous fibers in this layer is what produces leather in processed animal skins (these fibers also give skin toughness) • Elastic fibers make skin stretchable
Reticular Layer • Serves as a point of attachment for skeletal and smooth muscle fibers • Most of the structures such as muscle fibers, hair follicles, sweat and sebaceous glands are located in reticular layer • Several skeletal muscle are located in the skin of the face and scalp and permit various facial expressions
Reticular Layer • Distribution of smooth muscle is much more extensive than skeletal • Each hair follicle has a small bundle of involuntary muscle attached to it. Called arrector pili muscle • Contraction of these muscles makes hair stand on end and raises skin around hair • Goosebump • Have millions of somatic sensory receptors
Dermal Growth and Repair • Unlike epidermis, dermis does not continually shed and regenerate • Rapid regeneration of connective tissue in dermis occur only unusual circumstances such as wound healing • Fibroblasts quickly reproduce and begin forming a dense mass of new connective tissue that will either be replaced by normal tissue or become a scar
Dermal Growth and Repair • Dense white bundles of collagenous fibers orient themselves in patterns called cleavage lines or Langer’s Lines. Page 167 (203) • Surgical incisions are made parallel to cleavage lines the resulting wound has less tendency of gaping open and will tend to heal with a less noticeable scar • When elastic fibers are stretched too much (pregnancy) Fibers weaken and tear and initially resulting in pinkish or slightly bluish depressed furrows with jagged edges • When they heal and lose color they remaining furrows appear as glistening silver-white scar lines (stretch marks)
Hypodermis • Called subcutaneous layer, subQ or superficial fascia • Not part of the skin but usually discussed with the skin • Forms connection between skin and underlying structures of the body • Made up mostly of loose fibrous tissue and adipose tissue • Carries major vessels and nerves to skin above
Review • What is the name of the layer separating the dermis from the epidermis? • Which layer of the dermis forms the bumps that produce the ridges on palms and soles? • Which layer is vascular? Dermis or epidermis • What is the main function of the hypodermis?
Melanin • Main determinant of skin color is the quantity of melanin deposited in the epidermis cells • Melanin (pigment) is produced by melanocytes • Melanocytes scattered throughout the stratum basale is roughly the same amount for everyone • It is the amount and type of melanin pigment that melanocytes produce that account for skin color variations
Melanin • Two groups of melanin • Eumelanin • Pheomelanin • Eumelanin • “true Black” • Very dark brown, sometimes nearly black • Dark skinned and dark haired produce large quantities • Absorbs more UV radiation than pheomelanin