Skin Introduction It is easy enough to identify a basic tissue in isolation, but it takes further skill to incorporate the knowledge of these separate tissues and distinguish them as such in a compound tissue organ, such as the skin. These basic tissues will be found in some capacity in every tissue you encounter, and the function of this lab is to help you become more familiar in recognizing these specific tissues in organs. Skin is a great example of how the basic tissues combine to create a compound tissue and organ. It is a tissue composed of three distinct layers: epidermis, dermis and hypodermis. Each layer has specific functions, which are derived from their basic tissue components. Your job during this lab is to focus on identifying these basic tissues within these layers of skin and to think about the specific function they impart to the skin. Learning objectives and activities Using the Virtual Slidebox: A Examine the keratinized stratified squamous epithelium of the epidermis, and identify the modified epithelial exocrine glands. B Analyze the organization of collagen fibers and connective tissue cells in the dermis and hypodermis and interpret their function within the skin. C Locate muscle, peripheral nerve and modified nervous tissues in the skin. D Examine hair and hair follicles and determine that they are derived from the epidermis. E Investigate the anatomy of the growing fingernail and appreciate its relationship to skin. F Complete the self-quiz to test your understanding and master your learning. Epidermis: the epithelium of the skin The epidermis is a specialized epithelium: keratinized stratified squamous. The layering and apical specialization makes this epithelium prime for protection against outside forces (i.e. abrasion, bacteria, etc.) The specialized keratinocytes also prevent water loss. i. Stratum basale - single layer of cells on the basement membrane - mitotically active and responsible for the constant production of keratinocytes v - recall that hemidesmosomes anchor the . cells to the basal lamina of basement iv. membrane ii. Stratum spinosum iii. - thickest portion of the epidermis - cells synthesize keratin filaments - recall that cells are held together by desmosomes - shrinkage during preparation causes cells ii. to separate except where they are held together giving them their ‘spiny appearance. iii. Stratum granulosum - cells undergoing terminal keratinization - contain keratohyaline granules that i release filaggrin to bundle keratin . filaments - creates a water-proof barrier iv. Stratum lucidum - a layer of pale-staining keratinocytes - only seen in thick skin Examine Slide 1 (79) to locate the 5 layers v. Stratum corneum of the epidermis and its apical - flattened, dead keratinocytes and their specialization. bundled keratin - forms the protective apical specialization known as keratin Sweat and sebaceous glands: the exocrine glands of the skin In some areas of the skin, the epithelium invaginates to form an exocrine gland. There are three types of exocrine glands: eccrine/merocrine, apocrine and sebaceous. They vary in look, function and location. Be cautious not to confuse the sweat glands with blood vessels. The tubular appearance can trick the naïve eye. i. Eccrine glands - also known as merocrine glands - most widely distributed gland in the skin - coiled tubular ducts that stain dark - secretory portions have small lumens and pale- staining cells - produce what we commonly known as ‘sweat’ which has a thermoregulatory function Examine Slide 2a (78) & find examples of eccrine glands ii. Apocrine glands - vestigial glands found in the axillary and genital regions of the skin - coiled tubular ducts that stain basophilic - easily recognized by their secretory portions, which have large lumens with eosinophilic cells. - produce pheromones which trigger sexual responses and mating in many mammals Examine Slide 2b (78) & find examples of apocrine glands iii. Sebaceous glands - located throughout the skin - secretory portion is distinctly acinar and composed of pale, lipid rich cells - short ducts usually empty into hair follicle, but they can also secrete directly onto the skin - produce ‘sebum’, an oily mixture of lipids that maintains the keratin - excessive production of sebum can lead to blockage of the ducts and subsequent tissue inflammation resulting in acne. Examine Slide 2c (78) & find examples of sebaceous glands Dermis & Hypodermis: the connective tissue of the skin The dermis and hypodermis are connective tissue layers that provide support and stretch to the skin. i. Papillary dermis - is the superficial portion of the dermis composed of loose irregular connective tissue - is composed of Type I collagen fibers and Type III collagen fibrils - forms projections into the epidermis called dermal papillae This organization: - provides attached to the epidermis at the basement membrane (recall its components). - provides a large surface area for anchoring of the epidermis - provides cushioning support for: i. the overlying epidermis ii. capillary loops that feed the avascular epidermis and are involved in thermoregulation - provides a framework for the many leukocytes present and ready to respond to microbes that may infiltrate the skin barrier. Examine the papillary dermis in Slide 3a (76) and identify its major connective tissue components ii. Reticular dermis - is the deep portion of the dermis composed of dense irregular connective tissue - is composed of Type I collagen fibers and fiber bundles - also contains elastin fibers This organization: - allows the dermis to resist tensile forces from all directions - allows the skin to stretch and recoil Examine the reticular dermis in Slide 3b (76) and identify its major connective tissue components iii. Hypodermis - is the deepest layer of the skin - composed of loose irregular connective tissue and many adipocytes This organization: - provides a cushioning protection to underlying organs and supports vasculature - acts a nutritional reserve and insulation from the cold Examine the hypodermis in Slide 3c (76) and identify its major connective tissue components Depending on the region, skin will contain varying amounts of skeletal and smooth muscle. For example, in the lip you can see the skeletal muscle of the orbicularis oris muscle and smooth muscle surrounding the blood vessels that travel through the dermis and hypodermis. You can also see sections through the peripheral nerves that innervate the skin. Examine slides of skin to find areas of muscle and nerve to test your skills at observing these basic tissues in compound tissues. i. Skeletal muscle - has fibers defined by striations and peripheral nuclei in longitudinal section Examine Slide 4a - has fibers that are polygonal with peripheral nuclei in cross section (42) and find - has organizing layers of connective tissue that surround: examples of a. individual muscle fibers (endomysium) skeletal muscle in b. fascicles/collections of muscle fibers (perimysium) long & cross- c. the whole muscle (epimysium) section ii. Smooth muscle - is found circumscribing tubes (like blood vessels) Examine Slide 4b - does not have striations (42) and find - has spindle-shaped cells with cigar shaped nuclei in longitudinal section examples of - has circular cells with large round nuclei (that makes tissue look spotty) in smooth muscle in cross section long & cross- section iii. Peripheral nerve - is formed from the myelinated axons of motor and sensory neurons - has a distinctive wavy appearance in longitudinal section Examine Slide 4c - may be confused with smooth muscle (but doesn’t surround tubes!) (42) and find - may have a ‘washed out’ appearance due to lipid in myelin sheath examples of - contains the nuclei of Schwaan cells (and some fibroblasts) peripheral nerves in (mainly) iv. Modified nervous tissue in skin (touch receptors) longitudinal - there are two main types of touch receptor section a. Meissner’s corpuscles - touch receptors in the papillary dermis of fingertips, palms and soles Examine Slide 5a - composed of sensory axons surrounded by flat Schwaan cells and find - respond to light touch when they are temporarily deformed examples of Meissner’s corpuscle b. Pacinian corpuscles and Slide 5b to find - touch receptors found in the reticular dermis throughout the body Pacinian corpuscles - oval structures composed of concentric lamellae (layers) - respond to frrm pressure and vibration Hair follicles are tubular invaginations of the epidermis that project into the dermis. The hair itself forms within the follicles and is itself constructed of columns of keratin. Examine regions of hairy skin and locate the different regions of a hair follicle and hair when cut in longitudinal section (Slide 6) and in cross section (Slide 7) 1. Hair follicle a. Infundibulum Identify the extent of the - extends from the surface opening of the follicle to the infundibulum in Slide 6a opening of its sebaceous gland Find a cross section through this - is part of the pilosebaceous canal (the route for sebum region in Slide 7a discharge) Find a sebaceous gland in b. Sebaceous gland Slide 6b - its duct opens into the infundibulum where it secretes and Slide 7b sebum Identify the extent of the c. Isthmus isthmus in Slide 6c - extends from the infundibulum to the insertion of the arrector pili muscle Find a cross section through this region in Slide 7c d. Arrector pili muscle - a thin bundle of smooth muscle that extends from the papillary dermis of the skin to the connective tissue sheath surrounding