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Home , Apocrine sweat gland, Arrector pili muscle, Basal lamina, Basement membrane, Cortex (hair), Eccrine sweat gland

Skin

Introduction

It is easy enough to identify a basic 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 . 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: , 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 of the epidermis, and identify the modified epithelial exocrine .

B Analyze the organization of fibers and cells in the dermis and hypodermis and interpret their function within the skin.

C Locate muscle, peripheral and modified nervous tissues in the skin.

D Examine and hair follicles and determine that they are derived from the epidermis.

E Investigate the 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 also prevent water loss. i. - single layer of cells on the - mitotically active and responsible for the constant production of keratinocytes v - recall that anchor the . cells to the of basement iv. membrane ii. iii. - thickest portion of the epidermis - cells synthesize filaments - recall that cells are held together by - shrinkage during preparation causes cells ii. to separate except where they are held together giving them their ‘spiny appearance. iii. - cells undergoing terminal keratinization - contain keratohyaline granules that i release filaggrin to bundle keratin . filaments - creates a water-proof barrier iv. - a layer of pale-staining keratinocytes - only seen in thick skin Examine Slide 1 (79) to locate the 5 layers v. 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 . There are three types of exocrine glands: eccrine/, 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

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, rich cells - short ducts usually empty into , but they can also secrete directly onto the skin - produce ‘sebum’, an oily mixture of that maintains the keratin - excessive production of sebum can lead to blockage of the ducts and subsequent tissue resulting in .

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 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. loops that feed the avascular epidermis and are involved in - 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 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 . For example, in the lip you can see the 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 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 ) 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 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 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 the hair follicle Find evidence of erector pili - contraction of this muscle pulls the shaft of the hair into an muscles in Slide 6d erect position trapping air close to the skin so it can be and Slide 7d heated and fend of cold to a small extent - the contraction of the muscle distorts the papillary dermis where its superficial attachment is, creating tiny bumps on the skin surface called ‘goosebumps’.

Identify the extent of the inferior e. Inferior segment segment in Slide 6e - extends below the level of the arrector pili muscle Find a cross section through this - has a terminal dilation called the hair bulb region in Slide 7e f. Hair bulb Find the hair bulb in - the dilated base of the hair follicle Slide 6f - contains epidermis derived keratinocytes that form the Find a cross section nearest this - has a dermal papilla that inserts into the base of the bulb region in Slide 7f g. Dermal papilla Find the dermal papilla in - connective tissue that inserts into the base of the hair bulb Slide 6g - contains a capillary network that is required to sustain the It is not visible in Slide 7 hair follicle h. Matrix - is composed mainly of keratinocytes that proliferate during Find evidence of the cells growth of the elongating hair root forming the hair matrix in - is derived from the stratum basale and stratum spinosum of Slide 6h the epidermis and proliferates and undergoes keratinization and to form the hair Slide 7h - also contains responsible for pigmentation of the keratinocytes that go on to form the hair, they are therefore responsible for hair color i. Epithelial Find the epithelial root sheath in - is formed by the outermost keratinocytes of the hair bulb Slide 6i that continue up along the wall of the follicle Find a cross section of the - has two layers: internal and external epithelial root sheath in Slide 7i j. Internal root sheath Find the internal rooth sheath in - completely surrounds the region of the hair located in the Slide 6j isthmus and inferior segment Find a cross section of the - degenerates in the infundibulum above the level of the internal root sheath in Slide 7j sebaceous gland duct

Find the external rooth sheath in k. External root sheath Slide 6k - completely covers the internal root sheath but extends all the way to the dermis where it is continuous with the stratum Find a cross section of the spinosum and stratum basale external root sheath in Slide 7k l. Glassy membrane Find the glassy membrane in - is an acellular layer of that separates the Slide 6l hair follicle from the dermis and in Slide 7l - is the equivalent of a basement membrane Find the connective tissue m. Connective tissue sheath sheath in Slide 6m - The region of the dermis that immediately surrounds the and in Slide 7m entire hair follicle

2. The hair n. Hair root Find the hair root in Slide 6n - any part of the hair extending through the hair follicle to the and in Slide 7n surface of the skin

The hair shafts are absent from o. Hair shaft Slide 6

- The part of a hair that extends beyond the surface of the skin but you can find them in Slide 7o p. Hair Find the medulla of a hair in - The central part of the hair root composed of moderately Slide 6p keratinized cells and in Slide 7p q. Hair Find the cortex of a hair in - A region of more heavily keratinized, densely packed cells that Slide 6q surrounds the medulla of the hair root and in Slide 7q r. Hair Find the cuticle of a hair in - The most peripheral cells of the hair root forms this thin, Slide 6r heavily keratinized layer cells and in Slide 7r Nails are plates of hard keratin on the dorsal surface of each distal phalanx

Examine the fetal and locate the different regions of the growing fingernail in Slide 8

1. Fingernail a. root Find the nail root in - The most proximal part of the nail that arises from the nail Slide 8a matrix b. (cuticle) Find the cuticle in - A region of stratum corneum that extends out from the skin Slide 8b of the finger to cover the nail root c. Nail plate - As the nail root continues to mature and harden it forms the Find the nail plate in nail plate which is bound to the underlying epidermis (nail Slide 8c bed) d. Nail bed Find the nail bed in - A region of stratum basale and stratum spinosum Slide 8d (epidermis) underlying and supporting the keratinized nail plate. e. Nail matrix Find the nail matrix in - The cells of the nail matrix divide, move distally and become Slide 8e keratinized to form the nail root. - it has ventral and dorsal components either side of the nail root f. - The distal end of the nail plate becomes free of the nail plate Find the hyponychium in at the epidermal fold called the hyponychium. Slide 8f You must be able to identify all the basic tissues in skin in addition to the following skin and adnexa specific structures

Epidermis Hair (not testable) Stratum basale Medulla Stratum spinosum Cortex Stratum granulosum Cuticle Stratum lucidum Root Stratum corneum Shaft Keratinocytes Melanocytes Hair follicle (not testable) Keratin Infundibulum Isthmus Inferior segment Glands Hair bulb Eccrine Dermal papilla Epithelial root sheath Sebaceous gland Internal root sheath External root sheath Glassy membrane Dermis Connective tissue sheath Papillary dermis Hair matrix Reticular dermis Erector pili muscle Hypodermis

Nail (not testable) Thin skin Eponychium (cuticle) Thick skin Hyponychium Nail matrix Nail plate Nail bed Adnexa Nail root Peripheral nerve Meissner's corpuscle

Pacinian corpuscle Hair (in cross section) Hair (in long section)