Integumentary System: Skin Layers and Functions

• The integumentary system is composed of the skin and body coverings (nails, hair).

Skin

• Skin is also called the integument or cutaneous membrane.
• The skin is the largest organ in the body, making up about 7% of body weight.
• The skin functions include:
• Providing a protective covering.
• Regulating body temperature.
• Preventing water loss from deeper tissues.
• Housing receptors for cutaneous sensations (touch, pressure, heat, cold, pain).
• Activating a precursor molecule in the skin by UV light for Vitamin D synthesis.
• Excreting small quantities of wastes.
• Acting as a blood reservoir, housing about 8-10% of total blood flow in dermal vessels.

Distinct Layers

• The skin has three distinct layers: epidermis, dermis, and hypodermis.

Epidermis

• The epidermis is the top layer and is made of keratinized stratified squamous epithelium.
• It is avascular, meaning it has no blood vessels.
• It contains various cells:
• Langerhans' cells: phagocytize microbes; they are also called intraepidermal macrophages.
• Merkel cells: function in the sensation of touch, located deep within the epidermis, and connect to Merkel discs/tactile discs (sensory neurons), also known as tactile epithelial cells.
• Keratinocytes: produce the protein keratin, a waterproof sealant.
• Melanocytes: cells producing pigment (melanin).
• The number of melanocytes is about the same in all people; skin color is determined by the amount of melanin these cells produce and distribute.
• Patches of melanin are called freckles.
• Albinism is the genetic inability to produce melanin.
• Yellowish coloration indicates the skin contains the pigment carotene.
• Pinkish skin is apparent in fair-skinned people when oxygenated (bright red) blood is in the dermal capillaries.
• Other factors influencing skin color include the oxygenation of blood and sunlight exposure which stimulates melanocytes to produce melanin.
• There are four or five layers of the epidermis, depending on the degree of friction and mechanical pressure.
• The layers of the epidermis, from deepest to most superficial, are:
• Stratum basale: The deepest layer, a mitotic layer where new cells are forming, and melanocytes are found. It is also known as the stratum germinativum or germinating layer.
• Stratum spinosum: Cells are held together by desmosome cell connections, giving the appearance of spines sticking off cells.
• Stratum granulosum: A grainy-looking layer.
• Stratum lucidum: Found only in palms and soles; it is "thick" skin, lucid = clear, translucent.
• Stratum corneum: The most superficial layer, consisting of 25+ rows of dead, squamous keratinocytes.
• Stratum lucidum is not found in thin skin, which is everywhere except in the palms and soles.
• Mnemonic for remembering the order of epidermal layers (superficial to deepest): Come (Corneum) Let's (Lucidum) Get (Granulosum) Sun (Spinosum) Burned (Basale).

Dermis

• The dermis binds the epidermis to underlying tissues and contains fewer cells.
• It is strong, elastic, and composed largely of fibrous connective tissue, fibroblasts, collagen, and elastin fibers.
• The dermis is vascular; blood vessels supply nutrients to all skin cells and help regulate body temperature.
• The upper layer contains dermal papillae that form ridges, resulting in spiral and concentric patterns.
• These dermal ridges function to increase friction for a better gripping surface and are unique to each person.
• Nerve fibers are scattered throughout the dermal layer.
• Motor fibers carry impulses from the brain or spinal cord to the dermal muscles and glands (Efferent).
• Sensory fibers pick up information from the outside and carry it to the brain and spinal cord; this includes warmth, cold, soft touch, and deep pressure (Afferent).
• The dermis also contains muscle tissue (arrector pili), hair follicles, sebaceous (oil) glands, and sweat glands.

Hypodermis

• The hypodermis consists of masses of loose areolar connective tissue and adipose tissues that bind the skin to underlying organs.
• It is below the skin and is not a true layer of the skin.
• It is also called the subcutaneous layer.
• Insulates the body.
• Contains major blood vessels and deep pressure nerve endings.

Sweat Glands

• Sweat glands, also called sudoriferous glands, help regulate body temperature through evaporation.
• There are estimated to be 3000 sweat glands in a single square inch of skin. Two types:
• Eccrine: most numerous sweat glands in the body.
• Apocrine: do not enlarge or become active until puberty, associated with body odor.

Sebaceous Glands

• Sebaceous glands secrete oil (sebum) for the skin and hair, and a hair follicle always has a sebaceous gland associated with it.

Ceruminous Glands

• Ceruminous glands are found in the external auditory meatus and aid in the formation of a waxy brown substance called cerumen, "ear wax."

Nails

• Nails consist of the root, cuticle (fold of skin), lunula (white 1/2 moon), nail body, and free edge (tip).

Hair

• Hair is associated with sebaceous glands; the follicle is located in the epidermis layer which extends into the dermis.
• Hair color is influenced by the amount and type of melanin.

Types of Hair

• Lanugo: extremely fine, soft downy hair that covers the body of the fetus.
• Vellus: covers most of the body during childhood (except for eyebrows, eyelashes, and scalp).
• Terminal: Eyebrow, eyelash, and scalp hairs.
• At puberty, terminal hairs replace vellus hairs in the axillary and pubic regions of both adolescent boys and girls.
• Terminal hairs additionally replace vellus hairs on the face, chest, and limbs of boys, therefore, men have more terminal hair than women.

Aging and The Integument

• The epidermis is held less tightly to the dermis, causing the skin to sag.
• There are fewer and more disorganized collagenous and elastic fibers in the dermis and fewer fibroblasts, leading to wrinkles.
• The subcutaneous layer has less adipose tissue, especially in the face and hands, and there is less vasculature (fewer blood vessels), which makes it harder to maintain homeostasis of body temperature and increases the chance of feeling cold.
• There a fewer sweat glands, making it harder to maintain homeostasis of body temperature and increases the risk of heat exhaustion.
• The number of hair follicles decreases, causing hair thinning.
• Reduced sebaceous glands means less oil, causing the skin to become drier and possibly crack.
• A reduced number of melanocytes leads to gray or white hair and paler skin; some remaining pigment cells may enlarge, causing pigmented blotches.
• Effects of aging on the integument include wrinkling, slower growth of hair and nails, and dryness.

Skin Cancer

• Skin Cancer warning signs can be remembered using ABCDE's.
• A: Asymmetry- sides do not match.
• B: Border Irregularity- indentation/notches.
• C: Color-2 or more.
• D: Diameter- larger that tip of pencil eraser ~6mm.
• E: Evolving- any change to size, shape, elevation, color.
• Burns: differences exist between 1st, 2nd, 3rd degree burns.
• The Rule of 9's can be used to estimate the area burned.