Integumentary System: Epidermis and Skin Color

Epidermis

• Outermost layer of skin, composed of 4-5 layers of squamous epithelial cells
• Deepest layer is the stratum basale
• Not vascularized, nutrients diffuse through dermal blood vessels and tissue fluid
• Cells on the surface are constantly shedding and replaced by new cells from the stratum basale every 2-4 weeks
• Cells in this layer divide and grow, moving towards the skin surface and away from the dermis
• As they move upwards, they receive fewer nutrients and eventually die
• Older cells are called keratinocytes, which harden with age through keratinization (keratin protein fills the cytoplasm of these cells forming stratum corneum layer)
• Dead skin cells in this layer are eventually shed from the body

Skin Color

• Melanocytes are located deep in the epidermis
• Responsible for skin color
• All humans have the same number of melanocytes
• Variation in skin color is due to the amount of melanin produced and distributed, not the number of melanocytes

Dermis

• Mainly composed of adipose tissue, elastic and fibrous connective tissue
• Lipocytes produce fat that provides padding to protect deeper tissues and insulation for temperature regulation

Accessory Structures of the Integumentary System

• Include: sweat glands, sebaceous glands, nails, hair and hair follicles
• Most are located in the dermis and protrude through the epidermis

Accessory Structures - Sweat Glands

• Also called sudoriferous glands
• Consist of a small tube originating as a coil in the deep dermis
• Coiled portion is lined with sweat-secreting cells
• Sweat is carried out of the skin by tubes called pores that open at the skin surface
• Made up of 99% water and salts, including sodium chloride
• Normal acidic pH of between 4 and 6

Accessory Structures - Sweat Glands

• Contains dermicidin, a peptide that kills microbes
• Sweating, starting on the forehead, prevents overheating
• Cold sweating, caused by nervousness or fright, starts on palms, axillae, and soles
• Sweat has no odor, but bacteria degrade substances in the sweat into chemicals that give off strong smells

Accessory Structures - Sweat Glands

• The skin contains two types of sweat glands: merocrine (eccrine) glands and apocrine sweat glands
• Merocrine glands: predominant type, 2-5 million in adults, found all over the skin, important in temperature regulation
• Apocrine sweat glands: active at puberty, ~2,000 in number, mostly found in the groin, anal region, and armpits, produce smell in contact with skin bacteria, believed to act as sexual attractants

Accessory Structures - Sebaceous Glands and Follicles

• Also called oil glands
• Primarily located near hair follicles, largest on the face, neck, and upper chest
• Secrete sebum, a mixture of fatty material and cell debris
• Sebum is secreted through small hair follicle ducts
• Functions of sebum: keeps hair and skin pliable and waterproof, inhibits bacterial growth, lubricates hair and skin

Accessory Structures - Sebaceous Glands and Follicles

• Sebaceous follicles are large sebaceous glands, with ducts discharging sebum directly onto the epidermis
• Found on the face, chest, nipples, back, and external genitalia
• Overactive sebaceous glands, usually on the scalp, can lead to inflammation known as seborrheic dermatitis, a common cause of dandruff

Accessory Structures - Nails

• Protect the ends of fingers and toes
• Consist of a nail plate above a skin surface called nail bed
• A modification of the epidermis that contains hard keratin
• Part of the nail plate that grows most actively is covered by a whitish, half-moon-shaped lunula
• Nails appear pink due to underlying capillaries, with the white crescent-shaped lunula above the nail matrix

Accessory Structures - Hairs and Hair Follicles

• Approx. 2.5 million hairs on the human body, over 75% on the body surface
• Hairs project above the skin surface over most of the body, except for soles of the feet, palms of the hands, sides of fingers and toes, lips, and parts of the external genitalia
• Consist of a large amount of dead keratinized cells, dominated by hard keratin
• Structures produced in organs called hair follicles

Burns

• Two factors assess the degree of damage: depth and total surface of skin damaged
• Depth refers to the layers of skin affected

First Degree Burn

• Involves only the epidermis, most common type
• Symptoms include redness, pain, and swelling (edema)
• Pain subsides in 2-3 days and usually does not leave scars
• Most sunburns are first-degree burns

Second Degree Burn

• Also called partial-thickness burn
• Involves the entire depth of the epidermis and a portion of the dermis
• May appear red, tan, or white with blisters
• Symptoms include redness, pain, and blistering
• Very painful, slow healing and may leave scars

Second Degree Burn

• Extent of blistering depends on the depth of the burn
• Blisters heal within 10-14 days without complications, but deeper burns take 1-3.5 months
• Scarring is common in second degree burns

Third Degree Burn

• Also termed full-thickness burns
• Affects all three layers of the skin; epidermis and dermis are completely destroyed, deeper tissue is damaged
• Skin can only repair itself from the edges of the wound

Test your knowledge on the structure and function of the epidermis, the outermost layer of skin. This quiz explores the role of keratinocytes, the process of keratinization, and the factors affecting skin color, including the role of melanocytes. Challenge yourself to understand these essential concepts in skin biology.