integumentary system skin Flashcards
(12 cards)
the two layers of skin
As we mentioned, the skin is composed of two kinds of tissue (Figure 4.3). The outer epidermis (ep″ĭ-der′mis) is made up of stratified squamous epithelium that is capable of becoming hard and tough. The underlying dermis is made up mostly of dense irregular connective tissue. The epidermis and dermis are firmly connected and the dermis is fairly tear resistant. However, a burn or friction (such as the rubbing of a poorly fitting shoe) may cause them to separate, allowing interstitial fluid to accumulate in the cavity between the layers, which results in a blister.
hypodermis
Deep to the dermis is the subcutaneous tissue, or hypodermis, which essentially is adipose (fat) tissue. It is not considered part of the skin, but it does anchor the skin to underlying organs and provides a site for nutrient storage. Subcutaneous tissue serves as a shock absorber and insulates the deeper tissues from extreme temperature changes occurring outside the body. It is also responsible for the curves that are more a part of a woman’s anatomy than a man’s.
keritin in epidermis
Most cells of the epidermis are keratinocytes (keratin cells), which produce keratin, the fibrous protein that makes the epidermis a tough protective layer in a process called keratinization. These keratinocytes are connected by desmosomes throughout the epidermis (refer to Figure 3.3).
avascular skin
Like all epithelial tissues, the epidermis is avascular; that is, it has no blood supply of its own. This explains why a man can shave daily and not bleed even though he cuts off many cell layers each time he shaves.
the layers
The epidermis is composed of up to five layers, or strata (strah′tah), each with multiple layers of cells. From the inside out these are the stratum basale, spinosum, granulosum, lucidum, and corneum, illustrated in (Figure 4.4) (except the stratum lucidum, which is found only in thick skin).
stratum bassale
The deepest cell layer of the epidermis, the stratum basale (stra′tum ba-sah′le), lies closest to the dermis and is connected to it along a wavy border that resembles corrugated cardboard. This basal layer contains the most adequately nourished of the epidermal cells because nutrients diffusing from the dermis reach them first. Stem cells in this layer are constantly dividing, and millions of new cells are produced daily; hence its alternate name, stratum germinativum (jer″min-ah-tiv′um; “germinating layer”). Of the new cells produced, some become epidermal cells, and others maintain the population of stem cells by continuing to divide. The daughter cells destined to become epidermal cells are pushed upward, away from the source of nutrition, to become part of the epidermal layers closer to the skin surface. As they move away from the dermis and become part of the more superficial layers, the stratum spinosum and then the stratum granulosum, they become flatter and increasingly keratinized.
stratum lucidum
As these cells leave the stratum granulosum, they die, forming the clear stratum lucidum (lu′sid-um). This latter epidermal layer is not present in all skin regions. It occurs only where the skin is hairless and extra thick, that is, on the palms of the hands and soles of the feet. The combination of accumulating keratin inside them, secreting a water-repellent glycolipid into the extracellular space, and their increasing distance from the blood supply (in the dermis) effectively dooms the stratum lucidum cells and the more superficial epidermal cells because they are unable to get adequate nutrients and oxygen.
stratum corneum
The outermost layer, the stratum corneum (kor′ne-um), is 20 to 30 cell layers thick, but it accounts for about three-quarters of the epidermal thickness. The shinglelike dead cells, completely filled with keratin, are referred to as cornified, or horny, cells (cornu = horn). The common saying “Beauty is only skin deep” is especially interesting in light of the fact that nearly everything we see when we look at someone is dead! The abundance of the tough keratin protein in the stratum corneum allows that layer to provide a durable “overcoat” for the body, which protects deeper cells from the hostile external environment and from water loss, and helps the body resist biological, chemical, and physical assaults. The stratum corneum rubs and flakes off slowly and steadily as dandruff. The average person sheds about 18 kg (40 lb) of these flakes in a lifetime, providing a food source for the dust mites that inhabit our homes and bed linens. This layer is replaced by cells produced by the division of the deeper stratum basale cells. Indeed, we have a totally “new” epidermis every 25 to 45 days.
melanin
Melanin (mel′ah-nin), a pigment that ranges in color from yellow to brown to black, is produced by special spider-shaped cells called melanocytes (mel′ah-no-sītz), found chiefly in the stratum basale. Freckles and moles are seen where melanin is concentrated in one spot.
Epidermal Dendritic Cells (Langerhans cells)
Scattered throughout the epidermis, especially near the epidermal-dermal junction.
Act as the skin’s immune sentries or guards.
They detect invading microbes like bacteria and viruses.
Once they spot a threat, they activate the immune system to respond and protect the body.
Think of them as the skin’s security team keeping watch.
merkel cells
Found mostly in the stratum basale, near the junction of epidermis and dermis.
Connected to sensory nerve endings.
Function as touch receptors, known as Merkel discs.
Help you sense light touch and texture on your skin.
if too much uv lights touches skin
Despite melanin’s protective effects, excessive exposure to UV light (via sunlight or tanning beds) eventually damages the skin, leading to a leathery appearance. It also depresses the immune system. This may help to explain why people infected with the human herpesvirus 1, which causes cold sores, are more likely to have an eruption after sunbathing. Overexposure to the sun can also alter the DNA of skin cells, leading to skin cancer. People with naturally dark skin seldom have skin cancer, attesting to melanin’s amazing effectiveness as a natural sunscreen.
