The Integumentary System
Structure of the Skin
Structure – two primary layers called epidermis and dermis, subcutaneous layer of fat (hypodermis). 1 Sq. inch contains: 500 sweat glands, over 1000 nerve endings, “yards” of blood vessels, nearly 100 sebaceous glands, 150 pressure sensors, 75 heat sensors, 10 cold sensors, millions of cells total.
Epidermis
Outermost and thinnest primary layer of skin
Composed of several layers of stratified squamous epithelium
Stratum germinativum (or basale) – innermost layer of cells that continually reproduce, and new cells move toward the surface.
Outermost layer is stratum corneum because cells are cornified (cornu = horn).
As cells approach surface, cytoplasm is replaced with a tough waterproof protein called keratin (cells are called keratinocytes; kera = horn in Greek) and eventually flake off.
Protection is provided by tight junctions between cells, keratin, and intercellular glycolipid, which is waterproof.
Stratum lucidum - extra layer present only in thick skin on palms and soles of feet.
Cells of the epidermis:
Melanocytes – pigment containing cells, produce melanin.People all have the same basic number but some are more active than others, or melanin doesn’t breakdown as rapidly.
Skin color also influenced by exposure to UV – stimulates more melanin production – protective
Carotene is another pigment in epidermal cells, gives a yellow to orange tint to skin; prominent in Asians
Blood flow can influence skin color in light skinned people (cyanosis, flush)
Langerhans cells (macrophage-like cells) phagocytize microbes, travel to lymphoid organs, stimulate immune response
Also have cells (Merkel cells) for touch receptors
Dermis
Deeper and thicker, consists mostly of dense irregular CT
Dermal papilla – projections upward that help bind epidermis and dermis
In palms and soles of feet lie on top of dermal ridges, to produce epidermal ridges, which increase gripping ability (fingerprints)
Deeper area has collagen and elastin fibers – strength and recoil (aging, wrinkles)
Also has nerves and nerve endings for pressure, pain, temperature; muscle, blood vessels, hair follicles, sweat and oil glands.
Hypodermis
Subcutaneous layer, below the dermis
Consists of loose connective tissue, primarily adipose
Accessory Structures of the Skin
Hair
Lanugo – soft hair of fetus and newborn
Growth occurs in hair follicles – epidermal cells grow down into dermis and form specialized tube in early fetal development.
Growth begins from hair papilla – cluster of cells at base of follicle, with nourished by dermal blood vessel
Hair root lies hidden in follicle and visible part is called shaft
Arrector pili – smooth muscle that causes hair to stand up straight (goose bumps) in response to fear or cold (traps layer of warm air in furry animals, also makes small cute fuzzy scared animals look larger and hairier – and we all know how scary big hair can be).
Nails
Produced by epidermal cells over terminal ends of fingers and toes
Cells are filled with hard keratin (as opposed to soft keratin in skin) like hair
Hard keratin is tougher and the cells don’t flake
Visible part called nail body
Root lies in a groove and is hidden by cuticle
Crescent-shaped area nearest root called lunula, appears white because of the
Nail bed may change color with change in blood flow, usually appears pink because of extensive capillary bed underneath, may appear blue in cyanosis or cold.
Glands
Sudoriferous (Sweat)
Sweat or sudoriferous glands are classified into two groups:
Eccrine sweat glands
Most numerous, important, and widespread
Produce perspiration – 99% water (hypotonic) with some salt, ammonia, uric acid, etc.
Sweat released from ducts through pores on skin surface (not the same as pores of your complexion, which are hair follicles)
Heat regulation
Apocrine sweat glands
Located in axillary and genital regions
Thicker milky secretion (sweat + lipid and protein), different from eccrine perspiration
Odorless, but breakdown of secretion by bacteria produces odor
Begin to function at puberty under influence of androgens
Not thermoregulatory, but stimulated by sympathetic NS during pain and stress
Activity increased by sexual foreplay and also enlarge and recede with the phases of the menstrual cycle – may be analogous to sexual scent glands of other animals
Sebaceous
Secrete oil of sebum for hair and skin
Ducts open into hair follicles
Sebum prevents drying and cracking of the skin
Level of secretion increases during adolescence
Amount of secretion regulated by sex hormones
Sebum in sebaceous gland ducts may darken to form a blackhead
Mammary - modified sweat glands produce milk under the influence of the hormone prolactin
Disorders of the Skin
Include everything from infections (athlete’s foot, impetigo) to overactive cell division (eczema, psoriasis) to accelerated keratinization (dandruff) to allergic reactions resulting in hives (uticaria)
Skin Cancer
Absolutely correlates with UV exposure. Melanin is protective, so fair skinned people are more at risk, however anyone can be affected. While UV-A rays are less likely to cause skin cancer than UV-B they still can do it and cause more damage to connective tissue in the dermis. Tanning beds may claim that using UV-A rays makes them less dangerous than tanning in the sun but it is like the difference between crawling into an oven set at 400 degrees and one set at 450. You will cook, and come out wrinkled to boot.
Basal cell carcinoma – most common, arises from basal cells.The physical appearance of the lesion is varied, and may look like anything from a pale mark to an open sore that won’t heal. Usually slow growing and easily cured, but tend to reappear.
Squamous cell carcinoma – much less common than basal cell carcinoma, arise from keratinocytes in the upper epidermis. More likely to spread to other organs (metastasize) and thus more dangerous. Signs are similar to basal cell carcinoma.
Malignant melanoma – this is the killer. Arises from melanocytes, so the lesions look like unusual moles.May actually begin as a mole, which is just an overgrowth of melanocytes and not dangerous. Tend to metastasize rapidly and are very dangerous.Identified by the ABCD rule: A means asymmetry (they’re often lopsided and opposite sides of the spot look different), B means border irregularity – the edges are indented rather than smooth, C means color – typically exhibit a range of colors rather than just one, and D means diameter – larger than 6mm, or the size of a pencil eraser.
Burns
May be caused by heat, uv light, electric current, or chemicals
Immediate threat to life is catastrophic fluid loss:
Dehydration and electrolyte imbalance
Renal shutdown
Circulatory shock due to loss of blood volume and inadequate circulation
Although initially the wound is sterile, within about 24 hours infection becomes a threat:
Bacteria, fungi, other pathogens invade – no barrier to protect
Multiply rapidly in the nutrient rich environment
Within a day or two of injury the immune system also becomes suppressed
Treatment and recovery or survival depend on total area involved and severity or depth of the burn
Classification of Burns
By depth:
First-degree (partial-thickness) burns – only surface layers of epidermis involved
Second-degree (partial-thickness) burns – involve the deep epidermal layers and always cause injury to the upper layers of the dermis
Third-degree (full-thickness) burns – characterized by complete destruction of the epidermis and dermis
May involve underlying muscle and bone
Lesion is insensitive to pain because of destruction of nerve endings immediately after injury – intense pain is soon experienced however
Fourth-degree burns – charred to the bone
By surface area involved:
Estimating body surface area using “rule of nines” in adults helps estimate volume of fluid lost
Body divided into 11 areas of 9% each
Additional 1% of body surface area around genitals
Burn patients also need lots of extra calories to replace lost proteins and allow tissue repair – usually have to get supplementary nutrients via NG tubes and IVs
Wound Healing
After the clot is formed macrophages come into the area to clean away debris, new capillaries begin to grow into the area, fibroblasts lay down collagen, which serves as a scaffold for the epithelial cells of the epidermis to grow into the area and fill it in.
If the area isn’t too big, and if the edges of the wound are fairly close together, the wound heals without much scarring.
Epidermal tissue can regenerate quite well, but wound healing depends upon several factors
If the area is large, or nutrition is poor, or an infection sets in,epithelial cells can’t fill in as well and the wound is filled with collagen and other connective tissue elements, leaving a scar.
Some tissues don’t regenerate well because the adult cells aren’t mitotic, for example skeletal muscle, cardiac muscle, and nervous tissue.
Functions of the Skin
Protection– first line of defense
Infection by microbes
Ultraviolet rays from sun
Harmful chemicals
Cuts and tears
Fluid loss
Vitamin D Synthesis
Synthesized by keratinocytes when exposed to UV irradiation (sunlight) from a precursor molecule derived from cholesterol
Doesn’t take much exposure, so don’t try to use vitamin D synthesis as an excuse for visiting the tanning bed.
Converted to calcitriol by liver and kidneys, which regulates calcium and phosphorus metabolism – important for calcium absorption and proper bone development, helps prevent rickets.
Temperature Regulation
Skin can release almost 3000 calories of body heat per day
Mechanisms of temperature regulation
Regulation of sweat secretion – a rise in body temperature stimulates sweat gland activity.The evaporation of sweat pulls heat away from the skin (it takes one calorie to raise one gram of water one degree Celsius, but it takes 540 calories to change one gram of boiling water to steam, which is basically what happens when sweat evaporates, just on a microlevel, so while it may feel as if you are basting like a turkey you really aren’t.
Regulation of blood flow to body surface – a rise in body temperature causes capillary beds in the skin to open and blood to flood the surface of the body.Blood transports heat, which is then given up to the surrounding atmosphere. If the surface of the body is cooled the capillary beds shut down, shunting blood to the interior of the body to keep internal organs warm.Frostbite occurs when the flow to the surface is impeded for long periods of time and the tissue can both starve for oxygen and freeze.
Hyperthermia – body temperature above normal
Heat exhaustion – fatigue, headache, nausea, profuse sweating may lead to loss of salts and low blood pressure. Rest and fluid replacement.
Heat stroke – Temperatures reaching upwards of 110 degrees F, symptoms include disorientation, coma, dry skin due to inability to sweat, low blood pressure. Requires immediate treatment, including immersion in an ice bath to drop temperature.Temperatures this high damage brain proteins and can cause permanent damage.
Fever – controlled hyperthermia in response to infection. Both proteins from pathogenic microorganisms and inflammatory chemicals released as the body fights the infection cause the brain to reset the thermostat, so to speak. As the body tries to reach the higher setpoint shivering and chills usually occur. When the crisis is past, that is, the infection is under control, the signals that caused the “thermostat” to be set higher are removed. When the body returns to it’s normal temperature the excess heat is given off normally – skin is flushed and sweating occurs. So fever is good. If you are sick, don’t be a sissy, tough it out and you’ll get well sooner, although you’ll feel worse. Also – keep an eye on the temperature. You don’t want that heat driven denaturation of brain proteins to occur (think of an egg: the white is nearly pure protein, and the proteins have a shape that allows light to pass through. They should call it egg clear instead of egg white. It doesn’t turn white until you put it in a frying pan and apply heat, which denatures or unfolds the protein. Remember, the shape of a protein determines it’s function, if it loses it’s shape it loses it’s function. So you wouldn’t put your brain in a frying pan and apply heat would you?)
Hypothermia – low body temperature. When you drop to the 90-95 degree range you experience uncontrollable shivering, incoherent speech, lack of coordination. In the 80-90 degree range shivering soon stops, unconsciousness occurs, respiration is depressed, and eventually death occurs. This can be used for good in some surgeries that might take a long time, but remember, after being packed in ice, the surgeons would intubate you and breathe for you with a respirator. This situation would not be so good if you were to fall in Lake Murray in January. You’d probably end up breathing water, which is basically incompatible with life.
Sense Organ Activity
Skin functions as an enormous sense organ
Receptors serve as receivers for the body, keeping it informed of changes in its environment
There are receptors for touch, pressure, pain, and temperature. The great number of touch receptors in the fingertips allow them to be useful for manipulating objects and performing delicate tasks – just try some of the things your fingers do with your toes and you’ll see what this is about.
Meissner’s corpuscle – detect light touch; located close to surface in dermal papillae
Pacinian corpuscle – pressure; located deep in the dermis
Free nerve endings – pain, temperature, light touch (with Merkel cells)
Krause’s end bulbs – touch, probably modified Meissner’s
Effects of Aging
Subcutaneous fat layer (hypodermis) thins
More susceptible to cold
Collagen fibers become thicker, spaced further apart, elastin fibers decrease in number and lose elasticity
Skin wrinkles and and sags
Hair tends to fall out to some degree or another
Melanocytes decrease in number and sometimes are found bunched together
Skin becomes more pale, hair turns gray