Describe the layers and accessory structures of the integumentary system, relate them to its functions, and explain its homeostatic imbalances

Learning goal: Describe the integumentary system as skin plus accessory structures; identify the two layers of the skin (epidermis and dermis) and the hypodermis beneath it, the cells of the epidermis and the accessory structures (sweat glands, sebaceous glands, arrector pili muscles, hair follicles, nails); explain the functions of protection, temperature regulation, excretion and sensory perception and the processes of perspiration, skin production and shedding, vitamin D synthesis and repair; compare the integument of the vertebrate classes; and explain the cause of homeostatic imbalances such as burns, skin cancer, anhidrosis, acne, eczema and scleroderma.

The largest organ. The integumentary system consists of the skin and its accessory structures: hair, nails and glands. In an adult the skin covers roughly 1.5 to 2 square meters and is the body's boundary with the outside world, so its structure is built for defense, sensing and control of heat and water. It is an organ because it contains all four tissue types: stratified squamous epithelium on top, connective tissue beneath, smooth muscle in the arrector pili and vessel walls, and nervous tissue in its many receptors.

Epidermis. The outer layer, the epidermis, is a keratinized stratified squamous epithelium with no blood vessels of its own; it is fed by diffusion from the dermis below. Its main cell type is the keratinocyte, which makes the tough protein keratin. Keratinocytes are born by division in the deepest layer, the stratum basale, which sits on the basement membrane. As new cells push older ones upward, the cells pass through the stratum spinosum, where they are linked by strong junctions; the stratum granulosum, where they fill with keratohyalin granules, which help bundle keratin filaments, and with lamellar bodies that release waterproofing lipids, and begin to die; in thick skin of the palms and soles only, a clear stratum lucidum; and finally the stratum corneum, a layer of many rows of dead, flattened, keratin-packed cells that are shed continuously from the surface. This is skin production and shedding: the epidermis renews itself roughly every month, so a wound that damages only the epidermis leaves no scar. Three other cell types live in the epidermis. Melanocytes in the stratum basale make melanin, the pigment that absorbs ultraviolet light, and pass it into keratinocytes where it collects over the nucleus as a shield; differences in skin color come mainly from how much melanin is made, not from the number of melanocytes. Dendritic (Langerhans) cells are immune cells that detect invaders. Merkel cells at the base, together with a nerve ending, detect light touch.

Dermis. Beneath the basement membrane lies the dermis, a thick layer of connective tissue that gives skin its strength and stretch. Its thin upper papillary layer is loose areolar tissue thrown into small ridges, the dermal papillae, which interlock with the epidermis, carry capillary loops that feed it and contain touch receptors; on the fingertips the papillae follow patterns that produce fingerprints. The thicker reticular layer below is dense irregular connective tissue, whose collagen fibers in every direction resist tearing and whose elastic fibers let skin recoil. The dermis also holds the blood vessels, nerves, glands and hair follicles.

Hypodermis. The hypodermis, also called the subcutaneous layer, lies beneath the dermis and is made of areolar and adipose tissue. It anchors the skin to the muscles beneath while allowing it to slide, stores fat as an energy reserve, cushions against blows and insulates against heat loss. It is often described as a layer of the skin, though strictly it lies beneath it.

Accessory structures. Hair follicles are tubes of epidermis that dip into the dermis; the hair grows from dividing cells at the base of the follicle and, like the stratum corneum, is made of dead keratinized cells. Attached to each follicle is a small band of smooth muscle, the arrector pili, which contracts in cold or fear to pull the hair upright and dimple the skin, producing goose bumps; in furred mammals this traps insulating air, but in humans it has little effect. Sebaceous glands open into the follicles and secrete sebum, an oily substance that softens hair and skin and slows the growth of some bacteria. Sweat glands are of two kinds: eccrine sweat glands, found over most of the body and most dense on the palms, soles and forehead, secrete a watery fluid directly onto the surface for cooling; apocrine sweat glands, in the armpits and groin, become active at puberty and secrete a thicker fluid into hair follicles, which skin bacteria break down to produce body odor. Nails are hard plates of keratin growing from a nail matrix under the skin fold at the base; they protect fingertips and help with picking up small objects.

Functions, each tied to a structure. Protection: the keratinized stratum corneum is a physical barrier that resists abrasion and water loss and blocks most microbes; the slightly acidic film of sweat and sebum discourages bacterial growth; melanin absorbs ultraviolet light before it reaches the dividing cells below; dendritic cells raise the immune alarm. Temperature regulation: perspiration cools because evaporating sweat carries heat away, and the dermal blood vessels widen to shed heat or narrow to keep it, under the control of the hypothalamus described in the homeostasis lesson. Excretion: sweat carries small amounts of water, salts and urea out of the body, a minor route beside the kidneys. Sensory perception: the skin holds free nerve endings for pain and temperature, Merkel cells at the base of the epidermis and Meissner's corpuscles in the dermal papillae for light touch, and Pacinian (lamellated) corpuscles deep in the dermis and hypodermis for pressure and vibration; the type of receptor, its depth and its density decide what a region can sense. Vitamin D synthesis: ultraviolet light striking the epidermis converts a cholesterol-related molecule into a vitamin D precursor, which the liver and kidneys finish; vitamin D is needed to absorb calcium from food, so skin is part of bone health. Repair: after a cut, blood clots and a scab forms; inflammation brings white blood cells; cells of the stratum basale migrate across the wound beneath the scab while fibroblasts in the dermis lay down collagen; a wound confined to the epidermis heals without a mark, but a deep wound fills with collagen-rich scar tissue that lacks hair, glands and normal pattern.

Comparing vertebrate integuments. The integument varies by class. Fish have scales of bone or dentine in the dermis covered by a mucus-secreting epidermis. Amphibians have thin, moist, glandular skin that exchanges gases, which is why they must stay damp. Reptiles have a thick keratinized epidermis with overlapping scales that resist water loss. Birds have feathers, keratin structures grown from follicles, for insulation and flight. Mammals have hair, sweat glands and sebaceous glands. Human skin, with dense eccrine glands and sparse hair, is unusually suited to cooling by evaporation.

Homeostatic imbalances. Each imbalance is a failure of one structure or process. Burns destroy skin by heat, chemicals or radiation: a first-degree burn damages only the epidermis and reddens; a second-degree (partial-thickness) burn extends into the dermis and blisters, and a deep one may still need surgery; a third-degree burn destroys the full thickness, so no epidermal cells remain to regrow and, unless it is very small, grafting is usually needed; the danger of large burns is loss of the barrier, leading to fluid loss and infection. Skin cancer arises when ultraviolet light from the sun or a tanning bed damages the DNA of dividing epidermal cells; basal cell and squamous cell cancers arise from keratinocytes and are usually local, while melanoma arises from melanocytes and can spread. Anhidrosis is the inability to sweat, so the body cannot shed heat by evaporation and overheats in warm conditions. Acne occurs when sebaceous glands enlarge at puberty and their follicle openings become blocked with sebum and dead cells, which bacteria then inflame; it is not caused by dirt, and scrubbing does not clear it. Eczema is an inflammatory condition in which the barrier is impaired, so skin becomes dry, itchy and prone to irritation. Scleroderma is an autoimmune disease in which excess collagen is deposited, making the skin hard and tight. None of these descriptions is medical advice; they explain which structure has failed.

Investigation: two-point discrimination. With a partner's eyes closed, gently touch the skin with either one or two blunt points (the two ends of a bent paper clip, cleaned) at a set separation and ask whether one or two points are felt. Starting far apart and moving closer, record the smallest separation at which two points are still felt as two for the fingertip, the palm, the forearm and the back. A smaller threshold on the fingertip than on the back supports the claim that touch receptors are packed more densely where fine sensing is needed. Press lightly and never use sharp points. The test shows receptor density indirectly; it cannot show the receptors themselves, their depth or which receptor type is responding, and results vary with pressure and attention.

Misconceptions. "Skin is one layer" fails; it is three regions, and the epidermis alone has four or five layers. "Tanning is safe if you do not burn" is wrong; a tan is the skin's response to DNA damage, and tanning beds emit the same damaging ultraviolet light. "Acne comes from dirty skin" confuses a blocked gland with surface dirt.

Worked layer identification from a description: A slide shows, from the top, many rows of flat dead cells with no nuclei; below them a layer of cells full of dark granules; below that several layers of cells with spiny junctions; and at the base a single row of cuboidal dividing cells resting on a wavy boundary, with loose connective tissue and capillary loops beneath. Reasoning: dead flat cells are the stratum corneum; granules mark the stratum granulosum; spiny cells the stratum spinosum; the single dividing row is the stratum basale; the wavy boundary with capillaries is the papillary dermis. No stratum lucidum is visible, so this is thin skin, not palm or sole.

Worked function-to-structure trace, cooling on a hot day: Core temperature rises; the hypothalamus signals; eccrine sweat glands in the dermis secrete watery sweat through ducts to the surface; the sweat evaporates, absorbing heat from the skin; at the same time smooth muscle in dermal vessel walls relaxes, the vessels widen and warm blood flows near the surface, losing heat by radiation when the surroundings are cooler than the skin. Two structures (eccrine glands and dermal vessels) carry out two processes (perspiration and increased blood flow) for one function (temperature regulation). In anhidrosis the first process is missing, so cooling depends on the second alone and is inadequate in heat.

Worked burn classification: A hypothetical burn is red, painful and dry, with no blisters. Only the epidermis is damaged, so it is first-degree, and because the stratum basale survives, it will heal by normal cell production without a scar. A second burn is blistered and very painful: the damage reaches the dermis, so it is second-degree; fluid collects between the layers; healing occurs from surviving epidermal cells in hair follicles and gland ducts. A third burn is white or charred and painless in its center because the nerve endings are destroyed: it is third-degree, full thickness, and cannot regenerate epidermis from within, so it needs a graft. Classification uses depth, not surface area; the two are reported separately.

Worked two-point discrimination data: Hypothetical results for one partner: fingertip 3 millimeters, palm 10 millimeters, forearm 35 millimeters, back 45 millimeters. Interpretation: the fingertip can resolve two points fifteen times closer than the back, which supports a much higher density of touch receptors in the fingertip skin. The data cannot show whether the difference comes from receptor number, receptor type or the amount of brain tissue devoted to each region, all of which contribute.

Representation task: Create a labeled illustration of a block of skin showing the epidermis with its layers named (stratum basale, spinosum, granulosum, corneum), a melanocyte, a hair follicle with its arrector pili muscle and sebaceous gland, an eccrine sweat gland with its duct, the papillary and reticular dermis, the hypodermis, a Meissner's corpuscle near the surface and a Pacinian corpuscle deep. Beside each labeled structure, write the function it carries out.

Independent performance: Carry out the two-point discrimination investigation on four skin regions for at least two partners and write it up with a homeostatic-imbalance analysis. Success criteria: (a) thresholds are recorded in millimeters for each region and partner, with the method and pressure kept the same; (b) the results are ranked and the ranking is explained by receptor density, naming the receptor types involved and their depth; (c) two limits of the test are stated; (d) the write-up explains, for two imbalances from burns, skin cancer, anhidrosis, acne, eczema and scleroderma, which structure or process has failed and which skin function is lost as a result; (e) one common myth about ultraviolet exposure or acne is stated and corrected with the relevant structure.