Human skin is made of three distinct layers: the epidermis on the outside, the dermis in the middle, and the hypodermis (subcutaneous tissue) at the bottom. Each layer has its own mix of cells, proteins, and structures that work together to protect your body, regulate temperature, and detect touch. Skin ranges from 0.5 mm thick on your eyelids to 4.0 mm on the heels of your feet, and by weight, the dermis alone is roughly 70% water.
The Epidermis: Your Outer Shield
The epidermis is the layer you can see and touch. It’s built almost entirely from cells called keratinocytes, which produce a tough protein called keratin. This is the same protein that makes up your hair and nails. New keratinocytes form at the bottom of the epidermis and slowly migrate upward, changing shape and hardening as they go. In young adults, this journey takes about 28 days: 14 days traveling from the deepest layer to the surface, then another 14 days sitting at the top as a protective barrier before shedding.
The epidermis has five sublayers, each representing a different stage in a keratinocyte’s life cycle. At the very bottom is the basal layer, where fresh skin cells are born and where melanocytes produce melanin, the pigment that gives skin its color. Above that, cells are held together by sticky proteins that keep the layer strong. As cells rise higher, they develop internal granules, flatten out, and eventually die. The outermost sublayer is made entirely of dead, hardened keratinocytes called corneocytes. These form a tough, water-resistant sheet that shields you from bacteria, UV light, heat, and physical abrasion.
The epidermis contains no blood vessels. It gets nutrients by diffusion from the dermis below.
The Dermis: Structure and Sensation
Beneath the epidermis sits the dermis, which provides the bulk of your skin’s strength and flexibility. It’s made primarily of collagen and elastin, two structural proteins. Collagen accounts for 75 to 80% of the dermis’s dry weight and gives skin its firmness. Elastin fibers allow skin to stretch and snap back. Specialized cells called fibroblasts produce both of these proteins continuously, rebuilding the structural framework as older fibers break down.
The dermis has two sublayers. The upper papillary layer is thinner and contains fine collagen fibers, blood capillaries, nerve fibers, and fat cells. The lower reticular layer is denser, with a thick net-like weave of collagen and elastin that supports the skin’s overall architecture.
This middle layer is also where your skin’s sensory equipment lives. Several types of specialized receptors sit embedded in the dermis, each tuned to a different kind of touch. Meissner corpuscles detect light touch and texture. Merkel disks sense sustained pressure. Pacinian corpuscles pick up deep vibrations. Ruffini endings respond to stretching. Free nerve endings detect pain and temperature. Together, these receptors create the rich, layered sense of touch you experience every moment.
Glands and Hair Follicles
The dermis also houses your skin’s glands and hair follicles. Sebaceous glands, which develop on or near hair follicles, produce an oily substance called sebum that moisturizes and waterproofs your skin. You have thousands of them across your body, with the highest concentration on your face and scalp. The only places without sebaceous glands are the palms of your hands and the soles of your feet. Sweat glands also sit in the dermis and open at the skin’s surface through tiny ducts, releasing sweat to cool you down.
The Hypodermis: Insulation and Energy
The deepest layer is the hypodermis, sometimes called subcutaneous tissue. It’s made primarily of connective tissue and adipose tissue, which is fat. Adipose tissue consists of specialized fat cells called adipocytes that store energy your body can draw on later. This fatty layer also insulates you against heat loss and cushions your muscles, bones, and internal organs from impact. Collagen and elastin run through the hypodermis as well, connecting it to the structures above and below and giving it a supportive scaffolding.
The thickness of this layer varies widely depending on the body site and the individual. It’s thicker on the abdomen and thighs, for example, and much thinner on the forehead and the backs of your hands.
What Skin Is Made of at the Chemical Level
Zoom in past the cells and layers, and skin is overwhelmingly water. The dermis is approximately 70% water by mass, split roughly between free-flowing interstitial fluid (about 40%) and water bound to proteins (about 30%). The remaining dry weight is dominated by collagen, with smaller contributions from elastin, other proteins, and lipids that form the waterproof barrier in the epidermis.
Type I collagen is the most abundant form, making up 75 to 90% of all the collagen in the dermis. Type III collagen accounts for another 10 to 15%. This ratio shifts with age: younger skin has more type III collagen, which is softer and more pliable, while the proportion of stiffer type I collagen increases over time.
How Skin Renews Itself
Your skin is constantly rebuilding. The epidermis replaces itself roughly every four weeks in young adults, with new cells pushing old ones upward until they shed. This turnover rate slows as you age, which is one reason older skin can look duller and take longer to heal from cuts and scrapes. The dermis renews more slowly, relying on fibroblasts to steadily produce fresh collagen and elastin. After about age 30, collagen production declines by roughly 1% per year, which gradually thins the dermis and contributes to wrinkles and reduced elasticity.
The hypodermis also changes with age. Fat redistribution and loss of adipose tissue in certain areas, particularly the face and hands, account for much of the visible aging people notice in their 40s and beyond.