Despite all the proverbs to the contrary, human beings are quite naturally drawn to and concerned about our superficial selves. Hair has been described as a crowning glory and skin the place where beauty resides. And beyond the aesthetics of surfaces, we also realize that our superficial covering provides us with indispensable protection and merits care and concern.

Each square inch of our skin is equipped with about 700 sweat glands, 100 oil-producing sebaceous glands, and more than 20,000 nerve endings that sense heat, pressure, and pain. Our skin is both amazingly tough and incredibly sensitive. The outermost layer of our skin is composed largely of dead cells that are constantly worn away and replaced by new cells. We produce and shed so much skin detritus that entire ecosystems of microscopic organisms subsist on our skin flakes. Skin also plays a key role in thermoregulation. A person sitting still (and not in a sauna) might sweat only a fraction of an ounce in an hour, while that same person playing full-tilt tennis could easily produce a quart or more of sweat in equal amount of time.

An infant's skin is different than an adult's, but mostly by degree. Maturation of the skin occurs largely within the first weeks after birth, though subtle changes will continue to take place. The differences can be seen most strikingly in babies born preterm. By the third trimester, the structure of skin in a developing fetus is similar to that of an adult, although its capabilities as a barrier are still immature. A preterm infant has skin that is thinner and more transparent and is also characterized by fewer hair follicles and sebaceous glands. Many of the differences involve only the outer layer, however.

Even though an infant has more sweat glands per square inch than an adult, an infant requires greater stimuli to trigger sweating; preterm infants less than 28 weeks gestation basically don't sweat at all. The skin of a preterm baby has fewer dermal papillae, which are projections of the inner layer, the dermis, that reach up into the outer layer, the epidermis, and thereby anchor the two layers together. As a consequence of having fewer dermal papillae, there is less cohesion between the two layers. Preterm skin also contains more water and its underlying network of collagen and elastin fibers is more loosely organized. As a result, skin injury can occur with even normal handling and friction.

A key component of skin, hair, and nails is the sulfur-containing protein keratin, characterized by extremely strong chemical bonds. Another important structural protein in skin is collagen, which is also a major component in bone, cartilage, and connective tissue. There are actually several types of collagen present in skin, and the ratio of one type to another shifts during the perinatal period. Proteins also play a significant role in the healing of wounds.

A newborn's skin is covered with a yellowish waxy coating called the vernix caseosa, a lubricating and insulating layer made of sloughed-off skin cells, oil, and lanugo hair. Vernix is usually lost in the process of the first bathing. The removal of the protective vernix results in the skin drying out in the air of the postnatal environment. This is a natural process, but it leaves the skin vulnerable until it develops an acid pH and is colonized with harmless bacteria that help restore these protective barrier functions.

Because a baby's skin is thinner and blood vessels lie closer to the surface, the infant loses heat more easily. The thinness and higher water content of the infant's skin (water content of newborn skin is nearly 75%, while an adult's is about 65%) allows substances, including environmental irritants, to be more easily absorbed.

In addition to proteins, lipids are also major components of skin. Sebaceous glands secrete a lipid-containing fluid known as sebum, which serves to protect and lubricate skin and hair. The amount and type of lipids secreted by the skin change over the course of childhood, peaking in the teenage years in response to sex hormones.