The anatomy of skin

10 October 2010

The skin is the largest organ in the human body. Its vital role is the protection of internal organs and structures from the environment. This article explains that baby skin is not the same as adult skin, but a more delicate and vulnerable structure, and hence requires different care from mature skin.

Anatomy of skin

Skin contains three main layers: the epidermis, the dermis, and the subcutaneous layer. The outermost layer of the epidermis is the stratum corneum (SC) —commonly called the 'skin barrier, which consists of flattened dead skin cells that shed every two weeks or so (see Figure 2). Of the three main skin layers, it is the SC that provides protection and serves as the all-important first line of defence for the skin.

Key points

  • The SC acts as a barrier
  • Baby skin is not mature at birth and continues to develop over the first year of life
  • Baby skin SC is thinner and more permeable than in adults
  • Baby skin loses water more quickly than adult
  • Baby skin needs special - and difefrent - care compared to adult skin
  • Baby skin hydration is higher and more variable during the first year of life than in adults but decreases over time
  • Preserving the health and integrity of the infant skin barrier is crucial; it is known that an SC breakdown can lead to atopic dermatitis, which can trigger other allergies

How is baby skin different from that of adult skin?

The skin of infants is physiologically different from that of older children and adults in terms of structure, composition and function. At birth, baby skin undergoes a dramatic transition from an aqueous to a dry terrestrial environment. After birth and over time, neonatal skin continues to develop.

Baby skin is structurally different to adult skin; the cells are smaller and the collagen fibres are thinner." According to recent clinical findings, infant SC is 30%, and infant epidermis 20% thinner than in adults.12 Consequently, baby skin is more permeable and more prone to dryness than adult skin. Furthermore, a baby's body surface to volume/weight ratio is higher than that of adults," which increases baby skin's vulnerability to applied substances and to sun exposure.

It is not just the dimensions of baby skin that differentiates it from adults. The composition of baby skin is also different from that of adult skin, containing less natural moisturising factors (NMFs), less lipids and less melanin. The lower amounts of NMFs in baby skin could contribute to its faster rate of water desorption.

Baby skin also has a higher pH than adult skin, another indication that the baby skin barrier is less mature than that of adult skin. In a clinical study involving a large number of newborns, the skin was found to have an average pH of 6.34 immediately after birth. Within four days, the pH decreased to a mean of 4.95, and between 7 and 30 days it further decreased to 4.7.15 The acid nature of skin is important because it has antimicrobial properties that make the skin resist colonisation by harmful bacteria; it also buffers the activity of proteases, those enzymes that break-down the corneodesmosomes (holders of the skin cells). Therefore, higher pH contributes to the weakening of the skin barrier.

These differences in structure and composition lead to differences in the function of the skin as a barrier. One of the main measurements of skin barrier is the rate of trans-epidermal water loss (TEWL). Baby skin has a higher rate of TEWL than adult skin.14 Additionally, water handling properties such as absorption and desorption are different. So, although infant skin is able to absorb more water than adult skin, it loses water at a faster rate.14 We hypothesise that these differences leave baby skin more prone to dryness, and as a baby's immune system is developing, there is a greater chance of developing irritations and infections.

For these reasons, baby skin is more vulnerable to the environment than adult skin, and if not properly cared for, the skin becomes susceptible to dryness, conditions such as atopic dermatitis, nappy dermatitis, or even infections.

Cross section of human skin

When does baby skin become mature?

The notion that skin development occurs only during pregnancy and that the organ is fully mature and capable of fulfilling all of its functions has been challenged and revised.16 Today, scientists understand that baby skin continues to develop and change through the first years of life. When it comes to skin maturation, scientists have taken a particularly close look at how baby skin functions compared to adult skin, especially the SC water barrier function. Low values of TEWL are indicative of good barrier function while high values are associated with compromised or poor barrier, such as in diseases with skin barrier abnormalities (for example, atopic dermatitis or psoriasis).16 Clinical studies show that TEWL rates are highest in younger infant age groups (3-6 months old) and continue to show higher levels than mature skin through the first year of life.'

Why is the sc important?

The skin is the first line of defence for babies. It acts as a barrier against attack from outside pathogens, irritants and allergens and environmental attack, thereby protecting the body against infection and diseases. For babies that are still developing their immune system, a healthy skin barrier serves a vital purpose in preserving their wellbeing.

The structure of the SC can be compared to a 'brick wall. The skin cells are the 'bricks, the lipid layers are the `mortar,' and the corneodesmosomes within the SC are the 'iron rods' for support (see Figure 3). It is this SC 'wall' that acts as a barrier that protects the body from the outside environment.

Until about 40 years ago, scientists thought that the SC was more or less biologically inactive — acting like an inert plastic sheet that protects the lower layers of the skin. However, recent research has discovered that the skin barrier undergoes both chemical and biological activity that either weakens the protective elements in the 'brick wall,' or strengthens it.

How does a break in the skin barrier affect infants?

Atopic dermatitis means inflammation of the skin from an unknown cause. Scientists found that genetic changes increase protease activity, causing a premature breakdown of the skin barrier. Environmental factors such as dust mites or soap also play a role in weakening the skin barrier. When the barrier is broken, contaminants can enter and cause inflammations, allergies or other diseases. A broken barrier also allows water to leave, making the skin feel dry to the touch. Infants are born with a thinner SC than those in adults, which makes infants more vulnerable to attack from outside environmental influences (eg, bacteria, allergens, dust mites).

Brick wall model of the skin barrier
Figure 3: Brick wall model of the skin barrier. Skin cells are analogous to the bricks and lipid layers are analogous to the cement. In tall brick walls, iron rods are passed down through holes in the bricks to give the wall greater strength. these iron rods are analogous to the corneodesmosomes that lock the skin cells together. (Adapted from Cork MJ et al[17])