Study on the characteristics of baby skin
1. GENERAL INTRODUCTION
The functions of the skin remain essentially the same at all stages of life, including: barrier, photoprotection, thermoregulation, immune surveillance, hormone synthesis, prevention of water loss, and sensory perception. However, there are several important structural differences between the skin of babies and adults, the differences are due to the drastic transition from the intrauterine aquatic environment to the external gaseous environment as well as immaturity.
The skin is a complex structure composed of different layers namely the epidermis, the dermis, the epidermal junction and the innermost layer of the hypodermis. The epidermis is non-vascularized while the remaining three layers are vascularized. The appendages of the skin are located in the dermis and the hypodermis. The thickness of the epidermis varies according to the location on the body: the palms, the feet have thick skin while the eyelids have the thinnest skin.
Babies' skin differs from adults' skin in several important ways, primarily due to its development and maturation.
2. SKIN - GENERALITIES
The skin is the largest and one of the most complex organs in the human body. It covers the entire exterior of the body, protecting it from external aggressions, infections, and water loss. The skin also plays a crucial role in regulating body temperature, sensation, vitamin D production, and other vital functions. It is composed of four main layers, each with specific functions.
3. INFANT SKIN
During the fetal phase, the skin becomes functional. It develops a protective barrier residing in the stratum corneum. The maturation of the stratum corneum is very important for the health of premature infants. Skin development involves the arrangement of tissues of mesodermal and adjacent ectodermal origin.
Fetal skin refers to the skin of the fetus during pregnancy. Its development is a complex process that evolves over the weeks of gestation, passing through several stages to reach its mature form at birth. Fetal skin has unique characteristics that differentiate it from adult skin, particularly in terms of structure, function, and composition.
3.1. Development and characteristics
Skin development begins in the first weeks of gestation. Initially, fetal skin is very thin and transparent, allowing the underlying blood vessels to be seen.
As in adults, fetal skin consists of four main layers: the epidermis, the epidermis-dermis junction, the dermis, and the hypodermis. However, these layers undergo significant changes throughout fetal development.
Fetal skin arises from the ectoderm during embryogenesis. The ectoderm is the outermost of the three primary germ layers. By 21 days of gestation, the ectoderm differentiates into the neuroectoderm and epidermis. By week 4, the basal cell layer has formed and is covered by periderm, which protects the developing epidermis from amniotic fluid and manages glucose absorption.
Melanocytes appear in the basal layer from 5 to 8 weeks of gestation. Langerhans cells appear and express antigens at 6 weeks of gestation. From 8 to 14 weeks, keratinocytes proliferate to form the spinous layer between the basal layer and the periderm. From 14 to 17 weeks, the upper spinous cells are flattened.
The epidermis is completely keratinized at 26 weeks with a basal layer, 2 to 3 spinous layers, a granular layer, and 5 to 6 layers of stratum corneum. Between weeks 5 and 26 of gestation, eight stages of epidermal differentiation occur. The stratum corneum and vernix appear at the same time, during the last trimester of gestation. From the second trimester, the fetal skin begins to be covered with vernix caseosa, a whitish, waxy substance consisting of dead epidermal cells and lipids secreted by the sebaceous glands. The vernix protects the fetal skin from the amniotic fluid, preventing it from drying out and facilitating passage through the birth canal.
Around the fifth month of gestation, the fetus develops lanugo, a fine, soft hair that temporarily covers the body. Lanugo helps hold the vernix caseosa on the skin. It usually falls off before or shortly after birth.
3.2. Functions
During the fetal phase, the skin essentially performs 2 functions:
- Protection: Fetal skin, with vernix caseosa, provides protection against infections and abrasions. It also plays a role in regulating exchanges between the fetus and the amniotic fluid.
- Sensory development: The skin is one of the first sensory systems to develop, allowing the fetus to begin to feel tactile sensations as early as the second trimester.
3.3. Special features
Fetal skin has some particularities:
- Permeability: Fetal skin is more permeable than adult skin, meaning it can more easily absorb and lose water and other substances.
- Maturation at birth: At birth, the skin continues to mature and adapt to the external environment. Vernix caseosa and lanugo usually disappear shortly after birth, although some newborns may still have traces of them.
Fetal skin is therefore a dynamic and essential organ for intrauterine development, providing protection, regulation and first tactile sensations to the fetus. Its composition and function evolve considerably throughout pregnancy, preparing for the external environment at birth.
4. STRUCTURAL DIFFERENCES INFANT SKIN VS ADULT SKIN
A baby's skin differs from that of an adult in several ways, mainly due to its structure, function and chemical composition. These differences make babies' skin more sensitive and require specific care.
4.1. Structural differences: Skin surface
The skin surface of infant skin appears rougher than that of adult skin. Video microscopy and CLSM (confocal laser scanning microscopy) images demonstrated distinct differences between the surface of adult and infant skin. In infants, the network of lines (microrelief grooves) per area is observed denser than in adults, and the structures are more rounded. In contrast, these structures in adults appear to be flatter and with a larger surface area.
This contrast may be a result of the drier state of corneocytes at these sites. In contrast, infant skin appears well hydrated consistent with previously reported skin hydration measurements.
4.2. Structural differences: at the level of the Epidermis
It has been reported that the infant Stratum Corneum (SC) is approximately 30% thinner than adult skin. Additionally, differences have been found in the granular layer, the layer just below the SC. A smaller cell size in the upper layers of the skin is observed indicating a higher cell turnover rate in the infant epidermis; which also explains the improved wound healing properties in infants compared to adults.
Structural differences were also observed in the dermal layer. In vivo confocal laser scanning microscopy revealed that the infant dermis is composed of short collagen fibers. Microscopy further confirmed the absence of the reticular layer at the observed depths unlike adult skin. Furthermore, infant skin is observed to be softer and more elastic than adult skin. Further in vivo CLSM images revealed that the size distribution of dermal papillae is homogeneous in infants while they vary in size and are more irregular in shape in adult skin.
4.3. Structural differences: Corneocytes & Keratinocytes
Infant corneocyte size is smaller than adult corneocytes. Differences between adult and infant keratinocyte size were measurable at the level of the granular layer (SG). Cell densities in the granular layer SG (but not corneal layer - SC) can also be measured from confocal data. The smaller cell size in infants is presumed to be due to higher cell turnover rates. There is a progressive decrease in the rate of epidermal cell proliferation during the first year of life.
During the first 3 months of life, skin roughness decreases in correlation with an increase in SC hydration. Furthermore, scanning electron microscopy revealed 10 times more hair structures per unit skin surface in newborns than in adults.
The SC faces the external environment and is responsible for preventing water loss and the penetration of possible incriminated external agents. Maintaining a healthy SC ensures an adequate air-liquid barrier. The SG is a thin layer of differentiated cells from the underlying stratum spinosum. Keratohyalin granules are filled with histidine- and cysteine-rich proteins that bind the keratin filaments together. Profilagrin is the major histidine-rich protein involved in atopic dermatitis and ichthyosis vulgaris. Lipid- and protein-filled lamellar bodies are present in the extracellular space between the SC and SG and form a bilayer structure alternating with water between the cells. This hydrophobic lipid “mortar” in association with cornified keratinocytes (“bricks”) is responsible for a large part of the mechanical and physiological barrier properties of the SC.
4.4. Structural differences: Dermis-Epidermal Junction
In full-term infants, the dermal-epidermal junction becomes progressively more wavy from birth to 16 weeks of age. Recall that the dermal-epidermal junction is a crucial anatomical structure located between the epidermis, the upper layer of the skin, and the dermis. This junction plays a critical role in the adhesion of the two layers of the skin, facilitating the exchange of nutrients and oxygen between them, while preventing the passage of harmful substances.
In babies, the dermoepidermal junction is particularly important because their skin is thinner and more vulnerable than that of adults. Babies have a less developed skin barrier, making them more susceptible to infections and irritations. The dermoepidermal junction helps protect the skin by strengthening this barrier and supporting skin regeneration and repair.
Skin care in babies must therefore take into account the sensitivity of the dermo-epidermal junction, avoiding harsh chemicals, keeping the skin well hydrated and protecting the skin from extreme environmental conditions, to preserve the skin's barrier function and promote healthy skin growth.
5. DIFFERENCES IN COMPOSITION BETWEEN INFANT SKIN VS ADULT SKIN
The regenerative capacity of the skin, as well as its function as a protective barrier against harmful substances and against water loss is determined by its components. Typical for a biological system, the function of these components is interdependent.
5.1. Composition differences: Water content
Infants are born with relatively dry skin, indicating a low skin hydration level at birth compared to adults. Skin hydration increases during the first 2–4 weeks after birth and gradually stabilizes during life. It has been suggested that the increased skin hydration level may be a result of the increasing functional maturation of the eccrine gland. Consequently, this increase leads to significantly higher skin hydration in older infants (3–12 months and 8–24 months, respectively) compared to adults. As skin hydration increases, skin roughness parameters decrease.
The skin barrier function is located in the SC, so it is important to study possible compositional differences between adult and infant SC. In different studies, it was found that the water content of baby SC was overall higher than that of adult SC, but in the case of TEWL (TransEpidermal Water Loss or TEWL – Perte Insensible en Eau), it was much higher in baby skin.
Research has shown that infant skin contains less total lipids and less sebaceous lipids (carbonyl band) than adult skin, confirming the reduced activity of infant sebaceous glands compared to adult glands.
The results also suggest that skin elasticity increases in newborns up to 2 years of age and then decreases. The increase in elasticity may reflect the formation of collagen bundles.
Capacitance quantifies the water content of the SC, which influences the percutaneous absorption barrier function, the reaction to irritants and also the mechanical properties of the skin. Humidity and ambient temperature influence capacitance. Capacitance values may vary between sites for adults, it was observed to be very high on the forehead, soles, palms and very low on the legs.
The capacity varies in many skin pathologies, as in AD (Atopic Dermatitis) it becomes very low, and it has been observed very high in diaper dermatitis. Excessive water content in diaper rash compromises the barrier function by increasing the coefficient of friction and irritant permeability.
5.2. Composition differences: Natural Moisturizing Factor (NMF)
To retain water and keep the skin hydrated, corneocytes contain hygroscopic molecules that act as humectants. Some protein degradation products, such as small amino acids, urea, pyrrolidone carboxylic acid, ornithine, citrulline, urocanic acid play an important role as skin humectants by binding and retaining water and maintaining the SC at normal hydration levels. These are known in the scientific literature as natural moisturizing factor (NMF). It has been reported that the concentration of NMF is lower in infant skin than in adult skin. NMF also contains organic acids, sugars, and ions. In addition to its role in water retention and hydration, NMF also has a self-regulatory function by controlling the activity of proteases involved in NMF synthesis. Low humidity increases the activity of filaggrin-degrading enzymes, resulting in more NMF, which in turn increases the moisture content of the SC.
Similar to water, NMF concentration can be measured by confocal Raman microscopy as a function of skin depth. However, since infant skin is more hydrated than adult skin despite the low NMF concentration, there must be other mechanisms to regulate water homeostasis in infants. Potential candidates are the thinner SC, the relatively high desquamation rate, or the skin surface structure unique to infants (water may be trapped in the dense microrelief lines of infants.)
5.3. Composition differences: Lipid content
Several recent studies confirm that the skin surface of infants contains significantly less total lipids and less sebaceous lipids compared to adults. This is in correlation with the low sebum levels measured in infants. Intracellular lipids of the skin are regulators of SC hydration and barrier function. Reduced lipid levels at the infant skin surface may thus suggest an immature barrier function of infant skin. The major lipids in the lamellar membranes are palmitic acid, stearic acid, behenic acid, lignoceric acid and hexacosanoic acid. Other lipids present in the SC include oleic acid, eicosapentaenoic acid, arachidonic acid, docosahexaenoic acid and linoleic acid, as its derivatives linolenic acids.
5.4. Composition differences: Melanin content
Melanin is synthesized in melanosomes, in dendritic cells located in the epidermis, melanocytes. Melanosomes are then transferred to keratinocytes, the melanin pigment is considered to play a photo-protective role.
According to different studies, it has been found that infants have a significantly lower concentration of melanin than adults in sun-exposed body sites. This is of great importance because melanin functions as a UV filter by reducing the penetration of UV light through the epidermis. Lower melanin concentration, thinner SC in infants, as well as increased hydration of the SC, can lead to increased sensitivity to the harmful effects of UV light. Therefore, increased sun exposure and sunburns acquired in childhood correlate with an increased risk of skin malignancies. In addition, the effects of sun exposure are observed in infants as early as the first year of exposure in the form of an increase in melanin concentration in exposed skin. The increased UV-induced melanin production can damage DNA. Thus, early sun exposure may contribute to the cumulative effect and skin damage caused by UV light. The population density of melanocytes appears to be uniform in the newborn.
6. FUNCTIONAL DIFFERENCES INFANT SKIN VS ADULT SKIN
The skin acts as a physical barrier such as for thermal protection; a chemical barrier to keep pollutants out; and a biological barrier to protect against microbes and allergens. As a physical barrier, the skin also prevents dehydration of internal tissues by trapping moisture.
6.1. FUNCTIONAL differences: Skin Barrier
Trans epidermal water loss (TEWL) is an indicator of skin barrier function. TEWL measures the amount of water that evaporates from the skin surface to the atmosphere. It is a key parameter for assessing skin barrier integrity and skin health.
Different studies have shown that baby skin has been shown not only to have higher TEWL levels than adult skin, but also strong variations in the data are observed, which confirms that in infants the skin barrier is not fully developed.
It was found that infant skin absorbs more water than adult skin and loses more of this water due to faster kinetics. Interestingly, adult skin appeared to involve only one desorption mechanism, while infant skin apparently involved a second, faster one.
The rate of trans epidermal water loss (TEWL) from the epidermis is very low, 4-8 g/m2/h in adults while it is 6 to 12 g/m2/h in infants depending on the measurement sites.
Note that in adults, a high TEWL may indicate an impaired skin barrier, often associated with conditions such as dermatitis, psoriasis, or eczema. Healthy skin typically has a lower TEWL, meaning the skin barrier is working effectively to retain moisture and protect against pathogens, allergens, and environmental irritants.
6.2. FUNCTIONAL differences: Acid Mantle
The development of an acidic stratum corneum (SC), necessary for normal processing of SC lipids to form a competent epidermal barrier, is an essential feature.
Babies develop an acid SC (acid mantle) within 4 weeks of birth. The acid mantle facilitates the enzymatic processing of lipids to form competent lipid bilayers that provide the epidermal permeability barrier, it also helps regulate enzymes that control sloughing and cohesion, and finally in antimicrobial defense.
In 1923, Sharlit and Scheer measured the surface pH of normal skin using a colorimetric method and found that the average surface pH of adult skin was 5.5. In 1928, Schade and Marchionini introduced a new method, the Glockenelektrode, and found that the skin surface pH ranged from 3.0 to 5.0 and attributed this variation to sweat. Irvin Blank's study of skin pH confirmed that the skin surface pH ranged from 4.2 to 6.0 in children. Overall, the pH ranges from about 7.0 in the upper epidermis to 4.5 to 5.0 in the outer stratum corneum. The skin surface pH of term and preterm infants is less acidic than that of children and adults, but becomes rapidly acidic during the first 4 weeks. It is noted that premature boys have more alkaline pH values and a more impaired barrier function compared to girls of the same gestational age.
Clinically, desquamation increases when exogenous acids are applied, this property has been used by many therapeutic and cosmetic preparations. Pathological desquamation can be observed in skin conditions associated with an abnormally neutral or alkaline surface pH, such as irritant contact or atopic dermatitis or for certain ichthyoses.
An acidic SC pH is also important in defense against microbial invasion. Different studies have demonstrated that a less acidic skin pH leads to an increase in the number of organisms on the skin, particularly gram-negative bacteria and Propionibacterium, suggesting that pH may also mediate the immune response to infection.
6.3. FUNCTIONAL differences: Microbiology
The skin is a protective barrier and above all it serves as a barrier against infection. The balance between skin colonization and infection involves an interaction between the mechanical, biochemical, physiological and immunological properties of the skin and the characteristics of the microorganism.
In the neonatal period, this balance is important because it increases the susceptibility of low birth weight infants to pathogenic infection that can cause premature infant death. Skin colonization begins at the time of exposure to the extrauterine environment. The skin of babies born by cesarean section is usually sterile, but infants born vaginally are colonized during descent into the birth canal.
Staphylococcus epidermidis is the most common vaginal organism just before birth and immediately establishes itself on the skin of most newborns. Other organisms, such as Malassezia spp. and Proprioni bacteria, are also seen on the skin surface as the skin barrier matures. Under hygienic conditions, the resident flora resembles that of adults after the first 6 weeks of life.
Caregiver hygiene practices, bathing, maternal vaginal cleansing are factors involved in the alteration of the normal bacterial flora of the infant's skin and therefore of the barrier function of the infant's skin.
6.4. FUNCTIONAL Differences: Cell Proliferation
The renewal of epidermal cells in healthy skin is linked to the rate of desquamation because an imbalance would lead to a thinning or thickening of the SC. It was thus found that during the first 3 months of life, the rate of desquamation varied according to the anatomical location. It is high in the facial area while this rate of desquamation is very low in the buttocks area. This could be explained by the high hydration in this area due to the occlusive environment (diapers).
Epidermal cell proliferation decreases significantly during the first year of life and reaches adult levels during the second year of life. The higher cell proliferation rate in young infants may explain the smaller cell size and higher cell density observed in this age group.
6.5. FUNCTIONAL differences: Sebaceous Glands
In humans, the sebaceous glands secrete sebum, which is a mixture of nonpolar lipids. A sharp increase in sebum excretion occurs within a few hours after birth, with a peak during the first week.
A number of studies have confirmed changes in the lipid composition of sebum associated with age or activity of the sebaceous glands. Vernix caseosa covers the fetus during the last trimester of pregnancy and has a waterproofing function for the fetus in utero as a hydrophobic barrier. The superficial sebum film has a positive influence on skin hydration. The lipids on the skin surface retain their protective mechanisms, where sebum lipids play a major role.
Ongoing in vitro studies have shown that total sebum lipids can cause a reduction in the growth of Gram-positive (Staphylococcus aureus, Streptococcus salivarius) and anaerobic (Fusobacterium nucleatum) bacteria, while they are ineffective against most Gram-negative bacteria.
Vitamin E (tocopherol) is an important constituent of human sebum which is a natural antioxidant and there is a close correlation between squalene and tocopherol levels in sebum. While UVA and UVB exposure of skin surface lipids induces oxidation products, vitamin E may exhibit its antioxidant properties for the protection of the skin against lipid peroxidation. Therefore, children aged 6 months to 7-8 years with low levels of skin surface sebaceous lipids may need external application of vitamin E to increase their local antioxidant capacity. On the other hand, increased sebum excretion leads to the development of baby acne.
6.6. FUNCTIONAL differences: Ceramides
Various studies have concluded that the ceramide content in infants is lower than that of adults of the same ethnic origin. The difference in ceramide values involves the immature epidermal barrier.
When comparing the values between adults of different ethnicities, it was observed that very little amounts of ceramide are found in the African ethnicity (0.74 ± 0.25), the values for Caucasian skin were noted as 1.18 ± 0.46 and 1.14 ± 0.51 for Asian skin. From this, we can conclude that the ceramide content for the infant of the African ethnicity may be even lower than that of the adult of the same ethnicity.
6.7. FUNCTIONAL Differences: Vitamin D Levels
Several studies have demonstrated a high prevalence of vitamin D deficiency in infants, particularly infants of phototype IV to VI (Mediterranean, Arab, black or mixed race).
According to the study conducted in the Netherlands, it was found that the prevalence of vitamin D deficiency is higher in dark-skinned babies than in Caucasian babies if the mother is vitamin D deficient. In another Dutch study, severe vitamin D deficiency (25-hydroxyvitamin D3 <13 nmol/l) was found in 54% of newborns of non-European origin compared to 6% of Dutch/Western European newborns.
Vitamin D is hydroxylated to 25‐hydroxyvitamin D3 in the liver, regulated by parathyroid hormone, with further hydroxylation to 1,25‐dihydroxyvitamin D3 occurring in the kidney. 1,25‐dihydroxyvitamin D3 is the active metabolite of vitamin D that increases intra‐ and extracellular calcium concentrations by several mechanisms: it absorbs intestinal calcium, decreases renal calcium excretion, and, in the presence of parathyroid hormone, mobilizes calcium from bone.
The alkaline phosphatase levels found in newborns may reflect an effect on bone mass, suggesting that additional vitamin D is needed in babies with phototype IV to VI for growth and bone health.
7. CONCLUSION
Babies' skin differs from adults' skin in several important ways, primarily due to its development and maturation.
In this present study, we have presented the skin in its generalities and we have highlighted the fundamental differences between the skin of an infant and that of an adult.
Whether structural, compositional or functional, these differences require a healthy, effective and transparent formulation of baby products.
Here are some of the key differences between baby skin and adult skin:
- Thickness: Babies' skin is considerably thinner than adults'. This makes it more permeable and susceptible to external irritants, chemicals and bacteria.
- Hydration: Babies' skin tends to become dehydrated more quickly than adults' because it has fewer natural moisture-holding fibres and active oil glands. This means that babies' skin can dry out more easily, requiring regular moisturising care.
- Protection: The skin's barrier function, which helps protect the body from infections and harmful substances, is less developed in babies. Their skin is therefore more sensitive and susceptible to irritation.
- Pigmentation: Babies have less developed skin pigmentation compared to adults. This makes them more susceptible to sun damage, hence the importance of proper sun protection.
- Thermal regulation: The skin's ability to help regulate body temperature is less effective in babies. They can easily get too hot or too cold because their thermal regulation system and sweat production are not yet fully developed.
- Reactivity: Babies' skin is more reactive to irritants, which can lead to rashes, eczema and other skin problems more frequently than in adults.
Because of these differences, baby skin care requires gentle products specially formulated for their delicate skin, as well as special attention to avoiding irritants, maintaining proper hydration, and protecting their skin from environmental elements.
