Multiple lipid mediators are known to be important in skin health either as structural components (e.g., ceramides) or as having a key role in skin inflammation and immune regulation4. Accordingly, determining the skin lipid mediator composition at birth may provide insights into the integrity and functionality of this barrier in the newborn. However, given the ethical issues associated with the invasiveness of sample collection in newborns (i.e., skin biopsies)8, skin functionality is usually assessed by the measurement of other surrogates such as the transepidermal water loss9.

In the present study, we inspected the feasibility of VC to provide information related to skin maturity by quantifying a series of lipid mediators belonging to the sphingolipid, endocannabinoid and oxylipin families. Lipid mediator levels were observed to associate with the gestational period, gender of the newborn and the lifestyle of the mother. Previous works reporting the presence of ceramides1 and other lipids6 in VC suggested the possibility that VC may reflect fetal epidermal synthesis. In addition, ceramides along with other epidermal lipids, have been previously described in VC and their pattern mirrors that of mid-gestational and early postnatal epidermis1. Due to their role in skin barrier function, ceramides are associated with a number of skin diseases4 and are being promoted for therapeutic use in skin care products10. VC contains the same ceramide classes as stratum corneum and in similar proportions11. We analyzed sphingolipids belonging to the ceramide NS class (i.e., non hydroxylated fatty acid/sphingosine backbone), one of the most abundant in VC111 and one of the two sphingolipid families where ceramides can be derived from sphingomyelins in the stratum corneum12. Our observation of increases in the ceramide/sphingomyelin ratio with gestational age (Fig. 2b–g) supports the theory that VC may be used as a surrogate of fetal epidermis synthesis1. This may be a reflection of the cornification process in the stratum corneum formation of the fetus, where increased levels of ceramides are required to assist the newborn in adapting to the dry environment13. Sphingomyelins are the likely NS-ceramide source12, as evidenced by their concomitant decrease with increasing ceramide levels. Interestingly, although epidermal barrier lipid synthesis has been found to be delayed by androgens in rats14, we observed no differences in the dynamics of the ceramide/sphingomyelin ratio with the gestational period in boys relative to girls. However, one study did report decreased epidermal barrier lipids in VC from males15, findings which could not be corroborated in follow-up studies1. In the current investigation, although mild differences were observed for some sphingolipid species depending on the gender of the newborn (Fig. 1), no changes were observed in the ceramide/sphingomyelin ratios. The current work focused only on the NS class of sphingolipids, and gender may of course still exert influence on other families of sphingolipids. Specifically, it would be interesting to examine possible alterations in sphingolipids containing an esterified omega-hydroxy fatty acid (EO), because they are known to be especially relevant in maintaining epidermal barrier and function4.

Endocannabinoids are endogenous lipid mediators that target the cannabinoid receptors. They are known to be involved in the regulation of skin physiology16, and play an important role during pregnancy17. In the present study, we report for the first time the presence of a series of endocannabinoids and related molecules, including the two prototypical endocannabinoid molecules, AEA and 2-AG in VC. The fetus is exposed to changes in AEA levels in utero depending on the gestational period, with initial lower levels that increase as term approaches18. Our data show that changes of AEA and 2-AG can be observed in VC even within the short time span (36 to 42 weeks) of its collection relative to the total time of gestation (Fig. 1). In addition, levels of AEA (and also of 2-AG) positively correlated with all ceramide/sphingomyelin ratios (except in the case of AEA and ceramide/sphingomyelin 24:0). AEA has been shown to inhibit epidermal differentiation in keratinocytes19, and thus increasing levels with gestational period may be related to the ongoing cornification process reflected by the increased ceramide to sphingomyelin ratio. Moreover, the activation of cannabinoid receptors has been shown to induce ceramide generation by activation of sphingomyelinases in astrocytes20. Thus, VC arises as an additional non-invasively collected source for the study of endocannabinoids and their interaction with other lipid mediators early in life.

Finally, the use of samples belonging to the ALADDIN cohort7 enabled us to study the influence of anthroposophic lifestyle on the composition of VC. Anthroposophy involves a lifestyle that tends towards home deliveries, a vegetarian diet, and restricted use of antibiotics as well as several other features reported elsewhere7. In the present study, only differences in home deliveries and diet were observed (Table 2). A number of linoleic acid-derived oxidation products were found to increase with the mother’s degree of anthroposophic lifestyle (Figs 1 and 3). These compounds are joint products of cytochrome P450 and soluble epoxide hydrolase (sEH) activity (EpOMEs and DiHOMEs, respectively)21. In the anthroposophic lifestyle group there was an increased use of butter both on bread and relative to vegetable oils when cooking as well as a higher prevalence of vegetarian diet (Table 2). Accordingly, the contribution of diet to the fatty acid content of the VC is unclear, with a potential shift in saturated (e.g., stearic acid) as well as unsaturated fatty acid (e.g., linoleic acid) species. There is unfortunately little information in the literature on the levels of fatty acids in VC and future studies would clearly benefit from quantification of the fatty acid composition of VC. This information would in particular aid in interpreting the effect of diet on the biochemical pathways responsible for lipid mediator production.

It is also plausible that the observed lipid mediator changes reflect differential activities of the enzymes involved in metabolite biosynthesis rather than shifts in the levels of the fatty acid substrates. This observation is supported by the fact that no lifestyle-associated differences were observed for either the linoleic acid-derived potential downstream lipoxygenase products 9- and 13-KODE or the autooxidation product EKODE (Fig. 1, Supplementary Figure S1c,d). However, the activity of cytochrome P450s on arachidonic acid metabolites could not be assessed because the majority of the corresponding arachidonic acid epoxide and diol analogs (i.e., the epoxyeicosatrienoic and dihydroxyeicosatrienoic acids; EpETrEs and DiHETrEs) were either not detected or did not pass the quality criteria (with the exception of 5(6)-EpETrE, which did not evidence a lifestyle-associated shift; Fig. 1). The lower levels of arachidonic acid metabolites relative to linoleic acid metabolites can be explained by the relatively higher abundance of linoleic acid in the epidermis22. No shifts were observed in the linoleic acid-derived linoleoyl ethanolamide (LEA) or linoleoyl glycerols (1- and 2-LG); however, this can be explained by a higher prevalence of linoleic acid incorporation into the sn-2 position of phospholipids (leading to the eicosanoid biosynthetic pathway). These results suggest that lifestyle-based differences may result in specific fluctuations in cytochrome P450-mediated linoleic acid metabolism, the implications of which should be explored in more detail.

In addition, gender-specific effects in cytochrome P450 and sEH activity have been reported, especially in relation to hypertension and cardiovascular disease23. In the current study, weak gender differences were observed in the cytochrome P450-derived 9(10)-EpOME and 5(6)-EpETrE (Fig. 1). Accordingly, future studies should take into account the influence of gender on the production of oxylipins and the associated subsequent potential pathobiological effects.

To summarize, the present study reports for the first time the presence of a series of bioactive lipid mediators in VC. The total gestational period, gender of the baby and lifestyle of the mother were found to differently affect the lipid mediator composition of VC. A number of potentially interesting associations were observed regarding their involvement in skin barrier development. Accordingly, levels and speciation of lipid mediators in VC may potentially be useful as early stage markers of the development of skin. The attractiveness of this matrix as a source of biomarkers is enhanced by the easiness and lack of invasion required for its collection.