Measurement of sebum secretion rates in young children

No skin off your back: the sampling and extraction of sebum for metabolomics

INTRODUCTION

Sebum-based metabolomics (a subset of "sebomics") is a developing field that involves the sampling, identification, and quantification of metabolites found in human sebum. Sebum is a lipid-rich oily substance secreted by the sebaceous glands onto the skin surface for skin homeostasis, lubrication, thermoregulation, and environmental protection. Interest in sebomics has grown over the last decade due to its potential for rapid analysis following non-invasive sampling for a range of clinical and environmental applications.

OBJECTIVES

To provide an overview of various sebum sampling techniques with their associated challenges. To evaluate applications of sebum for clinical research, drug monitoring, and human biomonitoring. To provide a commentary of the opportunities of using sebum as a diagnostic biofluid in the future.

METHODS

Bibliometric analyses of selected keywords regarding skin surface analysis using the Scopus search engine from 1960 to 2022 was performed on 12th January 2023. The published literature was compartmentalised based on what the work contributed to in the following areas: the understanding about sebum, its composition, the analytical technologies used, or the purpose of use of sebum. The findings were summarised in this review.

RESULTS

Historically, about 15 methods of sampling have been used for sebum collection. The sample preparation approaches vary depending on the analytes of interest and are summarised. The use of sebum is not limited to just skin diseases or drug monitoring but also demonstrated for other systemic disease. Most of the work carried out for untargeted analysis of metabolites associated with sebum has been in the recent two decades.

CONCLUSION

Sebum has a huge potential beyond skin research and understanding how one's physiological state affects or reflects on the skin metabolome via the sebaceous glands itself or by interactions with sebaceous secretion, will open doors for simpler biomonitoring. Sebum acts as a sink to environmental metabolites and has applications awaiting to be explored, such as biosecurity, cross-border migration, localised exposure to harmful substances, and high-throughput population screening. These applications will be possible with rapid advances in volatile headspace and lipidomics method development as well as the ability of the metabolomics community to annotate unknown species better. A key issue with skin surface analysis that remains unsolved is attributing the source of the metabolites found on the skin surface before meaningful biological interpretation.

Paper-Based Sandwich-Structured Wearable Sensor with Sebum Filtering for Continuous Detection of Sweat pH

Wearable sweat sensors, a product of the development of flexible electronics and microfluidic technologies, can continuously and noninvasively monitor abundant biomarkers in human sweat; however, sweat interferences, such as sebum, can reduce sensor reliability and accuracy. Herein, for the first time, the influence of sebum on the potentiometric response of an all-solid-state pH sensor was studied, and the obtained experimental results show that sebum mixed in sweat can decrease the potential response of the sensor and the slope of its calibration curve. A paper-based sandwich-structured pH sensor that can filter the sebum mixed in sweat was proposed based on commonly used oil-control sheets. Moreover, the hydrophilic properties, microstructure, and microfluidic performance of the sensor were investigated. The detection performance of the paper-based sandwich-structured pH sensor was comprehensively evaluated in terms of calibration in the presence of sebum and potentiometric response upon the addition of sebum. Furthermore, the anti-interference ability of the sensor was evaluated using different analytes under various deformation conditions. On-body trials were conducted to verify the performance, and their results showed that the proposed sensor can filter over 90% of the sebum in sweat, significantly enhancing sensor reliability and accuracy. Additionally, microfluidic channels could be simply fabricated using a scissor and paper, obviating the need for complex micromachining processes, such as photolithography and laser engraving. Overall, this work illustrates the influence of sebum on the detection performance of traditional potentiometric wearable sensors and paves the way for their development for real-world applications.

Evolution of the facial skin microbiome during puberty in normal and acne skin

BACKGROUND

The composition of the skin microbiome varies from infancy to adulthood and becomes most stable in adulthood. Adult acne patients harbour an 'acne microbiome' dominated by specific strains of Cutibacterium acnes. However, the precise timing of skin microbiome evolution, the development of the acne microbiome, and the shift to virulent C. acnes strain composition during puberty is unknown.

OBJECTIVES

We performed a cross-sectional pilot study in a paediatric population to understand how and when the skin microbiome composition transitions during puberty and whether a distinct 'acne microbiome' emerges in paediatric subjects.

METHODS

Forty-eight volunteers including males and females, ages 7-17 years, with and without acne were enrolled and evaluated for pubertal development using the Tanner staging criteria. Sebum levels were measured, and skin microbiota were collected by sterile swab on the subject's forehead. DNA was sequenced by whole genome shotgun sequencing.

RESULTS

A significant shift in microbial diversity emerged between early (T1-T2) and late (T3-T5) stages of puberty, coinciding with increased sebum production on the face. The overall relative abundance of C. acnes in both normal and acne skin increased during puberty and individual C. acnes strains were uniquely affected by pubertal stage and the presence of acne. Further, an acne microbiome signature associated with unique C. acnes strain composition and metabolic activity emerges in late puberty in those with acne. This unique C. acnes strain composition is predicted to have increased porphyrin production, which may contribute to skin inflammation.

CONCLUSIONS

Our data suggest that the stage of pubertal development influences skin microbiome composition. As children mature, a distinct acne microbiome composition emerges in those with acne. Understanding how both puberty and acne influence the microbiome may support novel therapeutic strategies to combat acne in the paediatric population.

Clinical characteristics of Demodex-associated recurrent hordeola: an observational, comparative study

Our study evaluated the association between Demodex infestation and recurrent hordeola and examined the clinical features associated with these eyelid lesions. This was an observational, comparative study. We reviewed 250 patients and divided them into the recurrent hordeolum (n = 153) and control (n = 97) groups. Demodex infestation was detected by epilating eyelashes around the lesion/s and viewing them under a light microscope. Patient medical records and photographs were retrospectively analyzed to identify the clinical characteristics of Demodex-associated recurrent hordeola. Demodex was detected in 91 (59.5%) and 17 (17.5%) patients in the recurrent hordeolum and control groups (p < 0.001), respectively. In the recurrent hordeolum group, Demodex mites were found in 74 (68.5%) and 17 (37.8%) of the adult and pediatric patients (p < 0.001), respectively. Among patients with recurrent hordeola, patients in their 20s were most likely to have concomitant Demodex infestation. Patients with Demodex infestations were also more likely to develop recurrent lesions within a shorter period of time from the primary incision and curettage. The most common presentation of Demodex-associated recurrent lesions was external hordeola (67%) (p = 0.002). Demodex infestation may cause recurrent hordeola in adults and children. These mites may play a greater role in the development of lesions in adult patients. The strongest association between Demodex infestation and recurrent lesions was seen in patients in their 20s. Our results suggest that if the hordeola recur within a short period of time with the clinical characteristics of external location of eyelid, multiple numbers of lesions, or anterior blepharitis, eyelash epilation should be performed to identify the presence of Demodex mites.

Age and Mothers: Potent Influences of Children's Skin Microbiota

The evolution of a child's skin microbiome is associated with the development of the immune system and skin environment. As only few studies have analyzed the microbiota in young children, we investigated changes in the skin microbiota of children (158 subjects; ≤10 years old) and compared the microbiota structures between children and their mothers using 16S rRNA gene amplicon sequencing. Sample location and age were the primary factors determining a child's skin bacterial composition, which differed significantly among the face, ventral forearm, and calf. Relative abundances of Streptococcus and Granulicatella were negatively correlated with age, and the alpha diversity at all body sites examined increased during the first 10 years of life, especially on the face. The facial bacterial composition of 10-year-old children was strongly associated with delivery mode at birth. Among mother-child pairs (50 pairs), the relative abundances of most bacterial genera in children were more similar to those of their own mothers than those of unrelated women. The data indicated that age and site were significantly associated with microbial composition and that maternal factors determine the child's microbiome. Further research is needed to characterize the effects of maturation of the infant microbiome on health in adulthood.

Sebomic identification of sex- and ethnicity-specific variations in residual skin surface components (RSSC) for bio-monitoring or forensic applications

Background

“Residual skin surface components” (RSSC) is the collective term used for the superficial layer of sebum, residue of sweat, small quantities of intercellular lipids and components of natural moisturising factor present on the skin surface. Potential applications of RSSC include use as a sampling matrix for identifying biomarkers of disease, environmental exposure monitoring, and forensics (retrospective identification of exposure to toxic chemicals). However, it is essential to first define the composition of “normal” RSSC. Therefore, the aim of the current study was to characterise RSSC to determine commonalities and differences in RSSC composition in relation to sex and ethnicity.

Methods

Samples of RSSC were acquired from volunteers using a previously validated method and analysed by high-pressure liquid chromatography–atmospheric pressure chemical ionisation–mass spectrometry (HPLC-APCI-MS). The resulting data underwent sebomic analysis.

Results

The composition and abundance of RSSC components varied according to sex and ethnicity. The normalised abundance of free fatty acids, wax esters, diglycerides and triglycerides was significantly higher in males than females. Ethnicity-specific differences were observed in free fatty acids and a diglyceride.

Conclusions

The HPLC-APCI-MS method developed in this study was successfully used to analyse the normal composition of RSSC. Compositional differences in the RSSC can be attributed to sex and ethnicity and may reflect underlying factors such as diet, hormonal levels and enzyme expression.

Evaluation of the Quantitative and Qualitative Alterations in the Fatty Acid Contents of the Sebum of Patients with Inflammatory Acne during Treatment with Systemic Lymecycline and/or Oral Fatty Acid Supplementation

Background. Acne is a dermatosis that involves an altered sebum pattern. Objectives. (1) To evaluate if a treatment based on antibiotics (lymecycline) can alter fatty acids contents of the sebum of patients with acne; (2) to evaluate if oral supplementation of fatty acids can interfere with fatty acids contents of the sebum of patients with acne; (3) to evaluate if there is any interaction in fatty acids contents of the sebum of patients with acne when they use both antibiotics and oral supplementation of fatty acids. Methods. Forty-five male volunteers with inflammatory acne vulgaris were treated with 300 mg of lymecycline per day, with 540 mg of γ-linolenic acid, 1,200 mg of linoleic acid, and 510 mg of oleic acid per day, or with both regimens for 90 days. Every 30 days, a sample of sebum from the forehead was collected for fatty acids' chromatographic analysis. Results. Twelve fatty acids studied exhibited some kind of pattern changes during the study: C12:0, C14:0, C15:0, C16:1, C18:0, C18:1n9c+C18:1n9t, C18:2n6t, C18:3n6, C18:3n3, C20:1, C22:0, and C24:0. Conclusions. The daily administration of lymecycline and/or specific fatty acids may slightly influence some fatty acids levels present in the sebum of patients with inflammatory acne vulgaris.

Effect of ethnicity, gender and age on the amount and composition of residual skin surface components derived from sebum, sweat and epidermal lipids

BACKGROUND/PURPOSE

The superficial layer on the skin surface, known as the acid mantle, comprises a mixture of sebum, sweat, corneocyte debris and constituents of natural moisturizing factor. Thus, the phrase 'residual skin surface components' (RSSC) is an appropriate term for the mixture of substances recovered from the skin surface. There is no general agreement about the effects of ethnicity, gender and age on RSSC. The aim of this human volunteer study was to evaluate RSSC in relation to ethnicity, gender and age. A suitable acquisition medium for RSSC collection was identified and samples of RSSC were subsequently analysed using gas chromatography-mass spectrometry (GC-MS) and gravimetry.

METHODS

A total of 315 volunteers participated in the study from a range of self-declared ethnic backgrounds. Six acquisition media were compared to determine the most suitable media for RSSC collection. The effect of age, gender and ethnicity on RSSC collection was evaluated by gravimetric analysis while GC-MS was used to determine the composition of RSSC.

RESULTS

Of the six candidate materials assessed, cigarette paper provided the most practical and reproducible sample acquisition medium. There was no significant difference in the amount of RSSC collected when based on gender and ethnicity and no significant correlation between RSSC recovery and age. Up to 49 compounds were detected from human RSSC when analysed by GC-MS.

CONCLUSIONS

The results of the present study suggest that RSSC can be effectively collected using cigarette paper and analysed by GC-MS. Ethnicity, gender and age had no significant impact on the quantity of RSSC recovered from the skin surface.

Percutaneous absorption and exposure assessment of pesticides

Dermal exposure to a diverse range of chemicals may result from various uses. In order to assess exposure and estimate potential risks, accurate quantitative data on absorption are required. Various factors will influence the final results and interpretations of studies designed to assess the ability of compounds to penetrate the skin. This overview will discuss skin penetration by pesticides, emphasizing key parameters to be considered from the perspective of exposure assessment.

Fatores etiopatogênicos da acne vulgar

A acne vulgar é uma das dermatoses mais freqüentes na população em geral. Encontra-se na literatura grande número de trabalhos científicos referentes sobretudo a sua etiopatogenia. No entanto, dado o grande número de informações geradas a respeito, dificilmente consegue-se reuni-las em entendimento comum. Esta revisão literária foi proposta a fim de abordar os mecanismos etiopatogênicos clássicos da acne vulgar (produção sebácea, hiperqueratinização folicular, colonização bacteriana folicular e inflamação glandular) e o mecanismo coadjuvante principal, a influência hormonal.

Acne and sebaceous gland function

The embryologic development of the human sebaceous gland is closely related to the differentiation of the hair follicle and the epidermis. The number of sebaceous glands remains approximately the same throughout life, whereas their size tends to increase with age. The development and function of the sebaceous gland in the fetal and neonatal periods appear to be regulated by maternal androgens and by endogenous steroid synthesis, as well as by other morphogens. The most apparent function of the glands is to excrete sebum. A strong increase in sebum excretion occurs a few hours after birth; this peaks during the first week and slowly subsides thereafter. A new rise takes place at about age 9 years with adrenarche and continues up to age 17 years, when the adult level is reached. The sebaceous gland is an important formation site of active androgens. Androgens are well known for their effects on sebum excretion, whereas terminal sebocyte differentiation is assisted by peroxisome proliferator-activated receptor ligands. Estrogens, glucocorticoids, and prolactin also influence sebaceous gland function. In addition, stress-sensing cutaneous signals lead to the production and release of corticotrophin-releasing hormone from dermal nerves and sebocytes with subsequent dose-dependent regulation of sebaceous nonpolar lipids. Among other lipid fractions, sebaceous glands have been shown to synthesize considerable amounts of free fatty acids without exogenous influence. Sebaceous lipids are responsible for the three-dimensional skin surface lipid organization. Contributing to the integrity of the skin barrier. They also exhibit strong innate antimicrobial activity, transport antioxidants to the skin surface, and express proinflammatory and anti-inflammatory properties. Acne in childhood has been suggested to be strongly associated with the development of severe acne during adolescence. Increased sebum excretion is a major factor in the pathophysiology of acne vulgaris. Other sebaceous gland functions are also associated with the development of acne, including sebaceous proinflammatory lipids; different cytokines produced locally; periglandular peptides and neuropeptides, such as corticotrophin-releasing hormone, which is produced by sebocytes; and substance P, which is expressed in the nerve endings at the vicinity of healthy-looking glands of acne patients. Current data indicate that acne vulgaris may be a primary inflammatory disease. Future drugs developed to treat acne not only should reduce sebum production and Propionibacterium acnes populations, but also should be targeted to reduce proinflammatory lipids in sebum, down-regulate proinflammatory signals in the pilosebaceous unit, and inhibit leukotriene B(4)-induced accumulation of inflammatory cells. They should also influence peroxisome proliferator-activated receptor regulation. Isotretinoin is still the most active available drug for the treatment of severe acne.

Is acne an infection of blocked pilosebaceous follicles? Implications for antimicrobial treatment

A model is proposed which is based on the assumption that acne is due to infection of functionally blocked pilosebaceous follicles by propionibacteria. Noninflamed lesions, which are first visible during the adrenarche in acne-prone individuals, do not contain propionibacteria. Comedogenesis appears to be independent of bacterial infection and may be driven by high levels of bioactive interleukin-1 alpha derived from ductal hyperkeratinocytes. The stimulus which triggers interleukin-1 alpha production is unknown. Formalin killed Propionibacterium acnes failed to stimulate production of the cytokine by cultured human keratinocytes in vitro. Inflamed lesions are thought to arise from microcomedones, but the initiating events are unknown. Evidence that propionibacteria are involved in the generation of inflammatory lesions is inconclusive. The cellular infiltrate is consistent with a type IV hypersensitivity response to one or more persistent lesional antigens, not necessarily bacterial. The potent adjuvant activity of P. acnes would up-regulate the immune response to any antigen which came into contact with the mononuclear cell infiltrate. Antibiotics are widely used in the treatment of acne, and their effects in selecting a predominantly resistant commensal population are well recognized. Although they reduce numbers of propionibacteria on the skin, other modes of action may contribute to or explain their therapeutic efficacy. At a time when there is global concern that antibiotic resistance rates in common bacterial pathogens may threaten our future ability to control bacterial infections, practices which promote the spread of antibiotic-resistant bacteria must be fully justified. A thorough reappraisal of the role of propionibacteria in acne is overdue. It is likely that further experimental work is needed to confirm or refute that P. acnes is aptly named.

Sebaceous gland secretion is a major physiologic route of vitamin E delivery to skin

Skin plays an important part in the protection against oxidative stressors, such as ultraviolet radiation, ozone, and chemicals. This study was based on the observation that upper facial stratum corneum contained significantly higher levels of the antioxidant alpha-tocopherol than corresponding layers of arm stratum corneum. We hypothesized that the underlying mechanism involves sebaceous gland secretion of vitamin E. To test this, we examined in eight human volunteers: (i) stratum corneum levels and distribution profiles of vitamin E in sites with a different sebaceous gland density (arm versus cheek); (ii) whether vitamin E is a significant constituent of human sebum; and (iii) if there is a correlation between levels of vitamin E and squalene, a marker of sebum secretion, in skin surface lipids. Using standardized techniques for stratum corneum tape stripping and sebum collection, followed by high-performance liquid chromatography analysis of tocopherols and squalene, we found that: (i) the ratio of cheek versus upper arm alpha-tocopherol levels was 20 : 1 for the upper stratum corneum and decreased gradually with stratum corneum depth; (ii) vitamin E (alpha- and gamma-tocopherol forms) is a significant constituent of human sebum and is continuously secreted at cheek and forehead sites during a test period of 135 min; and (iii) vitamin E correlates well with levels of cosecreted squalene (r2 = 0.86, p < 0.001). In conclusion, sebaceous gland secretion is a relevant physiologic pathway for the delivery of vitamin E to upper layers of facial skin. This mechanism may serve to protect skin surface lipids and the upper stratum corneum from harmful oxidation.

Quantitative evaluation of skin surface lipids

More than 50 years of noninvasive studies of epidermal and sebaceous lipids has revealed a vast amount of information concerning secretion and regulation of SSLs. The first techniques developed (solvent extraction and cigarette paper) required very long experimental procedures to obtain valuable parameters; however, a great part of the knowledge on SSL production and regulation was obtained by these pioneers. Because of the difficulties encountered in these multistep procedures, applicability was restricted to a limited number of research centers. The sampling procedures in the newly developed techniques are significantly reduced and are followed by a fast and accurate evaluation. Moreover, the new sampling procedures permit us to study other aspects of sebum secretion, as demonstrated with the Sebutape method. When using photometric techniques an estimate of the SSL can be obtained within minutes (casual level measurements). After such a screening procedure, more standardized parameters can be accurately determined. Whatever the parameter under investigation, strictly controlled experimental procedures are required. Control of temperature and relative humidity of the experimental room, of the acclimatization periods, and of the volunteers during the collection periods is of capital importance. The studies discussed in this article clearly demonstrate the applicability of the new techniques not only for research and cosmetic purposes but also for clinical use.

Sebaceous gland activity and urinary androgen levels in children

Sebum and 24-h urine samples were collected from 17 children, aged 10-17 years. Wax ester (WE) secretion rates were measured as the index for sebaceous gland activity. The fatty acid methyl esters were analyzed by capillary gas chromatography. Urinary testosterone as well as 17-ketosteroids (androsterone, dehydroepiandrosterone (DHEA), and etiocholanolone) were separated from the urine samples and analyzed by gas chromatography. WE secretion rates increased with age and there were correlated changes in the monounsaturated fatty acids of WEs. Straight chain types such as 14:1 and 16:1 tended to increase, while 18:1 tended to decrease. There was a negative correlation between iso even fatty acids and WE secretion rates. However the other chain types such as straight odd, iso odd and anteiso odd showed no statistically significant change in relation to WE secretion rates. WE secretion rates were correlated positively with testosterone levels in both sexes, with DHEA in males, and with etiocholanolone in females. The results suggest that sebaceous gland activity is stimulated by adrenocortical androgens in prepuberty, in addition to the strong effect of testosterone during puberty.

Stratum corneum lipid abnormalities in atopic dermatitis

Patients with atopic dermatitis (AD) often present with a dry skin. To clarify the relationship between dry skin and lipid abnormalities within stratum corneum, stratum corneum lipids were collected from six AD patients aged 15 to 25 years and from sex- and age-matched controls. All major stratum corneum lipid classes were separated and quantitated by high-performance thin-layer chromatography/photodensitometry. Six ceramide fractions were also isolated and quantitated by thin-layer chromatography/photodensitometry. Esterified fatty acids of both ceramide 1 (acylceramides) and wax esters were analysed by capillary gas chromatography. The relative amounts of all the stratum corneum lipid classes including squalene, cholesterol esters, wax esters, triglycerides, free fatty acids, cholesterol, ceramides, cholesterol sulphate and phospholipids did not differ statistically between AD patients and controls. However, a significant decrease in proportion of ceramide 1, which is believed to be a carrier of linoleate responsible for a water-barrier function, and increased levels of esterified C18:1 fatty acids (oleate) of ceramide 1 were observed in AD patients. On the other hand, the fatty acid compositions as well as the proportions of C16:1 straight-chain component in sebum wax esters of AD patients were very similar to those of controls. These results suggest that a significantly reduced amount and/or structural alterations of ceramide 1 deriving from epidermal keratinocytes may be responsible for the impaired water-barrier function of the skin in AD.

Unusual cholesterol esters in the sebum of young children

Cholesterol esters (CE) having fatty acids of more than 18 carbons are a prominent feature of fetal skin surface lipid (vernix caseosa), but are a minor component of adult lipid. The difference may be related to the fact that fetal sebaceous glands generally synthesize little lipid. If so, it would be expected that prepuberal children, who also have very inactive glands, would secrete CE with a large proportion of very-long-chain fatty acids. To test this conjecture, skin surface CE from young children were isolated and analyzed. Sebum was extracted from the hair of 38 children, aged six to nine. To obtain a measure of sebaceous lipogenesis, the class composition of the lipid was determined by quantitative thin-layer chromatography and the ratio of wax esters/[cholesterol + cholesterol esters] (WE/[CH + CE]) was calculated. CE were then isolated from the lipid and hydrolyzed. The freed fatty acids were converted to fatty acid methyl esters (FAME) and analyzed by capillary gas chromatography to determine the proportion with more than 18 carbons. FAME from five of the subjects were then separated into saturated and monounsaturated fractions and analyzed again by gas chromatography to identify chain types. Ratios of WE/[CH + CE] ranged from 0.08 to 2.8 in the subjects. The proportion of CE FAME with more than 18 carbons ranged from 15 to 72%, with the highest proportion being found in the children with the lowest WE/[CH + CE]. The saturated FAME were mostly iso- or anteiso-branched, whereas the monounsaturated FAME were mostly straight-chain extension products of 16: 1 delta 9 or 18: 1 delta 9.

Changes in the relative amounts of endogenous and exogenous fatty acids in sebaceous lipids during early adolescence

Skin surface lipid samples were collected from the scalps of 40 males, aged 9-15, and the lipid class composition of each was analyzed by quantitative thin layer chromatography. The ratio of wax esters/[cholesterol + cholesterol esters] (WE/[CH + CE]) increased with age. The wax ester, cholesterol ester, triglyceride, and free fatty acid classes were isolated from each sample and the fatty acid compositions were determined by capillary gas chromatography of fatty acid methyl esters (FAME) prepared from each lipid class. The concentrations of most of the different types of fatty acids were found to be correlated with the WE/[CH + CE] ratio. Those straight chain fatty acids that are thought to be synthesized mainly within the sebaceous glands, such as 14:0, 14:1, 16:1, and 18:2 delta 5, 8 tended to increase with increasing WE/[CH + CE], while fatty acids which circulate in the blood, such as 18:0, 18:1, and 18:2 delta 9, 12, tended to decrease with increasing WE/[CH + CE]. For the majority of straight chain fatty acid types, the data could be fitted to the equation y = a + b/[x + 1], which can be derived from simple assumptions concerning the origins of the various sebum components. The FAME from the wax esters were separated into saturated and monounsaturated fractions and analyzed by capillary gas chromatography to determine the concentrations of the different types of branched chain FAME present. In the wax esters, straight chain fatty acids tended to increase with increasing WE/[CH + CE], while terminally branched (iso and anteiso) fatty acids tended to decrease. Other branched chain fatty acids increased up to a WE/[CH + CE] ratio of about 2 and then decreased at higher ratios.

Skin lipid content during early fetal development

Although little is known about changes in the lipid composition of the skin during fetal development, information regarding the developmental sequence of fetal skin lipid content could be important for understanding the emergence of epidermal barrier function, as well as providing baseline criteria for prenatal diagnosis of certain inherited disorders of cornification. In these studies, epidermis was separated from dermis in fetal skin samples ranging from 50 to 140 d, estimated gestational ages (EGA), and its lipid composition was analyzed by quartz rod microchromatography/flame-ionization and thin layer chromatography. Lipid biochemical data were correlated with developmental milestones observed by electron microscopy (morphologic studies). The lipid composition of epidermal and dermal fractions from skin samples between 50 and 110 d EGA was similar, with both tissues exhibiting a predominance of free sterols and phospholipids. After 110 d EGA dermis became enriched in triglycerides, corresponding to the progressive development of adipocytes after this time. EGA epidermis after 110 d was enriched not only in triglycerides, but also sterol esters. Moreover, ceramides and glycosphingolipids also became increasingly prominent, changes that were greatest in epidermis from older fetuses and from cephalad regions. These changes in epidermal lipid composition corresponded morphologically to the progressive emergence of both folliculocentric epidermal cornification and sebaceous gland development.

Effect of aging on sebaceous gland activity and on the fatty acid composition of wax esters

Using fused-silica capillary gas chromatography, we investigated sebum samples from 55 healthy individuals to discover the effects of aging on the sebaceous gland activity and on the fatty acid composition of wax exters. The sebaceous gland activity, which was expressed by the ratio of wax esters/[cholesterol + cholesterol esters] (WE/[C + CE]), showed a distinct change from infancy through maturity to senescence; the curve of the ratio made a peak in our subjects's 20s. Using the fatty acid analyses, we found an interesting relationship between C16:1 straight and C16:1 iso-branched chains, each of which occupied a large proportion in the fatty acids of wax esters; the former increased in proportion from infancy toward the 20s, with a correlation with aging (r = 0.788, p less than 0.01), and decreased thereafter until our subject's 50s (r = -0.611, p less than 0.01). In contrast, the proportion of the latter followed an entirely reversed course with advancing age. The percentages of C16:1 straight chain components were correlated positively with the WE/[C + CE] ratio (r = 0.642, p less than 0.01), while there was found to be a negative correlation between the proportion of C16:1 iso-branched chain components and the WE/[C + CE] ratio (r = -0.556, p less than 0.01). The results suggest that more active sebaceous glands in lipid production excrete lipids with a higher proportion of C16:1 straight chain fatty acid and a lower proportion of C16:1 iso-branched chain fatty acid. As well as the sebaceous gland activity, the fatty acid composition in sebum wax esters is affected by advancing age in Japanese.

Evaluation of a method for measuring the sustainable rate of sebaceous wax ester secretion

Sebaceous wax ester secretion rates were measured in six subjects on six occasions each, using absorption into bentonite clay and analysis of the collected lipid by quantitative thin-layer chromatography. On each occasion eight samples were collected, four from the left and four from the right side of the forehead in four successive intervals. The first two intervals, which totaled 14 h, were intended to deplete the follicular reservoir of sebum so that a constant rate of absorption could be obtained during the third and fourth intervals, which were 3 h each. Thin-layer analysis of each sample was done in triplicate. The data were examined using analysis of variance techniques to determine the reproducibility of the measurement method and to identify possible sources of variability. The intraclass correlation coefficient (rI) for all 432 post-depletion determinations was 0.80. The reproducibility was considerably better for three of the subjects (rI = 0.93) than for the other three (rI = 0.75). Variability within the latter three subjects did not seem to be attributable to lack of reproducibility in the thin-layer analysis. Real biological variability also seems unlikely considering the holocrine mechanism of sebum secretion. Therefore, the variability probably arises from non-representative collection of sebum into the bentonite absorbent.

Skin lipids: an update

The stratum corneum lipids, responsible for the epidermal water barrier, consist principally of ceramides, cholesterol, cholesteryl sulfate, and free fatty acids. These lipids are arranged in multiple intercellular lamellae that provide an efficient water barrier because of the crystalline array of the straight and predominantly saturated lipid chains. Interlamellar linkages provided by lipids based on 30-carbon omega-hydroxyacids may be responsible for holding together the intercellular lamellae as well as for assembly of the lamellar granules of the granular cells. The normally ordered exfoliation of corneocytes as they arrive at the surface seems to require hydrolysis of the cholesteryl sulfate to free cholesterol. The sebaceous glands secrete continuously, producing sebum that consists predominantly of triglycerides, wax esters, and squalene. High rates of sebum production per sebocyte result in low levels of linoleate in the sebaceous esters, subjecting the follicular epithelium to essential fatty acid deficiency and the characteristic hyperkeratosis that results in comedo formation. Suppression of sebum production by drugs elevates sebum linoleate concentration and relieves follicular hyperkeratosis. Thus, sebum continues to be a prime suspect in the crime of acne. Low levels of sebaceous gland activity are not correlated with the occurrence of dry skin.

Dilutional effect of increased sebaceous gland activity on the proportion of linoleic acid in sebaceous wax esters and in epidermal acylceramides

Sebaceous wax esters and epidermal acylceramides were isolated from skin surface lipid obtained from children and from young adults. Fatty acid methyl esters (FAME) were prepared from the esterified fatty acids of these lipid classes and analyzed to ascertain the proportions of methyl linoleate (18:2 delta 9,12), methyl sebaleate (18:2 delta 5,8), and methyl sapienate (16:1 delta 6). On the same subjects, 2 measures of sebum secretion rate were obtained, namely the sustainable wax ester secretion rate (WESR) on the forehead and the ratio of wax esters/(cholesterol + cholesterol esters) [WE/(CH + CE)] in the surface lipid. The proportions of methyl linoleate in FAME from the wax esters decreased, and the proportions of methyl sebaleate increased, with increased rates of sebum secretion. For both methyl linoleate and methyl sebaleate, a better correlation was obtained when the ratio of WE/(CH + CE) was used as a measure of sebum secretion rather than the WESR. The proportions of methyl linoleate in the FAME from the acylceramides were also inversely related to ratios of WE/(CH + CE). In acylceramides, linoleate was replaced by sapienate, a major fatty acid of human sebum. It appears, therefore, that sebum fatty acid composition may change with changes in sebaceous gland activity, and that sebum fatty acids can enter the epidermis and be incorporated into epidermal lipids.

Effect of cyproterone acetate-ethinyl estradiol treatment on the proportions of linoleic and sebaleic acids in various skin surface lipid classes

The effect of high-dose cyproterone acetate-ethinyl estradiol treatment on rates of sebum secretion and on the proportions of linoleic acid (18:2 delta 9,12) and sebaleic acid (18:2 delta 5,8) in the skin surface lipids of three female acne patients was examined. Changes in rates of sebum secretion were evaluated indirectly by measuring the ratio of wax esters/(cholesterol + cholesterol esters) in the subjects' skin surface lipid. In two of the subjects, this ratio indicated a reduction of sebum secretion rates to the childhood range. Concomitantly, there was an increase in linoleic acid and a decrease in sebaleic acid in all lipid classes. In the third subject, in whom there was only a small reduction in sebum secretion rate, the proportion of linoleic acid in the cholesterol esters more than doubled, but the changes in the other lipid classes were small or nonexistent. The results indicate that the proportions of linoleic acid and sebaleic acid in sebum are influenced by sebum secretion rates.

Age-related changes in sebaceous wax ester secretion rates in men and women

Sustainable rates of sebaceous wax ester secretion were measured on the foreheads of 109 men and 167 women, aged 15-97. Each measurement was made after first depleting the cutaneous sebum reservoir by overnight absorption of lipid into a layer of bentonite clay. Lipid was then absorbed for 3 h into fresh clay in which two 2-cm cloth disks were embedded. The absorbed lipid was extracted from the disks with ether and analyzed for wax esters by thin-layer chromatography. For both men and women there was a wide range of wax ester secretion rates at all ages. Rates were highest in the 15- to 35-year-olds and appeared to decline continuously throughout the adult age range. Values of log(wax esters) were better correlated with age than the untransformed values of wax ester secretion. The equations of best fit of log(wax esters) vs age suggested that sebum secretion declines about 23% per decade in men and 32% per decade in women.

Proportions of various straight and branched fatty acid chain types in the sebaceous wax esters of young children

The fatty acid composition of sebaceous wax esters was examined in 20 children, ages 6-8, using quartz capillary gas chromatography. The results were compared with a previously reported study of wax ester composition in 10 adults, in which the same analytical techniques were used. The children's wax esters showed more variability in the proportions of all fatty acid chain types than was found in adults. The most striking difference between the 2 age groups was in the proportions of iso-branched fatty acids with odd numbers of carbon atoms. These ranged from 1-26% of fatty acids in the children's wax esters but were not more than 3% of fatty acids in the adults' wax esters. Some of the variability in wax ester fatty acid composition in children seemed to be related to variation in rates of sebum production. The highest percentages of terminally branched (iso-even, iso-odd, and anteiso) and the lowest percentages of internally branched fatty acids were found in the children with the lowest rates of sebum secretion. However, even in children with similar rates of sebum secretion there were considerable differences in wax ester fatty acid composition, especially with respect to iso-even fatty acids. The results suggest that sebum fatty acid composition is influenced both by genetic factors and by limitations on the supply of precursors for terminally branched, but not internally branched, fatty acids.