The predominant monounsaturated fatty acid of sebum is the sapienic acid (16:1, Δ6), which has its single double bond at the sixth position from the carboxyl end (Nicolaides 1974; Wertz 2007). In nature, long-chain fatty acids with similar chain length are the most abundant, however, their initial double bond is preferably inserted in the ninth position from the carboxyl end. The 16-carbon isomer with the single double bond at the ninth position is the palmitoleic acid, a naturally found and abundant in many tissues and organisms fatty acid. Sapienic acid in the human body is truly unique to sebum and is not found as previously mentioned in other organs. In addition, most likely humans do not obtain it from the diet since very few plant species have been reported to manufacture this unusual fatty acid (Nicolaides 1971, 1974). Remarkably enough sapienic acid can be elongated by two carbons in human sebaceous cells and further accommodate an additional double bond between the fifth and sixth carbon; process that yields the sebaleic acid (18:2Δ5, 8), which is the most specific reaction and metabolite that has been reported only in human sebaceous cells. Sapienic acid is still the most prevalent and in much higher levels than any of its derivatives, isomers, or other monounsaturated fatty acids found in sebum. However, the potential role of sapienic acid in the etiology of acne is still remained to be decoded and somehow controversial. There are certainly reports with a conflicting message to the presence of sapienic acid in sebum where it could correlate with elevated sebum levels (Smith et al. 2008), while other reports demonstrate its potency against bacteria commonly associated with acne (Drake et al. 2008; Georgel et al. 2005; Wille and Kydonieus 2003).

A more comprehensive summary on the sapienic acid biology and roles is included within this book (Chapter “Sapienic Acid and Sebum Specific Fatty Acid Metabolism of the Human Sebaceous Gland”).

Wax esters are also unique molecules to sebaceous cells and are not produced by any other cell type in the human body. They account for about 25–30 % of the total sebaceous gland lipids and their production correlates with sebaceous gland differentiation (Strauss et al. 1991; Wertz 2007). Several animal models have demonstrated a strong correlation between atrophic-sebaceous gland, impaired-wax ester synthesis, and skin and hair abnormalities (Chen et al. 2002; Miyazaki et al. 2001).

Wax ester synthases (Cheng and Russell 2004; Lardizabal et al. 2000) have recently been discovered, however, additional reports (Yen et al. 2005a, b) provided evidence that another family of enzymes can also synthesize waxes. Therefore, there is not a single and unique wax synthase and besides, the wax ester biosynthesis is still not fully explored in humans. DGAT1 and DGAT2 acyl transferases catalyze acylation of alcohols, diols, retinol, etc., as well as a diacylglycerol. The human wax synthases AWAT1 and AWAT2, belonging to the DGAT2 family, can transfer acyl groups to mono or diacylglycerol, fatty alcohol, and retinol (Turkish and Sturley 2009). Most probably, these multifunctional acyl tranferases can account for the majority of the mono- and diester waxes.

Although active wax synthesis correlates with the differentiation of sebaceous cells, it is still uncertain if they are the reason or the outcome of the differentiation process. In vitro, it is one of the distinct pathways that is usually downregulated no matter if explanted sebaceous glands, tissues, cell preps, or transformed cell lines are used, since in vitro experiments that report the synthesis of similar to 25 % of waxes in lipid excretions are missing. Age and sex related differences have been reported in wax ester synthesis, which also correlate with total sebum production (Downing et al. 1989; Jacobsen et al. 1985; Stewart et al. 1982). In nature, waxes always act as protective layers against excessive dehydration or water permeability and therefore are found on the surface of leaves, fruits, plant stems, algae, or even the skin, feathers, and fur of animals and additionally they also coat the wall of bacteria and fungi (Kolattukudy 1980). Waxes are very stable molecules and are far more resistant to oxidation, hydrolysis, and heat than triglycerides or phospholipids. Their primary role of protection is not the only one as they could also have lubrication properties. They are the most hydrophobic molecules found on the surface of organisms and that is how they help in sealing in the internal moisture of tissues while they are preventing their excessive hydration (Kolattukudy 1980). In certain instances, the packing and physicochemical properties of the wax crystals demonstrate unusual surface self cleaning properties that repel not only moisture, but together with water any kind of physical or biological invader, from dust to pollen, mites, and bacteria. This phenomenon has been termed as the “lotus effect” and is a fundamental property of the way that waxes can pack and form microstructures that could induce water repelling (Koch et al. 2007). For a more comprehensive summary on the wax ester biology and roles the reader should read the included chapter within this book (Chapter “Wax Esters: Chemistry and Biosynthesis”).

It will not be an understatement to claim that there is nothing unique about the synthesis of squalene since it is a precursor of cholesterol, therefore, most mammalian cells that have the capacity to synthesize cholesterol could also synthesize squalene. Cholesterol is an indispensable molecule for cell membranes fluidity and structure. Squalene is its precursor that is a long-unsaturated hydrocarbon. In any other tissue of the human body other than the sebaceous gland, it will readily get converted to the intermediates that would eventually yield the final product, cholesterol (Elias and Feingold 1992; Nicolaides 1974). The uniqueness in human sebum is the presence and accumulation of this cholesterol precursor to unusually high levels (12–15 %); and such levels have never been reported to any other tissue or organ. On the other hand, cholesterol does not account more than 2 % of the total sebaceous gland lipids; although is abundant and to the level of 30 % in the epidermal keratinocyte origin lipids and therefore it could be a contaminating source of cholesterol in sebum. Squalene synthase is the enzyme responsible for the production of squalene and squalene epoxidase or monooxygenase for its further processing to cholesterol. It is speculated that in sebaceous cells the activity of these two enzymes regulates the observed accumulation of squalene.

Squalene is very hydrophobic as a long hydrocarbon, however, highly unsaturated, it attributes fluidity to the molecule, therefore it becomes a natural lubricant with high penetration efficiency; therefore its role could be more diverse than just being a precursor of cholesterol. Past reports demonstrated, but not fully validated, possible roles of squalene oxidation products on UV protection (Ohsawa et al. 1984) and also irritation (Chiba et al. 2000). These oxidation products, together with oxidation products from unsaturated free fatty acids, have been reported to be comedogenic (Kligman et al.1970; Motoyoshi 1983). Perhaps this could be a reason as to why human sebum transports lipophilic antioxidants as vitamin E (Thiele et al. 1999) or humectants as glycerol (Fluhr et al. 2003), which orchestrate important roles in protecting skin from lipid oxidation and proper barrier function, respectively.

A more comprehensive summary on the squalene biology and roles is included within this book (Chapter “Squalene Chemistry and Biology”).

Genetic knock out (KO) animal models of lipid synthesis have clearly demonstrated the importance of sebaceous lipids in skin and hair physiology. In most of these studies, skin and fur abnormalities became the common denominator, once a certain sebaceous lipid pathway is disturbed. One of the first models to be reported, the melanocortin-5 receptor (MC5-R) KO, resulted in severe defects in water repulsion and thermoregulation due to decreased production of sebaceous lipids (Chen et al. 1997). The effect of the MC5-R on sebaceous lipid metabolism unveiled an additional path for the melanocortin receptors, besides their anticipated role on pigmentation, obesity, or body weight regulation.

Two years after the MC5-R KO was reported, Zheng et al. (1999) demonstrated by positional cloning that the dramatic alopecia manifested in the asebia mouse is due to the lack of a functional stearoyl-CoA desaturase (Scd1) enzyme activity. The absence of mature sebaceous glands demonstrated the evident significance of the SCD1 gene and its products (monounsaturated fatty acids) to normal sebaceous gland function and additionally their role in hair development and health. The same findings were further confirmed 2 years later, in 2001, by what we can call as the reverse experiment; where the SCD1 KO mice were constructed which evidently bared a similar phenotype (Miyazaki et al. 2001). The revelation that in both models the SCD1 activity is solely responsible for scant to absent hair and hypoplastic to absent sebaceous glands was further supported by the fact that sebaceous glands are also scant in certain forms of alopecias (Sundberg 1994). The skin of the SCD1 KO mice demonstrated lower than normal levels of triglycerides and wax esters, besides the expected lower than normal levels in the SCD1 direct products as oleic acid and monounsaturated fatty acids.

An amazingly similar phenotype was demonstrated in the acyl CoA:diacylgylcerol acyltransferase 1 (DGAT1) KO mouse, where sebaceous gland atrophy and hair loss were also evident and the most severe skin manifestation (Chen et al. 2002). DGAT is the primary triglyceride synthase and there are two isoforms, DGAT1 and DGAT2 in mice. However, these two isoforms differ in sequence and localization (Headington 1996). DGAT1 has a distinct role and expression than the DGAT2; since it is also involved in the synthesis of wax esters, unlike DGAT2 (Yen et al. 2005a), and is expressed in most tissues, including the sebaceous gland (Chen et al. 2002; Cases et al. 1998). The demonstrated involvement of DGAT1 in wax ester synthesis is consistent with the observation that there are no or little wax esters in the fur lipids of the DGAT1 KO mouse.

The DGAT2 KO animals (Stone et al. 2004), in a similar fashion to SCD2 KO mice (Miyazaki et al. 2005), do not survive due to an impaired skin barrier function. The animals deficient in SCD-2 demonstrated abnormal skin barrier function due to abnormal lamellar bodies and epidermal maturation. This further proved that the presence of monounsaturated fatty acids is vital also for skin’s barrier component and besides the formation of the sebaceous glands.

Additional animal models that demonstrated the importance of sebaceous and also epidermal lipids to skin functions are the KO of the elongases 3 and 4 that are responsible for the synthesis of very-long-chain fatty acids (VLCFA) (Westerberg et al. 2004; Vasireddy et al. 2007). The Elovl3 gene product is involved in the formation of VLCFA and has a distinct expression in the skin that is restricted to sebaceous and epithelial cells of hair follicles. Disruption of that gene generated a KO model with impaired formation of neutral lipids necessary for skin functions but in addition resulted in disturbed water barrier and increased transepidermal water loss. This was caused to a certain extent from a disruption in normal lamellar body formation that the deficiency of Elovl3 has caused but most profoundly the Elovl3-ablated mice demonstrated sparse hair coats and hyperplastic sebaceous glands with unusual lipid content mainly in monounsaturated fatty acid with 20 carbons.

Furthermore, the importance of the elongated fatty acids was recently further confirmed in a similar animal model where deficiency of the elongase of very-long chain fatty acid -like 4 (ELOVL4) displayed a scaly and wrinkled skin (Vasireddy et al. 2007). In addition, this model demonstrated a severely compromised epidermal permeability barrier function, which resulted in death within a few hours after birth (and similar to SCD2 and DGAT2 KO models). However, in this particular model the skin histology showed an abnormally compacted outer epidermis (SC), and electron microscopy revealed deficient epidermal lamellar body contents, decreased levels in VLCFA (> C28) in ceramide, glucosylceramide, and the free-fatty acid fractions, demonstrating the necessity of VLCFA for the synthesis of skin ceramides rather than a sebaceous gland deficiency.

In addition to the above preclinical models, few other models have additionally demonstrated the importance of sebaceous and epidermal lipid pathways for skin’s integrity and physiology. Fatty acid transport proteins are fundamental to nonpolar lipid synthesis and out of the six mammalian FATPs, FATP4 seems to be the most important one involved in skin lipid biosynthesis (Schmuth et al. 2005). Deletion of FATP4 in mice resulted in perturbations in the biosynthesis of skin lipids mainly by keratinocytes that caused barrier dysfunction and neonatal fatality (Lin et al. 2013). One of the pathways that was drastically reduced by the absence of FATP4 was the wax-ester synthesis.

In addition, mammalian sebaceous glands also produce some wax diesters that require 2-hydroxylation of fatty acids which is catalyzed by the mammalian fatty acid 2-hydroxylase (FA2H). When FA2H gene, that was originally thought to be important for the synthesis of hydroxylated fatty acid containing sphingolipids of the murine skin, was knocked out, the FA2H deficient mice did not show any effect on the level of such sphingolipids (Maier et al. 2011). FA2H expression was found to be restricted to sebaceous glands and FA2H deficiency caused a drastic reduction not only in 2-hydroxylated glucosylceramides but in addition to diester waxes. The subsequent change in surface lipids caused a blockage in hair canals and thus severely interfered with hair growth and health.

Besides the lipid processing enzymes, we also have evidence that the most important lipid transcriptional factor, the peroxisome proliferating activated receptor gamma (PPARγ) is also required for the formation of sebaceous glands (Fu et al. 2010), but in addition regulates sebocyte functions such as cellular responses to oxidative stress (Zhang et al. 2006) and eotaxin production (Nakahigashi et al. 2012).