Based upon their specific adaptations, the surface lipids have unique structural characters and
differ fundamentally from the internal lipids.
Thus, branched hydrocarbon chains are common
in the surface lipids but rare in the internal lipids.
The hydrocarbon compounds and the mono- and
diester waxes of the surface waxes play only a
minor role in the body fat of most organisms. Surface lipid preparations may be contaminated with
internal lipids during isolation, leading to considerable errors in analytical results. Extensive
analytical results are available for the skin fats of
mammals, the uropygial-gland lipids of birds and
the cuticular lipids of insects, but there are fewer
results for the surface lipids of the reptiles and the
terrestrial arthropods other than the insects.
There have been almost no experimental
investigations of the metabolism of most surface lipids. Conclusions about the synthesis of
the most important components, such as
hydrocarbons, fatty alcohols and alkane diols,
have been mainly drawn by analogy. This is
particularly regrettable as the unique structure
of many of these substances makes their synthesis especially interesting. It must be remembered, however, that most surface lipids are
produced close to their functional sites in epidermal cells or small skin glands, which are difficult to isolate. An exception is the uropygial glands of birds, with the result that they
have become a model for lipid biosynthesis
[133].
15.3.1 Epidermal Lipids of Terrestrial
Vertebrates
In the epidermis of mammals, birds and reptiles,
the innermost layer, e.g. the stratum germinativum of mammalian epidermis, is made up of living cells (keratinocytes) which are similar to other
living cells in the complexity of their lipid content. During keratinization, they are transformed
into the flattened, dead cells of the outer layer,
e.g. the stratum corneum of mammalian epidermis. In the process, lipids are released from the
epidermal cells into the intracellular space and
become an important barrier to water diffusion.
These epidermals lipids include mainly shingolipids of the type O-acyl-glucosylceramide (Fig.
15.13 d) and cholesterol, as well as smaller
amounts of free fatty acids and polar composite
lipids [133, 276, 277]. To date, little is known
about the epidermal lipids of the other terrestrial
vertebrates, i.e. birds and reptiles, but they
15.3.2 Skin Fats of the Mammals
587
appear to be relatively similar to those of the
mammals. Acyl-glucosylsterols were discovered
as a component of chicken epidermal lipids ; they
are probably also present in reptiles and amphibians but not in mammalian epidermis [277]. A
relatively uniform spectrum of glycosylceramides,
cholesterols, free fatty acids and various phospholipids is found in the discarded skin of various
snakes [30, 163].
15.3.2 Skin Fats of the Mammals
The fat substances that can be isolated from
human skin or from the fur of mammals arise
mainly as secretions of the sebaceous glands. In
contrast to the epidermal lipids, the sebaceous
gland lipids show large species-specific differences in both the content of the various lipid
classes (Table 15.10) and the structure of the aliphatic components, i.e. fatty acids, fatty alcohols
and diols. Some mammalian families have uniform lipid spectra, e.g. in the dog (Canidae), cattle (Bovidae) and horses (Equidae), but others
have widely different species-specific spectra, e.g.
raccoons (Procyonidae), squirrels (Sciuridae) and
martens (Mustelidae). The water-repellent properties of the sebaceous lipids are uniform and
cannot explain the structural variety. However, if,
as is now generally accepted, the skin fats play an
important role in chemical communication, even
closely related species should show distinct differences [48]. The lipids of mammalian hair make up
0.6-1.6 % of the fresh weight and consist mainly
of ceramides, cholesterol sulphates and glycosophingolipids [286].
In contrast to their role in the surface lipids of
the arthropods and plants, aliphatic hydrocarbons are not especially important in the surface
lipids of the mammals. A characteristic of humanskin fats is the presence of large amounts of squalene and triacylglycerols, which are otherwise seldom found (Table 15.10). The skin fat of the otter
Lutra canadensis in fact contains 44 % squalene,
which has especially effective water-repellent
properties [49]. Mono- and diester waxes
(Fig. 15.13a-c) are common components of mammalian skin fats. Two types of diester wax can be
distinguished: type I, which contains a hydroxy
fatty acid esterified with an unsubstituted fatty
acid and a fatty alcohol, and type II, which is a
1,2-diol carrying two un substituted fatty acids.
Most mammals, however, have only one of the
two types; the presence of both is seldom seen
(Table 15.11). The skin fats of horses (Equidae)
differ fundamentally from the internal lipids.
Thus, branched hydrocarbon chains are common
in the surface lipids but rare in the internal lipids.
The hydrocarbon compounds and the mono- and
diester waxes of the surface waxes play only a
minor role in the body fat of most organisms. Surface lipid preparations may be contaminated with
internal lipids during isolation, leading to considerable errors in analytical results. Extensive
analytical results are available for the skin fats of
mammals, the uropygial-gland lipids of birds and
the cuticular lipids of insects, but there are fewer
results for the surface lipids of the reptiles and the
terrestrial arthropods other than the insects.
There have been almost no experimental
investigations of the metabolism of most surface lipids. Conclusions about the synthesis of
the most important components, such as
hydrocarbons, fatty alcohols and alkane diols,
have been mainly drawn by analogy. This is
particularly regrettable as the unique structure
of many of these substances makes their synthesis especially interesting. It must be remembered, however, that most surface lipids are
produced close to their functional sites in epidermal cells or small skin glands, which are difficult to isolate. An exception is the uropygial glands of birds, with the result that they
have become a model for lipid biosynthesis
[133].
15.3.1 Epidermal Lipids of Terrestrial
Vertebrates
In the epidermis of mammals, birds and reptiles,
the innermost layer, e.g. the stratum germinativum of mammalian epidermis, is made up of living cells (keratinocytes) which are similar to other
living cells in the complexity of their lipid content. During keratinization, they are transformed
into the flattened, dead cells of the outer layer,
e.g. the stratum corneum of mammalian epidermis. In the process, lipids are released from the
epidermal cells into the intracellular space and
become an important barrier to water diffusion.
These epidermals lipids include mainly shingolipids of the type O-acyl-glucosylceramide (Fig.
15.13 d) and cholesterol, as well as smaller
amounts of free fatty acids and polar composite
lipids [133, 276, 277]. To date, little is known
about the epidermal lipids of the other terrestrial
vertebrates, i.e. birds and reptiles, but they
15.3.2 Skin Fats of the Mammals
587
appear to be relatively similar to those of the
mammals. Acyl-glucosylsterols were discovered
as a component of chicken epidermal lipids ; they
are probably also present in reptiles and amphibians but not in mammalian epidermis [277]. A
relatively uniform spectrum of glycosylceramides,
cholesterols, free fatty acids and various phospholipids is found in the discarded skin of various
snakes [30, 163].
15.3.2 Skin Fats of the Mammals
The fat substances that can be isolated from
human skin or from the fur of mammals arise
mainly as secretions of the sebaceous glands. In
contrast to the epidermal lipids, the sebaceous
gland lipids show large species-specific differences in both the content of the various lipid
classes (Table 15.10) and the structure of the aliphatic components, i.e. fatty acids, fatty alcohols
and diols. Some mammalian families have uniform lipid spectra, e.g. in the dog (Canidae), cattle (Bovidae) and horses (Equidae), but others
have widely different species-specific spectra, e.g.
raccoons (Procyonidae), squirrels (Sciuridae) and
martens (Mustelidae). The water-repellent properties of the sebaceous lipids are uniform and
cannot explain the structural variety. However, if,
as is now generally accepted, the skin fats play an
important role in chemical communication, even
closely related species should show distinct differences [48]. The lipids of mammalian hair make up
0.6-1.6 % of the fresh weight and consist mainly
of ceramides, cholesterol sulphates and glycosophingolipids [286].
In contrast to their role in the surface lipids of
the arthropods and plants, aliphatic hydrocarbons are not especially important in the surface
lipids of the mammals. A characteristic of humanskin fats is the presence of large amounts of squalene and triacylglycerols, which are otherwise seldom found (Table 15.10). The skin fat of the otter
Lutra canadensis in fact contains 44 % squalene,
which has especially effective water-repellent
properties [49]. Mono- and diester waxes
(Fig. 15.13a-c) are common components of mammalian skin fats. Two types of diester wax can be
distinguished: type I, which contains a hydroxy
fatty acid esterified with an unsubstituted fatty
acid and a fatty alcohol, and type II, which is a
1,2-diol carrying two un substituted fatty acids.
Most mammals, however, have only one of the
two types; the presence of both is seldom seen
(Table 15.11). The skin fats of horses (Equidae)