Traditionally it has been a standard practice to use numerical nomenclature within stratum corneum (SC) ceramide literature. This nomenclature is based on the relative polarity of ceramides separated by high performance thin-layer chromatography (HPTLC) Rf values to classify the ceramide species. However, this practice is fundamentally flawed and is continually undermined as improved separation techniques are revealing ceramides that co-migrate by HPTLC. As previously unrecognized ceramide species are discovered, it has become necessary to rename all the ceramide species according to their new relative Rf values. As it turns out, this numbering system then becomes complicated because it is no longer based on ceramide polarity alone.
The confusion regarding ceramide nomenclature can be eliminated by adopting a nomenclature based on the actual chemical structure of ceramides instead of their chromatographic location. This idea was first suggested some years ago1, 2 and is now widely practiced in the skin barrier literature. SC ceramide chemical nomenclature is based on four molecular structures corresponding to the base chains: sphingosine (S), dihydrosphingosine (DS), 6-hydroxy sphingosine (H), and phytosphingosine (P). The presence of a saturated fatty acid is represented by “N,” an alpha hydroxy group on the acid chain is represented by “A,” an omega hydroxy group is represented by “O,” and an ester linkage by “E”. The complete molecule is named “ceramide-acid-base,” i.e., the acid chain abbreviation precedes the base chain. In the case of an ester link in the acid chain, this is noted first. For example, a 6-hydroxy sphingosine with an alpha hydroxy fatty acid chain would be denoted as “Cer [AH].” A comparison of the old numbering system with the new nomenclature system is given in Figure 2.