Fig. 2. Distinctive structures of skin ceramides. Sphingosine (1b,2b) or phytosphingosine (3b,4b) backbone were linked to a long-chain fatty acid (typically C₃₀;1a, 3a) or has a terminal hydroxy group (2a,4a) which esterified with linoleate (2c) or 2-hydroxy acid (4c).