The lipid secretion of the meibomian glands

Lipid synthesis in vitro by rabbit Meibomian gland tissue and its inhibition by tetracycline

Rabbit Meibomian gland tissue was incubated with radioactive acetate, propionate, valine, leucine or isoleucine. The time-course of incorporation of radioactivity into total lipids from acetate and isoleucine was studied in Hanks' or Krebs medium. Incorporation of acetate or isoleucine into lipid classes was followed by TLC, and of acetate label into individual fatty acids and alcohols by GLC with radioactivity detection. Radioactivity was highest in the minor acid components C16:1 and C18:1. Levels in branched fatty acids fell with chain length, reflecting the time-course of successive chain elongations. Use of amino acids or propionate as precursors suggested that molecules containing a specific branched structure were preferentially incorporated; this indicates a binding preference at the level of the fatty acid synthase which might explain the very high proportion of anteiso-branched structures in the rabbit secretion. Incorporation of label into total lipids was significantly reduced by tetracycline in the medium, with 50% reduction at about 1.5 mg/ml for acetate and 0.7 mg/ml for isoleucine.

Identification by gas chromatography/mass spectrometry of long-chain fatty acids and alcohols from hamster meibomian glands using picolinyl and nicotinate derivatives

Meibomian secretions from the hamster were hydrolysed with base and examined as TMS, [2H9]TMS, methyl ester/TMS, picolinyl ester/TMS and nicotinate/TMS derivatives by capillary GC/MS. Over 90 compounds, representing over 89% of the hydrolysed fraction, were identified. Fatty acids with chain lengths from 10 to 32 carbon atoms were found, the most common of these were in the C15 to C18 and in the C25 to C30 regions. Chain types were predominantly iso or anteiso branched, mono-unsaturated (C16 and C18) and straight. Fatty alcohols were mainly from the iso or anteiso series and tended to have longer chain lengths; the major alcohols had anteiso-25 and 27 and iso-26-chains. In these respects the secretions were similar to those reported earlier from other species, although fewer mono-unsaturated compounds with longer chains (C20 to C30 region) were found than in the rat and human. The steroid fraction was characterized by a larger number of compounds than normally present in secretions of this type. The major compound was cholesterol, in common with that in all other examined species except the rabbit.

Long-chain fatty acids and alcohols from gerbil meibomian lipids

Capillary gas chromatography-mass spectrometry was used to identify constituent fatty acids, alcohols and steroids from gerbil meibomian glands. Over 80 compounds representing about 90% of the total fraction were identified. The major steroid was cholesterol accompanied by a lower concentration of 3 beta-hydroxy-5 alpha-cholestane. Fatty acids with chain lengths from 12 to 27 carbon atoms were present; they had predominantly straight, iso or anteiso chains with major concentrations in the C15-C18 and C25-C27 regions. Unsaturated acids had mainly 16 and 18 carbon atoms. The fatty alcohols were mainly branched-chain with the majority of compounds having chain lengths of 25-27 carbon atoms. Several alcohols, both branched and unsaturated, were found with chains of up to 33 carbon atoms long. The profile was similar to that found earlier in other species, but with lower concentrations of mono-unsaturated compounds than were found in rats and humans. Di-hydroxy compounds, on the other hand, tended to be more abundant although still of low relative concentration.