Glycero- versus sphingo-phospholipids: correlations with human and non-human mammalian lens growth

The human lens differs from other mammalian lenses in its very slow growth and unusual phospholipid composition of its cell membranes. Dihydrosphingomyelins (DHSMs) make up about half of all phospholipids in adult human fiber membranes. In all other membranes, sphingomyelins(SMs) with a trans double bond in their backbone, are prevalent. In our quest to understand the biological implications of such elevated DHSM levels, we analyzed membranes from various regions of human, elephant, giraffe, polar bear, pig and cow lenses. The levels of DHSMs were minor in non-human lens membranes. A strong correlation was observed between growth rate and relative contents of phosphatidylcholines(PCs) in epithelia and outer cortical fibers. Sphingomyelins became increasingly predominant in differentiated fibers and this increase was age dependent. Indeed, nuclear fiber membranes of aged non-human mammals were composed, almost exclusively, of (SMs). Although human lens membranes followed comparable compositional trends, the magnitude of the changes was much smaller. We postulate that the high relative contents of DHSMs provide a biochemically inert matrix in which only small amounts of PCs and SMs and their metabolites, known to promote and arrest growth, respectively, are present. This compositional difference is proposed to contribute to the slow multiplication and elongation of human lens cells.