Elongation of fatty acid chains occurs in the endoplasmic reticulum.

This pathway (the microsomal system) converts fatty Acyl-CoA derivative having two carbons more, using malonyl-CoA as acetyl donor and NADPH as reductant and is catalysed by the microsomal fatty acid elongase system of enzymes. Elongation of stearyl-CoA in brain increases rapidly during myelination in order to provide C₂₂ and C₂₄ fatty acids for sphingolipids.