Complex sentence: However, the exact process by which the oligodendrocytes were stimulated to produce myelin was not yet fully understood, but early myelination in the optic nerve has been linked to a rise in the production of various lipids--cholesterol, cerebroside, and sulfatide.
Split into simpler sentences: However, the exact process by which the oligodendrocytes were stimulated to produce myelin was not yet fully understood. Although, early myelination in the optic nerve has been linked to a rise in the production of various lipids--cholesterol, cerebroside, and sulfatide.