Monogalactosyl diglyceride is a minor galactolipid present in oligodendrocytes and myelin, as well as comprises a major fraction of the lipids in photosynthesizing plants and green microorganisms. [1][2] It is used as a marker for myelination and has also been shown to stimulate PKCα activity in oligodendrocytes. [2]

Reference:
[1]. Schmidt-Schultz, T., and Althaus, H.H. Monogalactosyl diglyceride, a marker for myelination, activates oligodendroglial protein kinase C. Journal of Neurochemistry 62(4), 1578-1585 (1994).
[2]. Rosenberg, A., Gouaux, J., and Milch, P. Monogalactosyl and digalactosyl diglycerides from heterotrophic, hetero-autotrophic, and photobiotic Euglena gracilis. Journal of Lipid Research 7(6), 733-738 (1996).