Lipid accumulation and metabolism in polychaete spermatogenesis: Role of the large discoidal lipoprotein.

In most oviparous animals, lipoprotein-mediated lipid transport plays an important role in the nutrient supply for the oocyte. In male gametes, lipids are used as energy substrates in spermatozoa but nothing is yet known about their origin and metabolism throughout spermatogenesis. The lipid profiles analyzed from different stages of male germ cell development in the marine annelid Nereis virens were found to undergo a dramatic change from primary triacylglycerides at the beginning of germ cell development to cholesterol and phospholipids at the end of development as demonstrated by HPLC with evaporative light scattering detection and mass spectrometry. The uptake of a large discoidal lipoprotein into the developing germ cells could be demonstrated by fluorescence labeling and electron microscopic techniques as well as by the presence of a lipoprotein receptor in the germ cells, thus establishing its role in lipid supply. The incorporated lipoprotein discs were found to be stored as intact complexes indicating that they are not readily degraded upon endocytotic uptake. The change in lipid composition during germ cell development reflects their metabolic activity, especially in spermatogonia. The high concentration of lipids maintained by spermatogonia during the early phase of gametogenesis seems to be required for the later rapid processes of meiosis and spermatocyte differentiation. At times when peak demand of lipids arises for membrane synthesis and increased metabolism, this may be met more efficiently by a rapid on-site mobilization of lipids instead of an external supply.