Fig.1.

Energy metabolism of oocytes. Oocyte metabolism relies on glucose metabolites provided by cumulus cells. The majority of glucose is metabolized in cumulus cells through anaerobic glycolysis, resulting in lactate production. Cumulus cells can convert glucose into pyruvate, lactate, or nicotinamide adenine dinucleotide phosphate (NADPH) through anaerobic glycolysis and the pentose phosphate pathway. These metabolites are then transferred to oocytes through paracrine signaling and gap junctions, providing energy substrates for oocyte metabolism. Oocytes generate ATP through the tricarboxylic acid (TCA) cycle and oxidative phosphorylation (OXPHOS). Additionally, some glucose can be directly transported to oocytes and metabolized through the pentose phosphate pathway and hexosamine synthesis pathway. Oocytes acquire free fatty acids from the follicular fluid and gap junctions with cumulus cells, and they can also synthesize fatty acids endogenously. After entering the cells, free fatty acids can be converted and stored in lipid droplets or enter mitochondria for β-oxidation. Fatty acids in lipid droplets are esterified and stored as neutral triglycerides (TAGs). Fatty acyl-CoA is synthesized by acyl-CoA synthetases, which catalyze triglycerides into fatty acyl-CoA. Carnitine transports fatty acyl-CoA to mitochondria. The TCA cycle and OXPHOS in mitochondria process fatty acids into acetyl-CoA that is then oxidized, producing ATP once they enter the mitochondrial matrix. Glutamine enters oocytes through the follicular fluid and gap junctions, and oocytes can also synthesize glutamine. Glutamine is metabolized in the mitochondrial matrix as a fuel source for the cycle. Oocytes may possess the ability to convert Adenosine monophosphate (AMP) to ATP through the adenosine salvage pathway. Cumulus cells can also produce ATP through the adenosine salvage pathway and directly supply ATP and AMP to oocytes through gap junctions