Life without cytoplasm: sperm sustain protein production utilizing cytoplasmic droplets and mitochondria as translational apparatuses.

Sperm present a unique paradox: heavily compacted chromatin silences transcription, and loss of cytoplasm during spermiation eliminates the conventional translational apparatus, yet sperm require ∼10 days of protein-demanding epididymal maturation to acquire motility and fertilization competence. How sperm produce the necessary proteins has remained enigmatic. Here, we show that mammalian sperm sustain protein synthesis through a biphasic translational program. In testicular sperm, a residual cytoplasmic droplet retains the full complement of translational machinery, including ribosomes and intact tRNAs, and supports nascent protein synthesis. As sperm enter the epididymis, this droplet is progressively fragmented, and protein synthesis shifts to the midpiece, where mitochondrial translation machinery becomes active. Proteomic and functional analyses indicate that both systems contribute to sperm maturation. Inhibiting cytoplasmic translation in testicular sperm or mitochondrial translation in epididymal sperm abolishes motility. These findings reveal how cytoplasm-free sperm overcome transcriptional silence to complete maturation and support male fertility.