ИСТИНА

Spermatogenesis is a complex process during which cells undergo multiple changes including the transition from spermatogonia to spermatocytes, round spermatids and, finally, spermatozoa. At this time the reassembly of the nucleoli is also taking place. Number and size of nucleoli reduce and it undergoes gradual degradation in meotic prophase I. During degradation part of the material migrates into cytoplasm and participates in formation of a specific to male germ cells structure – chromatoid body (CB), which is involved in acrosome formation and mitochondria migration. Almost complete absence of transcriptional activity of late stage spermatozoa’s nuclei is a characteristic feature of spermatogenesis. At this point the chromatin is highly condensed due to substitution of histones with protamines. Multifunctional proteins fibrillarin, nucleolin, nucleophosmin and SURF6 take part in ribosome biogenesis, participating in rDNA transcription and rRNA processing. We have showed that all these proteins were present in multiple nuceoli of spermatogonia. During meiosis in spermatocytes we detect several chaoticlly located structures of irregular shape that contain all four proteins. At the same time we observe the formation of the CB and its marker protein DDX4 colocalizes with SURF6. In round spermatids only small dots containig fibrillarin remain present. Our attention was drawn to the unexpected presence of SURF6 in acrosomes and tails of spermatozoa. Following immunocytochemical and westernblot studies of mature epididymal spermatozoa confirmed our observations. Thus, we demonstrate that SURF6 leaves the nucleoli earlier that other nucleolar proteins and is situated in the CB. It is shown for the first time that this participating in ribosome biogenesis protein SURF6 remains in spemratogenic cells until the stage of mature spermatozoa, which are transcriptionally inactive. The study was supported by the Russian Foundation for Basic Research (grant 18-34-00767).