Mammalian spermatogenesis is a complex and highly coordinated process that drives a stem cell through meiosis and cell differentiation to form the specialised genetic cargo and shape of the spermatozoon. One of the critical features of this process is the sperm-specific packaging of DNA, that sees the normal histone organisation replaced with protamines, leading to an almost crystalline compaction of the genome and loss of transcription. To overcome this, haploid spermatids produce the necessary mRNA and hold these transcripts for up to a week prior to protein translation. While the processing and localisation of individual/total mRNA has been characterised at some stages of development, an overall picture of mRNA distribution throughout spermatogenesis has not been described.
Using RNA-FISH we have shown the localisation of mRNA during both murine and human spermatogenesis. The abundance of mRNA transcripts begins to increase in spermatocytes which subsequently form discrete foci, known as chromatoid bodies, in the late pachytene stage. Following this, the localisation of mRNA in spermatids changes dramatically during their 16 steps of development. In earlier stages, the mRNA present within nuclear speckles and colocalises in the cytoplasm with DDX4, a highly conserved RNA helicase that is critical for spermatogenesis. As the spermatid nuclei begin to elongate (steps 8-12) the cytoplasmic staining of mRNA is significantly reduced, however nuclear mRNA staining remains within the speckles. Finally, prior to the release of sperm into the seminiferous lumen, there is very little nuclear mRNA staining present.
It is likely that the tight spatial regulation of mRNA is a critical component of spermatogenesis, given that several knockout models affecting its localisation in the testis are infertile (DDX4, MIWI, PIWI). By proving a comprehensive map of the dynamics, we hope to allow easier identification of spermatogenetic disorders that disrupt normal mRNA biology.