Functional heterogeneity of spermatogenesis revealed by >27,000 single cells of mouse testis
Qianyi Ma1,3, Christopher D. Green1,3, Jun Z. Li1, 2*, and Sue Hammoud1,*
1Department of Human Genetics, University of Michigan, Ann Arbor, 2Department of Computational Medicine and Bioinformatics
Spermatogenesis is a well-organized and tightly regulated biological process composed of three distinct biological activities: 1) Continuous stem cell self-renewal and the expansion of progenitor cells by mitosis, 2) the production of haploid cells from diploid progenitor cells by meiosis, and 3) the orderly differentiation of haploid cells into spermatozoa (spermiogenesis). The transition between these developmental stages is precisely timed, and is dependent upon both germ cell intrinsic programs and extrinsic factors secreted by supporting cells (e.g. Sertoli, Leydig, Myoid and Immune cells). In past studies, the spatial organization of these cells across the seminiferous tubule has been determined histologically, and their functional contribution to spermatogenesis has been examined through the purification of predefined cell subpopulations, followed by gene expression profiling using microarrays or bulk RNA-seq. Despite the invaluable insights gained from these approaches, our understanding of the functional heterogeneity of cell populations and their interactions required for spermatogonial stem cells to develop into mature sperms in vivo is limited. To overcome these limitations, we have obtained gene expression profiles from ~27,500 single cells of the mouse testis. Unbiased clustering analyses of these data successfully revealed up to 18 distinct subpopulations, including those corresponding to known and/or rare interstitial, Sertoli, and germ cell populations. The rare populations have been further characterized by smFISH. Lastly, unsupervised ordering of the discovered cell populations reflected the differentiation trajectory from spermatogonia to spermatocytes, and to round and mature sperm. Taken together, these findings provide a molecular portrait of major cell types in the testis at an unprecedented resolution.The cell type markers developed from this resource will be valuable for studying and cell-cell interactions and their spatiotemporal patterns during different stages of spermatogenesis.