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Neonatal, Adult Testes Cell Populations Characterized by Single-Cell RNA Sequencing

NEW YORK (GenomeWeb) – Using single-cell RNA sequencing, a team from the US, the Netherlands, and Italy has characterized the cell populations in adult and neonatal testes tissues, providing a look at the different cell types and the timing behind testes development.

The researchers used scRNA-seq, immunofluorescence, immunohistochemistry, and fluorescence-activated cell sorting to assess thousands of cells from testicular biopsy samples obtained from two fertile men — a 37-year-old and a 42-year-old — during vasectomy reversal procedures. They compared those results to RNA sequences from individual cells in infant testes samples, taken from two-day-old and week-old newborns that died of non-testes-related conditions.

In addition to clustering the testicular cells based on their expression features, the team went on to compare cell types in the adult and neonate testes, retracing their developmental trajectories from primordial germ cells in embryos to primordial germ cell-like cells and pre-spermatogonia cells in neonates and spermatogonial stem cells in adult testes.

"We map the timeline of male germ cell development from [primordial germ cells] through fetal germ cells to differentiating adult [spermatogonia] stages," senior and corresponding author Miles Wilkinson, an obstetrics, gynecology, and reproductive sciences researcher at the University of California at San Diego, and his colleagues wrote in a study they published online today in Cell Reports.

The researchers identified four undifferentiated spermatogonia clusters in the adult testes, for example, along with potential markers for finding primordial germ-like cells as well as the most primitive forms of undifferentiated cells in adults, which are enriched for spermatogonial stem cells.

"Our data provide a blueprint of the developing human male germline and supporting somatic cells," Wilkinson wrote, noting that "[primordial germ-like cells] and [spermatogonial stem cell] markers are candidates to be used for [spermatogonial stem cell] therapy to treat infertility."

Using 10x Genomics Chromium cell capture and library prep, in combination with Illumina RNA sequencing, the researchers analyzed 18,723 adult testes cells and 14,862 neonatal testes cells.

"Although much has been learned about these various steps in rodents," the authors explained, "we are only just beginning to unravel male germ cell development and spermatogenesis in humans."

With these and other data, the team found and characterized five neonatal cell clusters and more than half a dozen cell clusters from adult testes, including Leydig cells that prompt sperm development to spermatocytes, which form from the most differentiated spermatogonia cells. In adults, for example, the results pointed to at least four clusters of the latter spermatogonia cells, dubbed SSC-1, SSC-2, early-differentiating SPG, and differentiating-SPG populations, along with transition cells that assist with their differentiation.

For their subsequent analyses, the researchers took a closer look at the expression profiles, predicted functional features, and discovered potential cell type markers in the adult and neonatal datasets. They uncovered key similarities and differences found at these different stages of testes development and developed preliminary approaches to enrich for specific cell types.

"Our identification of genes expressed in testicular somatic cells, including those encoding cell-cell signaling factors, may prove useful in developing cocktails containing 'testes niche' factors that allow for the propagation and expansion of human [spermatogonical stem cells] in vitro for clinical application in vivo," the authors concluded.

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