Stage-specific expression of Sal-like protein 4 in boar testicular germ cells

Spermatogenesis, the complex process of sperm cell development including mitotic cell division and meiosis, relies on spermatogonial stem cells (SSCs). While markers for developing germ cells have been well investigated in mice, developmental stage-specific markers of germ cells in domestic animals have not been identified. Sal-like protein 4 (SALL4) is known as a putative marker for undifferentiated spermatogonia in rodents; however, its expression in domestic animals has not been investigated. The objective of this study was to characterize the expression of SALL4 in the developmental stages of boar testes and SSCs. Interestingly, all SALL4-expressing cells responded positively to PGP9.5, which is known as a spermatogonia marker in boar testes, while some PGP9.5-positive cells did not express SALL4 in pre-pubertal boar testes. At this stage, the expression of SALL4 was observed in GFRα1-positive cells, and its expression was maintained in cultured pSSCs in vitro, suggesting that SALL4 is a marker of early-stage boar spermatogonia that express GFRα1 in pre-pubertal testes. Additionally, SALL4 expression was observed in c-Kit-positive but not in PGP9.5- or SCP3-positive cells in post-pubertal testes. In conclusion, SALL4 is expressed in early undifferentiated spermatogonia in pre-pubertal boar testes and in primary spermatocytes in post-pubertal boar testes. Therefore, SALL4 can be used as a stage-specific marker of developing germ cells in boar testes.