Expression of paired box protein PAX7 in prepubertal boar testicular gonocytes

Spermatogenesis involves mitosis, meiosis, growth, and differentiation of spermatogonial stem cells (SSCs), which are capable of self-renewal and differentiation into spermatozoa. Markers of spermatogonia and other spermatogenic cells have been extensively studied in rodents, whereas physiological characteristics and stage-specific markers of germ cells remain largely unknown in large domestic animals. In rodents, paired box protein 7 (PAX7) is known to be a specific marker of a rare spermatogonial subpopulation in adult testes, while being expressed by a large proportion of neonatal testicular germ cells. However, the expression of PAX7 has not yet been investigated in domestic animals. The objective of this study was to characterize PAX7 expression during boar testis development and in in vitro cultured porcine SSCs (pSSCs). Notably, the expression of PAX7 was positively correlated with that of a known boar testis spermatogonial and gonocyte marker, protein gene product 9.5 (PGP9.5), in prepubertal (5-day-old) boar testes but was not observed during or following puberty. Furthermore, the early-stage spermatogonial markers GDNF family receptor alpha-1 (GFRα1) and Sal-like protein 4 (SALL4) were coexpressed in PAX7⁺ testicular cells from 5-day-old boars. PAX7 expression was also maintained in in vitro cultured undifferentiated porcine spermatogonia, with both PAX7 and PGP9.5 strongly expressed in pSSC colonies but not in feeder cells (testicular somatic cells). These data demonstrated that PAX7 expression only occurred in boar testes during prepuberty and was mainly restricted to very early-stage spermatogonial germ cells, such as gonocytes, which implies that PAX7 can be used as a boar gonocyte marker.