Gonocytes, from the Fifties to the Present: Is There a Reason to Change the Name?

Historically, the precursor cells to spermatogonia have been identified as "gonocytes," a term created in the fifties to encompass fetal and neonatal germ cells from the time they become resident in testis primordia to the time they relocate at the basement membrane of the seminiferous cords and differentiate. During this period, spreading over several days in rodents and months in humans, germ cell morphology and central location within the cords remain relatively unchanged. Another common trait is the intensive DNA methylation taking place in fetal to neonatal gonocytes. It is only when they reach the periphery of the cords after birth that germ cells acquire the characteristic appearance of spermatogonia. Studies showed that fetal and neonatal germ cells undergo progressive developmental changes comprising three major phases, a fetal mitotic phase followed by a quiescent period during which most of DNA methylation occurs and a neonatal mitotic phase associated with migration to the basement membrane, morphological changes, and differentiation to spermatogonia. Efforts to associate a distinctive gene expression profile to each of these phases have failed, revealing instead gradual changes in gene and protein expression and the coexistence within each period of unsynchronized cells at different phases of development. In the seventies, the terms pre- or prospermatogonia appeared as alternatives for the term gonocytes, but the definition of these terminologies varied between studies. Thus far, the term gonocyte remains the most commonly used, corresponding to a specific location of the cells, morphological appearance, and functional traits, which are distinct from the prior and subsequent developmental phases. In view of the present knowledge, one could further distinguish gonocyte subsets by the prefixes M, Q, and T, describing, respectively, fetal mitotic, quiescent, and transitional neonatal mitotic/migratory gonocytes, in conjunction with emerging methods allowing better discrimination of these subsets.