Mouse minipuberty coincides with gonocyte transformation into spermatogonial stem cells: a model for human minipuberty

Abstract

As the transient postnatal hormone surge in humans, known as 'minipuberty', occurs simultaneously with key steps in germ-cell development, we investigated whether similar changes occur in the hypothalamic-pituitary-testicular axis of neonatal mice at a time that would coincide with gonocyte transformation into spermatogonial stem cells (SSC). Serum and testes were collected from C57Bl/6 mice at embryonic Day 17 (E17), birth (postnatal Day 0; P0) and daily until P10. Serum FSH and testosterone levels in both serum and testes were analysed and gene expression of FSH receptor (Fshr), luteinising hormone receptor (Lhr), anti-Müllerian hormone (Amh), octamer-binding transcription factor 4 (Oct-4), membrane type 1 metalloprotease (Mt1-mmp), proto-oncogene C-kit and promyelocytic leukaemia zinc finger (Plzf ) was quantified by real-time polymerase chain reaction. We found a transient surge of serum and testicular testosterone levels between P1 and P3 and a gradual increase in FSH from P1 to P10. Testis Lhr expression remained low from P0 until P10 but Fshr expression peaked between P3 and P6 (P<0.01). The same was found for Oct-4 expression (a gonocyte marker), which surged between P3 and P6 (P<0.01). Mt1-mmp expression peaked at P3 (P<0.05). The expression pattern of both C-kit and Plzf (SSC markers) was similar with a steady increase from P1 to P10. These results show a transient activation of the hypothalamic-pituitary-testicular axis postnatally with increases in serum and testicular testosterone at P1-P3 and testicular Fshr (but not Lhr) at P3-P6. These changes coincide with increases in gene expression of Oct4, Mt1-mmp, Plzf and C-kit, reflecting gonocyte activation, migration and transformation into SSC. In conclusion, these findings suggest that 'minipuberty' does occur in mice and that gonocyte transformation may be driven by a transient FSH signalling pathway.

Bcl2-Associated X (BAX) Knockout in Mice Resulted in Persistence of Neonatal Testicular Germ Cells (Gonocytes)

“…The potentially important role for apoptosis in eliminating persistent gonocytes from the testicular tubules is relevant to understanding the potential cause of testicular cancers in men with a history of cryptorchidism. In humans, it is known that gonocyte transformation occurs in the first few months after birth during minipuberty, similar to mice [Li et al, 2017], and that the total number of germ cells in the testis halves by 6-12 months of age [Hadziselimovic and Herzog, 2001;Hutson and Hasthorpe, 2005;Hutson et al, 2012Hutson et al, , 2013Hutson et al, , 2015. It has been assumed that this loss of germ cells is caused by apoptosis, but this has been hard to confirm in the human.…”

“…At birth, the testis in both humans and rodent models contains undifferentiated germ cells, known as gonocytes. At about 2-6 months in humans and 2-6 days in mice [Li et al, 2017], the gonocytes migrate from the centre of the testicular tubules to the periphery and come in contact with the basement membrane (BM), transform into type A spermatogonia, which represent the undifferentiated spermatogonial stem cells (SSC) [Hadziselimovic and Herzog, 2001;Phillips et al, 2010;Li et al, 2015b;Busada and Geyer, 2016] to establish the initial pool of SSC. Only about half of the gonocytes transform, and the remaining cells are thought to undergo apoptosis [Hutson et al, 2012].…”

“…Only about half of the gonocytes transform, and the remaining cells are thought to undergo apoptosis [Hutson et al, 2012]. Both the transformation and apoptosis are likely to be regulated by a short burst of gonadotrophin secretion (follicle-stimulating hormone and luteinising hormone), now known as "minipuberty" (human and mouse) [Hadziselimovic and Herzog, 2001;Li et al, 2017]. Neonatal germ cell loss is more significant in undescended testes (UDT) than in normally descended testes, especially in unilateral UDT [Cortes et al, 2007;Hadziselimovic et al, 2007;Hutson et al, 2012;Li et al, 2014].…”

“…However, there is evidence that immature germ cells persisted in UDT after minipuberty, suggesting these germ cells failed apoptosis [Li et al, 2019]. Lack of a suitable animal model has hindered investigation of early germ cell development, but the recent recognition of minipuberty in mice has enabled studies to examine this crucial stage of germ cell development experimentally [Li et al, 2017].…”

Minipuberty of human infancy – A window of opportunity to evaluate hypogonadism and differences of sex development?

“…54) In addition, recent rodent studies revealed that minipuberty also occurs in mice and that gonocyte transformation is influenced by the transient FSH signaling pathway. 55) Thus, minipuberty is not limited to the human neonate but is observed in other species. However, humans differ compared to other species in their much longer duration of the childhood period spanning the interval between minipuberty and puberty.…”