Human spermatogonial stem cells and their niche in male (in)fertility: novel concepts from single-cell RNA-sequencing

Abstract

The amount of single-cell RNA-sequencing (scRNA-seq) data produced in the field of human male reproduction has steadily increased. Transcriptional profiles of thousands of testicular cells have been generated covering the human neonatal, prepubertal, pubertal and adult period as well as different types of male infertility; the latter include non-obstructive azoospermia, cryptozoospermia, Klinefelter syndrome and azoospermia factor deletions. In this review, we provide an overview of transcriptional changes in different testicular subpopulations during postnatal development and in cases of male infertility. Moreover, we review novel concepts regarding the existence of spermatogonial and somatic cell subtypes as well as their crosstalk and provide corresponding marker genes to facilitate their identification. We discuss the potential clinical implications of scRNA-seq findings, the need for spatial information and the necessity to corroborate findings by exploring other levels of regulation, including at the epigenetic or protein level.

Identification of two hidden clinical subgroups among men with idiopathic cryptozoospermia

“…First insights pointing to the stage of germ cell differentiation failure in cryptozoospermic testes were provided by single cell RNA-sequencing data ( Di Persio et al , 2021 ; Di Persio and Neuhaus, 2023 ). These revealed that the proportions of spermatogonia and early meiotic stages were still comparable to the control situation.…”

Human spermatogonial stem cells retain states with a foetal-like signature

“…Furthermore, in a previous study applying hematoxylin-staining to human seminiferous tubules, many cells were found to have an intermediate morphology, simultaneously resembling both A pale (by staining lightly with hematoxylin) and A dark (possessing a non-staining nuclear rarefaction zone) 13 . There is now an emerging consensus that the undifferentiated fraction of human spermatogonia can be sub-classified into at least four subtypes (reviewed in 14 and based largely on the work of two research groups 15,16 ), each stably maintained throughout reproductive life: state 0 ( EGR4 + PIWIL4 + TSPAN33 +), state 0A ( FGFR3+ ), state 0B ( NANOS2 +), and state 1 ( GFRA1 + NANOS3 +). Among these, state 0 is considered the most undifferentiated, standing at the apex of the differentiation trajectory.…”

Favorable culture conditions for spermatogonial propagation in human and non-human primate primary testicular cell cultures: a systematic review and meta-analysis

“…Within cultures of primary testicular cells, spermatogonia morphologically appear as relatively small, round cells, initially floating within the culture medium or connected to the attached endogenous somatic cells, in time progressing to colonies with individually visible cells ( Sadri-Ardekani et al, 2009 ; Mirzapour et al, 2017 ). The manner of verifying the presence of these SSCs in propagation cultures by use of protein or gene markers is hindered by the current lack of an unambiguously established SSC marker for human culture ( Di Persio and Neuhaus, 2023 ). Therefore, a wide array of spermatogonial markers for PCR or immunofluorescence is used by researchers in an attempt to characterize the cultured cells, though these markers are often not specific for just one cell type and some markers have since been disputed due to their observed presence in somatic cells as well ( Kossack et al, 2013 ; Struijk et al, 2020b ).…”