There is a current trend to delay parenthood, due to sociopolitical and cultural reasons (Khandwala et al., 2017; Mills et al., 2011; Prioux, 2005). The reproductive implications for delayed fatherhood have been consistently undervalued due to the lifelong production of sperm. Nevertheless, the effects of ageing on male fertility are obvious, albeit often disregarded.

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Background: Ageing in men is believed to be associated with fertility decline and elevated risk of congenital disorders for the offspring. The previous studies also reported reduced germ and Sertoli cell numbers in older men. However, it is not clear whether ageing in men with normal spermatogenesis affects the testis and germ cell population dynamics in a way sufficient for transmitting adverse age effects to the offspring. Objectives: We examined men with normal spermatogenesis at different ages concerning effects on persisting testicular cell types, that is the germ line and Sertoli cells, as these cell populations are prone to be exposed to age effects. Material and methods: Ageing was assessed in testicular biopsies of 32 patients assigned to three age groups: (i) 28.8 AE 2.7 years; (ii) 48.1 AE 1 years; and (iii) 70.9 AE 6.2 years, n = 8 each, with normal spermatogenesis according to the Bergmann-Kliesch score, and in a group of meiotic arrest patients (29.9 AE 3.8 years, n = 8) to decipher potential links between different germ cell types. Besides morphometry of seminiferous tubules and Sertoli cell nuclei, we investigated spermatogenic output/efficiency, and dynamics of spermatogonial populations via immunohistochemistry for MAGE A4, PCNA, CREM and quantified A-pale/A-dark spermatogonia. Results: We found a constant spermatogenic output (CREM-positive round spermatids) in all age groups studied. In men beyond their mid-40s (group 2), we detected increased nuclear and nucleolar size in Sertoli cells, indirectly indicating an elevated protein turnover. From the 7th decade (group 3) of life onwards, testes showed increased proliferation of undifferentiated spermatogonia, decreased spermatogenic efficiency and elevated numbers of proliferating A-dark spermatogonia. Discussion and conclusion: Maintaining normal sperm output seems to be an intrinsic determinant of spermatogenesis. Ageing appears to affect this output and might provoke compensatory proliferation increase in A spermatogonia which, in turn, might hamper germ cell integrity.

Delayed family planning and increased parental age increases the risk for infertility and impaired offspring health. While the impact of ageing on oogenesis is well studied, on spermatogenesis this is less understood. Assessing ageing effects on the male germline presents the challenge of differentiating between the effects of ageing-associated morbidities, infertility, and “pure” ageing. However, understanding the impact of ageing on male germ cells requires a separation of age from other factors. In this review we therefore discuss the current knowledge on healthy ageing and spermatogenesis. Male ageing has been previously associated with declining sperm parameters, disrupted hormone secretion, and increased time-to-pregnancy, among others. However, recent data show that healthy ageing does not deteriorate testicular function in terms of hormone production and spermatogenic output. In addition, intrinsic, age-dependent, highly specific processes occur in ageing germ cells that are clearly distinct from somatic ageing. Changes in spermatogonial stem cell populations indicate a compensation for stem cell exhaustion. Alterations in the stem cell niche and molecular ageing signatures in sperm can be observed in ageing fertile men. DNA fragmentation rates as well as changes of DNA methylation patterns and increased telomere length are hallmarks of ageing sperm. Taken together, we propose a putative link between the re-activation of quiescent Adark spermatogonia and molecular changes in aged sperm descending from these activated spermatogonia. We suggest a baseline of “pure” age effects in male germ cells which can be used for subsequent studies in which the impact of infertility or co-morbidities will be studied.

SUMMARYThe 'selfish spermatogonial selection'-model was proposed to explain the paternal age effect (PAE) of some congenital disorders associated with point mutations in male germ cells. According to this, spermatogonia carrying pathogenic mutations gain a selection advantage over non-mutated spermatogonia which leads to an increased number of mutated spermatogonia and consequently spermatozoa over time. Recently, an immunohistochemical approach using the premeiotic marker melanoma antigen family A4 (MAGE A4) was undertaken by the Wilkie group to confirm the presence of microclones of putatively mutated spermatogonia in testes of elderly men. The objective of our study was the age-dependent assessment of testes from men with normal spermatogenesis using MAGE A4 immunohistochemistry to identify and corroborate cellular clusters indicative for 'selfish spermatogonial selection' in our cohort. We analyzed testicular tissues obtained from men with normal spermatogenesis assigned to three age groups [(1) 28.8 AE 2.7 years; (2) 48.1 AE 1 years; (3) 71.9 AE 6.8 years, n/group = 8]. We could detect very similar distribution patterns of MAGE A4-positive cells and the presence of several types of microclusters as reported previously. However, these cellular clusters, indicative for clonal expansion, were not only present in testes from elderly men but also in those from age group 1 and 2. Using graphical three-dimensional modelling, we identified that cross-section directions e.g. longitudinal sections might provoke misleading interpretation of spermatogonial clusters, in particular when the tissue processing is limited. Thus, appropriate fixation and embedding is needed for reliable analysis of testicular sections. We therefore propose a more careful interpretation of such spermatogonial clusters and recommend a 3-D analysis to unequivocally determine 'selfish spermatogonial selection'-manifestations.

Children of older fathers have higher risk for certain diseases. Nevertheless, how ageing specifically affects male germ cells is so far not completely understood. In a cohort of 197 healthy men (18-84 years), we found that semen and reproductive parameters remained normal over six decades. Along with an age-dependent increase in telomere length in sperm (r=0.41, p>0.001), we found accelerated DNA fragmentation, more prominent after the sixth decate of life, and with around 60% of men older than 66 showing abnormal levels of DNA breaks. At the epigenetic level, by whole genome bisulfite sequencing we identified 236 sperm-specific differentially methylated regions between the youngest and oldest group, affecting mostly regions associated with homeobox genes and nervous system development. Therefore, we propose that during ageing, male germ cells are affected by an intrinsic and specific ageing process, distinguishable from the soma. These age-dependent changes might have consequences for fertility and offspring of older men.

Molecular aging markers provide the opportunity for biological age determination in humans and to study factors, such as genetic determinants, affecting the ageing process. In males with Klinefelter syndrome (KS, non-mosaic karyotype 47, XXY), which is the most common sex chromosome aneuploidy, age-related morbidity and mortality are increased, and a significantly reduced life span has been observed. The aim of this study was to investigate whether Klinefelter patients exhibit molecular signs of premature ageing. We studied, specifically, age-associated DNA methylation patterns (by pyrosequencing) and relative telomere length (TL; by quantitative polymerase chain reaction) in blood in a cohort of Klinefelter patients (n=178 and 266 for DNA methylation and TL, respectively) aged 18-71 years and compared them to the data of age-matched healthy male (n = 184 and 196 for DNA methylation and TL, respectively) and female controls (n = 50). Age-associated DNA methylation patterns were not indicative of accelerated ageing in Klinefelter men. Significantly longer telomeres were found in the young Klinefelter subjects aged 18-24 years (mean=1.51 vs. 1.09 and 1.26 in female and male controls, respectively). However, telomere length in subsequent age groups showed no difference to controls. Gonosomal aneuploidy in Klinefelter syndrome is associated with higher baseline TL at adolescent age, but comparable TL with progressive age in other age groups.