The pathological effects of SARS-CoV-2 infection on testicular tissue have been studied through molecular tests on deceased COVID-19 patients. The most common are Reverse transcription polymerase chain reaction (RT-PCR), Immunohistochemistry (IHC), and Transmission electron microscopy (TEM). Bian et al. (14) reported during the earlier stages of the pandemic that in deceased patients, SARS-CoV-2 was detected through RT-PCR. Since that study, conflicting evidence on the presence of SARS-CoV-2 in testicular histology has been published. Despite another study also confirming positive SARS-CoV-2 RT-PCR in the testis, three more recent studies reported a negative test in either all or the majority of their subjects (15–18). Masterson et al. (18) hypothesize that their adverse finding conflicted with previous studies due to the method of harvesting testicular tissue. At the same time, they used an open fashion in contrast to previous studies, which used a percutaneous biopsy which may raise concerns for potential contamination. Bian et al. (14) also reported positive IHC staining. However, IHC in the context of SARS-CoV-2 positive participants remains ambiguous. This is because there is no positive control for IHC staining, studies that reported positive findings have not shown a significant correlation within positive SARS-CoV-2 RT-PCR participants, and the molecules targeted in IHC staining have not been consistent throughout all the previous studies (15–18). Despite these higher levels of ACE2, IHC staining has been correlated with worse clinical outcomes regarding testis injury severity (19).

Similarly, reported data on the histopathological effect of SARS-CoV-2 on the testis have been conflicting. While these differences could be secondary to different methodologies in conducting the biopsies and reporting them, other theories should be considered. Bian et al. (14) initially said various degrees of injury to the testis and reduced spermatogenesis. Shortly after that study, another publication reported negative histopathological findings, which the authors attributed to the duration after the infection of which they obtained the biopsy. They hypothesize that the injury to the testis might have occurred at the earlier stage of the disease and have resolved later (15). Other studies with a shorter duration between the onset of symptoms and biopsy showed varying degrees of injury, including interstitial edema, vascular changes, and germ cell loss (16, 17). Finally, Masterson et al. (18) found no injury to testicular tissue in postmortem biopsies of deceased patients following SARS-CoV-2 infection. Their study was the first to conduct the biopsy using an open fashion, contrary to a percutaneous biopsy performed by a previous study. The authors hypothesize that positive findings reported previously could be secondary to contamination during a percutaneous biopsy. Summary for reported studies about impact of SARS-CoV-2 infection on testicular function at ( Table 1 ).

The limited expression of ACE2 in the epididymis has been reported (20). However, highly expressed receptors such as Neuropilin 1 (NRP1) and Cluster of Differentiation 147 (CD147) in the epididymis has been reported (21). Evidence suggest that the expression of the mentioned receptors play a significant role in the entry of SARS CoV 2 to host cells (22, 23). In addition, SARS-CoV-2’s spike protein can bind to epididymal sperm (24). Analysis of the epididymis in deceased COVID-19 patients found many immature spermatocytes and sperm accumulated in the cauda (25). Orchiepididymitis was diagnosed in a pediatric patient with COVID-19 who presented with testis swelling and epididymal inflammation with reactive hydrocele (26). Similarly, La Marca et al. (27) reported COVID-19-induced epididymitis symptoms, such as slight swelling and vascularization accentuation in the epididymis. COVID-19-induced epididymitis presents as a reactional hydrocele with nonuniform echo or microcyst dissemination, resulting in caput augmentation (>1.2 cm) and scrotum incrassation (28).

Prostatic fluid, also known as expressed prostatic secretion (EPS), is a vital component of semen secreted by the prostate gland, and it accounts for around 33% of the volume of ejaculation. Since ACE2 and TMPRSS2 are highly expressed in the epithelium of the human prostate, it is plausible to assume that SARS-CoV-2 could impact the prostate (29). The Androgen Receptor (AR) is vital in managing cellular activities associated with prostate function and physiology. Prostate cancer is directly connected to imbalances in androgen and AR signaling (30). Moreover, new studies suggest that ACE2 could be controlled by AR signaling (31, 32). Behavioral factors like smoking and obesity, as well as comorbidities such as diabetes, hypertension, and alcoholism, are known to affect COVID-19 severity as well as the progression and outcomes of prostate cancer (33–37). The most prominent shared risk factor for prostate cancer and COVID-19 complications and mortality is age, with men over 50 being at higher risk for prostate cancer and more prone to severe outcomes from COVID-19 (38–41). Several studies have demonstrated that androgens can influence the range of immune responses by modifying the behavior of particular immune subsets responsible for removing viruses (42, 43). Studies involving 84 subjects found no evidence of SARS-CoV-2 RNA in the EPS (44, 45).

Over the past few decades, a good amount of evidence supports erectile function as an excellent indicator of systemic health in general and vascular health in particular (46), effects of COVID-19 on the cardiovascular system (i.e., acute cardiac injury, myocarditis) as well growing evidence in the role of endothelial cell dysfunction during COVID-19 infection most importantly, the endothelium expresses the ACE2 led many have hypothesized there may be an increased risk of Erectile dysfunction (ED) following COVID-19 (47).

Three studies have assessed the association between COVID-19 and ED on a population level. Compared men with prior COVID-19 infection to men without documented COVID-19 infection. They all reported that newly diagnosed erectile dysfunction is higher in men with prior COVID-19 compared to age-matched control (47–49).

The pathophysiology behind this association between COVID-19 and ED has been described by a pilot study examined the histopathological features of two cases developing severe erectile dysfunction post-COVID-19 infection which revealed decreased expression of endothelial netric oxide synthase (eNOS), which is consistent with endothelial dysfunction (2). Moreover, this study reported positive spiked Coronavirus-like viral particles in the peri-vascular erectile tissue observed via TEM and Viral RNA was detected in the tissue samples using PCR (2).

A total of 10 observational studies that have been reviewed investigated the impact of SARS-CoV-2 infection and semen parameters (semen volume, sperm concentration, total sperm counts, percentages of total motile and progressively motile spermatozoa, percentage of normal morphology) in semen specimens collected from men who were acutely infected or those who were recovering/recovered from SARS-CoV-2 (1, 13, 45, 50–56), 9 of them reported a significant decline in one or more of the semen parameters in semen specimens of men with active or recent SARS-CoV2 infection in comparison to healthy controls or concerning WHO guidelines (57). However, the results of these studies were inconsistent regarding which semen parameter is affected, as two studies revealed a global decline in all semen parameters (50, 52), while the rest reported the change in only a few specific semen parameters that have been analyzed, Summary reported studies about the impact of COVID-19 on semen parameters are summarized in Table 2 .

Guo et al. (55) revealed that all semen parameters were within normal reference range according to WHO guidelines; what distinguishes this study from the others that have been reviewed is the median interval from confirmation of SARS-CoV2 infection to providing semen samples from study subjects was only 32 days, and 52% of the subjects were still tested positive by pharyngeal swabs. This conflict between the studies suggests that SARS-CoV-2 infection is unlikely to cause semen quality to decline at the onset but rather be delayed with indirect pathophysiology. They considered the duration of human spermatogenesis, which is 78 days (55, 59).

Several theories have been put forward to explain the Impact of SARS-CoV-2 infection on semen quality; one of them is direct invasion and damage of the testicular tissue by the virus since It is known that a broad range of virus families, including human immunodeficiency virus (HIV), mumps virus, influenza, Zika virus, etc., may attack testes and affect male reproductive function (59).

Preliminary studies on SARS-CoV-2 infection had indicated the possibility of SARS-CoV-2 outreach to male gonads, suggesting the role of ACE2 as the cellular receptor for SARS-CoV-2 may be the mechanism for access to the male reproductive organs where ACE2 is predominantly (51). Therefore, theoretically regarded as a vulnerable target to SARS-CoV-2. 8 studies reported the absence of SARS-CoV-2 in all semen specimens collected from men who were acutely infected or those who were recovering/recovered from SARS-CoV-2; these reports suggest that the testis might be not a target organ for SARS-CoV-2 (13, 45, 50, 51, 54, 55, 60, 61). Contrary to these reports, two studies demonstrated SARS-Cov-2 positivity in semen samples. In contrast, one study conducted in China revealed that SARS-CoV-2 was detectable in 6 out of 38 (15.8%) semen specimens collected from male COVID-19 patients, including the patients recovered from the infection (2 out of 23, 8.7%) (58). However, this study did not describe the semen collection or analysis in detail, nor was there evidence of SARS-CoV-2 in the urine of these patients, so the possibility of viral contamination from non-semen sources could not be excluded completely; the other study conducted in Italy reported detection of one positive SARS-CoV-2 genome in semen sample after 21 days after the second negative swab (53). Li et al. (51) suggests that detecting SARS-CoV-2 in the seminal fluid is not necessarily considered an absolute determinant of the impact of SARS-CoV-2 on male fertility and semen quality. And there are multiple other probable indirect mechanisms where SARS-CoV-2 infection could affect semen quality apart from a natural condition. Carlsen et al. (62) investigated the effect of febrile illness on semen parameters during the different phases of spermatogenesis; the study found that sperm concentration, morphology, and motility were significantly affected by fever occurring during the period of meiosis and the postmitotic period (spermiogenesis), but not by fever occurring during mitotic proliferation or after completion of spermiogenesis. An effect on sperm morphology and motility can only be seen when fever occurs during spermiogenesis, where the spermatids undergo morphological changes to sperm and acquire motility. However, the study couldn’t attribute the difference due to fever or the underlying cause of febrile illness (62).

Li et al. (51) investigated levels of proinflammatory cytokines and chemokines in semen samples in recovering COVID-19 men, the study shows increased seminal levels of IL-6, TNF- a, and MCP-1 compared to control males were observed. Although the absence of RNA virus detection was demonstrated by the studies discussed earlier including the aforementioned study. Altered seminal immune markers signifying immune impairment by COVID-19 illness Li et al. (51). This suggests that impaired semen quality among COVID-19 patients could be a result of the immune response in the testis and epididymis in COVID-19 patients.

In addition to these theories, several articles have been attributed the change in semen parameters to hormonal changes (low testosterone levels) (53, 54). The impact of SARS-CoV-2 on axis male reproductive hormonal function will be discussed below in further details.

Testosterone (T) and Follicle-stimulating hormone (FSH) serum levels were lower in infected males and serum Luteinizing Hormone (LH) levels were considerably higher. Also, a significant elevation in serum prolactin (PRL) levels was noted (63). It is important to point out that PRL can be influenced by many factors. Higher PRL levels may suppress the pituitary gland resulting in decreased gonadotropin levels (64). In a follow-up study of men recovering from COVID-19 over seven months, it was found that almost 90% of patients had increased total testosterone (tT) levels after recovery compared to baseline levels. However, further decreased tT levels were observed in 10% of the patients, suggesting persistent hypogonadism. Additionally, 55% of men had tT concentrations suggestive of hypogonadism, especially when comorbid conditions are present (65). Also, Apaydin et al. found that hypogonadism persisted in 48.2% of men with lower T concentrations over a six-month follow-up post-recovery (66). Even after 12 months of recovery, almost 30% of men still had serum T levels consistent with biochemical hypogonadism. Of clinical relevance, the lower the serum T at admission, the poorer the outcomes and the lower the probability of achieving a state of eugonadal, even after a long period of follow-up (67). Summary reported studies about the impact of COVID-19 on male reproductive hormones are summarized in Table 3 .

Furthermore, autopsy analysis from the testicular tissue of patients with COVID-19 found that Leydig cells were significantly reduced in the testicular interstitium, which suggests that SARS-CoV-2 may have caused ultrastructural damage to the cells. ACE2 was diffusely expressed in Sertoli cells and strongly expressed in Leydig cells, as revealed by immunostaining (15).