Cryptorchidism and infertility: what do we know so far?

Abstract

Cryptorchidism is recognized as a significant risk factor for male germ cell tumors and infertility, with a complex and multifaceted mechanism contributing to male infertility. When the testes fail to descend into the scrotum, increased local temperature and pressure lead to increased apoptosis of spermatogenic and Sertoli cells. Additionally, disruptions in the hypothalamic–pituitary–gonadal axis result in decreased testosterone levels within the testes, and abnormal secretion of follicle-stimulating hormone and luteinizing hormone, negatively impacting spermatogenesis. Cryptorchidism also induces increased oxidative stress within the testes, leading to sperm DNA damage and impairment of the sperm plasma membrane, hindering sperm–oocyte fusion. Unilateral cryptorchidism may cause injury to the ipsilateral genitofemoral nerve, further affecting the contralateral testis by increasing oxidative stress and apoptosis. Moreover, the production of antisperm antibodies can trigger autoimmune responses, potentially damaging germ cells and contributing to infertility. Damage to type A dark spermatogonia (type Ad spermatogonia) is also considered a high-risk factor for male infertility. Understanding the mechanisms by which cryptorchidism leads to male infertility may provide new avenues for enhancing fertility in affected patients.

INTRODUCTION

Cryptorchidism is a common developmental anomaly of the male reproductive system, with an incidence of approximately 3.4%–5.8% in full-term infants and as high as 45.3% in premature infants.1,2 The occurrence of cryptorchidism is closely associated with various factors, including abnormalities or absence of the gubernaculum, decreased testosterone (T) secretion due to endocrine dysfunction, family history, high-risk environmental exposures during pregnancy, and conditions such as prematurity or low birth weight.3,4,5 Research indicates that cryptorchidism leads to increased apoptosis of testicular cells, elevated oxidative stress, and endocrine alterations.3,6 Additionally, cryptorchidism may damage the genitofemoral nerve, increase antisperm antibodies, and disrupt type A dark spermatogonia (type Ad spermatogonia), all of which negatively impact male fertility (Figure 1).7,8,9,10

Figure 1.

Mechanism of male infertility caused by cryptorchidism. ABP: androgen-binding protein; type Ad spermatogonia: type A dark spermatogonia; Bax/Bcl-2: Bcl-2-associated X protein/ B-cell lymphoma 2; BTB: blood–testis barrier; CGRP: calcitonin gene-related peptide; FADD: Fas-associating protein with a novel death domain; Fas/FasL: factor associated suicide/Fas ligand; FSH: follicle-stimulating hormone; GFN: genitofemoral nerve; HPG: hypothalamic–pituitary–gonadal; INH B: inhibin B; LH: luteinizing hormone; LHRH: luteinizing hormone-releasing hormone; ROS: reactive oxygen species; T: testosterone; 8-OHdG: 8-hydroxy-2’-deoxyguanosine; nDNA: nuclear DNA; mtDNA: mitochondrial DNA. Created with BioGDP.com.

Early detection and treatment of cryptorchidism are crucial for preserving fertility and preventing tumor development.11,12 Current treatments for cryptorchidism primarily include endocrine therapy and surgical intervention.2,13 However, the optimal timing for surgical treatment, the necessity of postoperative hormone supplementation, and the exploration of alternative treatment options remain the subjects of ongoing investigation.13,14,15 This review aims to delve into the mechanisms by which cryptorchidism leads to infertility, providing a comprehensive understanding for future research in this area (Table 1).

Table 1.

Mechanisms of male infertility caused by cryptorchidism

Factor	Mechanisms	Reference
Apoptosis
High temperature	Spermatogonia: Bax/Bcl-2 activates the mitochondrial apoptotic pathway, increasing permeability, releasing cytochrome C, and recruiting procaspase-9 to activate caspase-3 and initiate apoptosis	24–26,75–80
Elevated external pressure	Spermatogonia: Bax/Bcl-2 mediates mitochondrial apoptotic pathway	29
	Sertoli cell: Fas binding to FasL activates FADD, subsequently activating caspase-8 and caspase-3, initiating in apoptosis	30
Endocrine changes
HPG	Decreased secretion of T, ABP, and INH B inhibits sperm production and disrupts the secretion of LHRH, FSH, and LH via the HPG axis	32–35,38,41,81–89
Oxidative stress
Sperm DNA damage	Nuclear DNA: ROS induces single- and double-strand breaks, DNA fragmentation, base modifications, and cross-linking within sperm DNA. These damages can either suppress or induce gene transcription, activate signaling pathways, accelerate telomere erosion, cause replication errors, lead to genomic instability, and promote GC-to-TA transitions	37,44–46,48–50,90–94
	Mitochondrial DNA: oxidative stress damages exposed mitochondrial DNA, resulting in deletions, variations, and mutations	37,44–46,48–50,90–95
Lipid peroxidation in sperm plasma membrane	Oxidative stress targets sperm membrane’s unsaturated fatty acids, causing lipid peroxidation and a chain reaction that loses up to 60% of membrane fatty acids, thereby disrupting membrane properties and reducing sperm viability and oocyte fusion	53,54,96,97
Germ cell apoptosis	ROS-induced DNA damage may accelerate the apoptotic process in germ cells	55
GFN damage
Contralateral testis in unilateral cryptorchidism	In unilateral cryptorchidism, sustained GFN stimulation reduces CGRP secretion, lowering local blood flow and affecting the nutrition and blood supply of the affected testis, with consequences for the contralateral testis	59,60,98,99
Antisperm antibodies
Sperm	Disruption of BTB leads to the production of antisperm antibodies, which bind to sperm and interfere with sperm function, affecting motility and ability to bind with the oocyte	62–66,100–102
Type Ad spermatogonia damaged
Type Ad spermatogonia	A reduction in type Ad spermatogonia directly decreases sperm production and disrupts the spermatogenesis cycle, affecting sperm quality and quantity	69,70,103,104

ABP: androgen-binding protein; type Ad spermatogonia: type A dark spermatogonia; Bax/Bcl-2: Bcl-2-associated X protein/B-cell lymphoma 2; BTB: blood–testis barrier; CGRP: calcitonin gene-related peptide; FADD: Fas-associating protein with a novel death domain; Fas/FasL: factor associated suicide/Fas ligand; FSH: follicle-stimulating hormone; GC-to-TA transitions: guanine-cytosine to thymine-adenine transitions; GFN: genitofemoral nerve; HPG: hypothalamic–pituitary–gonadal; INH B: inhibin B; LH: luteinizing hormone; LHRH: luteinizing hormone-releasing hormone; ROS: reactive oxygen species; T: testosterone

APOPTOSIS

Spermatogenesis refers to the process through which spermatogenic cells develop into spermatozoa in a conducive environment, involving growth, division, and differentiation.16 During this process, Sertoli cells play a crucial role by providing structural and nutritional support to spermatogenic cells while regulating hormone levels and fluid balance within the testes.16 Moreover, Sertoli cells contribute to reproductive function by secreting factors such as inhibin B (INH B). The proper growth, division, and differentiation of testicular cells are essential for successful spermatogenesis.17 However, studies have shown that cryptorchidism induces apoptosis within the testes, particularly affecting spermatogenic and Sertoli cells.18,19 This apoptosis directly disrupts the spermatogenic process, resulting in reduced sperm production and an elevated rate of abnormal sperm, leading to male infertility.20 Therefore, understanding the impact of environmental factors on spermatogenesis is crucial for enhancing male reproductive health.

Apoptosis is primarily mediated through two classic pathways. The first, known as the intrinsic pathway, involves the mitochondria and is characterized by the downregulation of Bcl-2-associated X protein/B-cell lymphoma 2 (Bax/Bcl-2), loss of mitochondrial membrane potential, release of cytochrome c from permeabilized mitochondria, and the subsequent activation of caspase-9, which in turn activates caspase-3 to induce cell death. The second pathway is the extrinsic apoptotic pathway, where death receptors bind to their ligands, such as factor associated suicide/Fas ligand (Fas/FasL), leading to the activation of caspase-8 and subsequently caspase-3, thus triggering apoptosis.21 Understanding these apoptotic mechanisms is essential for developing strategies to mitigate the adverse effects of cryptorchidism on male fertility.

High temperature

Spermatogenesis requires optimal temperatures, with the temperature within the scrotum typically 2°C–6°C lower than core body temperature.22 The occurrence of cryptorchidism results in the retention of the testes within the abdominal cavity or inguinal canal, exposing them to relatively higher environmental temperatures. Research indicated that elevated temperatures can induce apoptosis in germ cells, with sperm production capability decreasing by approximately 14% for every 1°C increase in temperature.23 Recent studies further confirmed that high temperatures lead to testicular damage, germ cell apoptosis, and functional impairment. For instance, Jorban et al.24 investigated the effects of 35°C and 37°C on spermatogenic and Sertoli cells extracted from immature mouse seminiferous tubules. The results revealed that increased temperature led to a pronounced decline in cell viability (P = 0.0002) and a rise in apoptosis (P = 0.0001). Additionally, spermatogenesis-related precursor cells, along with meiotic and post-meiotic populations, showed significant reductions in both number and gene expression (P < 0.05).24

In a study involving a surgically constructed bilateral cryptorchidism model in rabbits, surgical cryptorchidism resulted in a significantly higher surface temperature of testes compared with control testes (mean±standard deviation [s.d.]: 38.02°C ± 0.36°C vs 36.15°C ± 0.64°C, P < 0.05), accompanied by a markedly elevated apoptosis index of germ cells (mean±s.d.: 89.69% ± 3.76% vs 7.73% ± 4.95%, P < 0.05). This cryptorchidism significantly increased the expression of the pro-apoptotic protein Bax, while the expression of the antiapoptotic protein Bcl-2 was decreased, suggesting that the increased temperature in the inguinal region induced germ cell apoptosis.25

Further molecular investigations have explored the negative impacts of high temperature on germ cells in a mouse model of unilateral cryptorchidism. The cryptorchid model was established by surgically relocating the left testis of mice into the abdominal cavity. Testicular weight, morphological changes, and protein marker expression were examined at different postoperative time points (3 days, 5 days, 7 days, 14 days, 21 days, and 28 days). There was a significant decline in the weight of cryptorchid testes at 21 days and 28 days post-surgery (both P < 0.05), along with increased expression of the apoptosis executioner cleaved caspase-3 (P < 0.05).26 Cell–cell adhesion molecules, including cell adhesion molecule 1 (Cadm1), nectin cell adhesion molecule 2 (Nectin-2), and nectin cell adhesion molecule 3 (Nectin-3), are critical for maintaining the structural integrity of the seminiferous epithelium by mediating adhesion between spermatogenic and Sertoli cells. Extending these observations, the investigators demonstrated that the elevated temperature induced a downregulation of Cadm1 expression, concomitant with alterations in the expression patterns of Nectin-2 and Nectin-3. These changes disrupted the adhesion between spermatocytes and Sertoli cells, ultimately resulting in impaired spermatogenesis, characterized by reduced production of functional sperm and a corresponding increase in the proportion of abnormal sperm.26

Elevated external pressure

In addition to elevated temperatures, studies have shown that cryptorchidism can lead to increased pressure within the inguinal canal, exceeding normal atmospheric pressure by approximately 28.15 cmH₂O, equivalent to 20.71 mmHg.27,28 This increased temperature results in external pressure being higher than that within the scrotum. Such elevated external pressure can promote apoptosis in spermatogenic cells by upregulating factors associated with the intrinsic apoptotic pathway, including Bax/Bcl-2, caspase-9, and caspase-3.29 This external pressure also activates the Fas/FasL signaling pathway, leading to the upregulation of caspase-8 and caspase-3, which in turn promotes apoptosis in Sertoli cells. This process significantly reduces the secretion of functional proteins, such as androgen-binding protein (ABP) and INH B, adversely affecting spermatogenesis.30 These findings underscored the critical role of external pressure in the testicular microenvironment and highlighted its significance in furthering our understanding of reproductive dysfunction caused by cryptorchidism.

ENDOCRINE CHANGES

The hypothalamus releases gonadotropin-releasing hormone (GnRH), which stimulates the anterior pituitary gland via the hypothalamic–pituitary portal system to synthesize and release follicle-stimulating hormone (FSH) and luteinizing hormone (LH). These hormones stimulate the secretion of sex steroid hormones, such as T, INH B, and anti-Müllerian hormone (AMH) from the gonads which regulate the hypothalamic–pituitary–gonadal (HPG) axis through negative feedback mechanisms. The HPG axis is active during mid-pregnancy but becomes suppressed at term because of negative feedback from placental hormones. After birth, this suppression is lifted in neonates, resulting in a transient surge of T, which reactivates the HPG axis, elevating FSH and LH levels that peak between 1 month and 3 months of age before gradually declining. The HPG axis reactivates during puberty, promoting the development of the gonads and maturation of reproductive function.31

In the testes, Leydig cells produce T in response to LH stimulation, facilitating the development of male secondary sexual characteristics and maintaining reproductive function while promoting spermatogenesis. Elevated T levels initiate negative feedback, suppressing hypothalamic and pituitary activity to reduce GnRH, LH, and FSH secretion, thus lowering T production. Conversely, declining T weakens this feedback, stimulating hypothalamic and pituitary activity to boost hormone synthesis. This regulation maintains the hormonal balance critical for male reproduction. Research by Farrer et al.32 indicated that endogenous T levels in cryptorchid testes were significantly lower than those in normally descended testes (2.0 ng vs 71.2 ng, P < 0.001). In cryptorchid testes, the activities of several key enzymes involved in T biosynthesis (17α-hydroxylase, 17,20-lyase, and 17β-hydroxysteroid dehydrogenase) were inhibited by approximately 80% in adulthood. These findings suggested that cryptorchidism adversely affects Leydig cell T synthesis and may partially explain the abnormal morphology and resulting infertility in affected individuals.32

Additionally, the HPG axis is often disrupted in patients with cryptorchidism, with significantly lower serum LH and T levels compared with normal controls (P < 0.05), while FSH and estradiol levels are markedly elevated (P < 0.05).33 Estradiol exerts negative feedback on the hypothalamus and pituitary, helping to regulate hormone secretion and maintain gonadal function. The hormonal imbalance with cryptorchidism is associated with a decrease in spermatogonia and atrophy of the seminiferous tubules. Generally, higher serum FSH levels correlate with more severe spermatogenic impairment, and notably, serum FSH levels significantly decrease post-surgery.

Hadziselimović et al.34 randomized patients undergoing orchiopexy into two groups: a treatment group receiving a luteinizing hormone-releasing hormone analog therapy via Buserelin nasal spray (10 μg) every other day for six months and a control group receiving no additional treatment. All patients in the control group had severe oligospermia, whereas 86% of the treatment group exhibited semen quality restored to normal ranges, demonstrating a significant difference between the two groups.34 In another study, Jallouli et al.35 administered GnRH nasal spray (1.2 mg per day) to prepubertal boys with unilateral cryptorchidism for four weeks prior to orchidopexy. The fertility index, measured by the number of type Ad spermatogonia per seminiferous tubule, was significantly higher in the GnRH-treated group compared with the untreated group (mean ± s.d.: 0.88 ± 0.31 vs 0.49 ± 0.52, P = 0.02). These findings suggested that GnRH treatment enhances the maturation of germ cells, thereby improving fertility potential in adulthood.35

Studies have explored the predictive value of testicular histology, gonadotropin levels, and INH B in assessing the fertility risk in prepubertal boys with bilateral cryptorchidism.36,37 Approximately 40% of patients showed a reduction in germ cell numbers, with no significant elevation in FSH and LH levels, while 22% had post-operative INH B levels below normal, indicating a higher infertility risk. About 25% of patients exhibited elevated initial FSH levels, which gradually normalized post-surgery, suggesting the potential for hypergonadotropic hypogonadism and supporting the hypothesis of primary testicular dysfunction. However, 35% of patients demonstrated reduced inhibin B levels, correlating with moderate infertility risk. Another 35% maintained normal germ cell counts and preoperative INH B/gonadotropin levels, suggesting lower reproductive risk.38

Sertoli cells primarily secrete ABP and INH B. ABP binds T, facilitating its transport in the bloodstream and helping to stabilize hormone levels. INH B regulates FSH secretion from the pituitary through negative feedback and serves as an indicator of male fertility and testicular health. However, high-temperature conditions impair the functions of Sertoli and Leydig cells. Elevated temperatures significantly decrease the expression of ABP, INH B, and androgen receptors in Sertoli cells,39 while the expression of the Wilms’ tumor 1 (WT1) protein, which regulates steroid hormone production in Leydig cells, also declines, adversely affecting sperm production.40

Sertoli cell-derived AMH is a critical marker for assessing the development of the male reproductive system before puberty. AMH levels are high in early puberty but decline as the testes mature. In boys over one year old with cryptorchidism, serum AMH levels positively correlate with INH B levels.41

OXIDATIVE STRESS

Oxidative stress occurs when reactive oxygen/nitrogen species (ROS/RNS) production overwhelms the body’s antioxidant defenses, disrupting redox homeostasis and causing cellular damage.42 In the testes, ROS are produced from multiple sources, involving various cell types and physiological processes. White blood cells, particularly peroxidase-positive polymorphic leukocytes and macrophages, are major contributors to ROS generation during infection or inflammatory responses. Immature sperm also contribute to ROS production via the fructose-6-phosphate dehydrogenase mechanism. Mitochondria, crucial for energy metabolism, generate ROS, especially under dysfunctional conditions. Sertoli cells have a key role in maintaining the testicular microenvironment and also produce ROS under specific circumstances. Inflammation, resulting from infections or autoimmune responses, activates inflammatory cells and further increases ROS generation. Environmental factors, such as high temperatures, radiation, and chemical exposure, also stimulate ROS production in testicular cells,43 ROS primarily damage sperm through two mechanisms, one by degrading nuclear DNA and RNA, and thereby compromise paternal genomic contributions to the embryo, and the other by oxidizing lipids in sperm plasma membranes, which reduces sperm motility as well as sperm fusion with the zona pellucida of oocytes.

In cases of cryptorchidism, the elevated temperature surrounding the testes impairs blood supply, leading to localized hypoxia. Under hypoxic conditions, cells generate energy through glycolysis, potentially producing more ROS. Li et al.44 investigated ROS production and gene expression patterns following cryptorchidism induction in adult mice and found increased ROS levels associated with elevated germ cell apoptosis. This response was linked to alterations in gene expression related to energy and lipid metabolism, stress responses, and redox reactions.44 Notably, the NM23 family member 5 (Nme5) gene, which is specifically expressed in mouse testicular tissue and involved in spermatogenesis, was found to have significantly higher expression in cryptorchid testes compared with normal testes, further demonstrating the imbalance between oxidative and antioxidant systems in cryptorchidism.45

Multiple lines of research have established a causal relationship between oxidative stress and sperm DNA fragmentation.37,42 Telomeres, which consist of guanine-rich hexameric repeat sequences, are particularly susceptible to oxidative stress because guanine has a relatively low oxidation potential. Consequently, telomeres are prime targets for free radical attack. The oxidative product 8-hydroxy-2’-deoxyguanosine (8-OHdG) serves as a widely used biomarker for assessing DNA oxidative damage resulting from ROS exposure. Hosen et al.46 assessed semen from 66 (26 healthy and 41 infertile) men using spectrophotometry and enzyme-linked immunosorbent assay (ELISA) methods to estimate levels of lipid peroxidation end-product malondialdehyde, phospholipid hydroperoxides, superoxide dismutase, total antioxidant status (TAS), and 8-OHdG, while examining their associations with sperm parameters. Their results indicated that infertile men had significantly lower percentages of motile and morphologically normal sperm (both P < 0.001). Furthermore, malondialdehyde, phospholipid hydroperoxides, and 8-OHdG levels were significantly elevated (P < 0.001, P < 0.001, and P = 0.02, respectively) in the semen of infertile men, while superoxide dismutase and TAS levels were significantly lower (both P < 0.001). Notably, sperm parameters were negatively correlated with malondialdehyde, phospholipid hydroperoxides, and 8-OHdG, while positively correlated with superoxide dismutase and TAS.46 Smith et al.47 investigated sperm DNA damage and oxidative stress in men with oligospermia, cryptorchidism, and normal fertility. Sperm DNA damage was assessed using sperm chromatin structure analysis and the terminal dUTP nick end labeling (TUNEL) assay. ROS levels and total antioxidant capacity were measured using chemiluminescence and colorimetric methods. The results indicated that sperm DNA damage was significantly higher in patients with idiopathic oligospermia and cryptorchidism compared with the control group, with TUNEL-positive cell proportions of 26.9% and 29.1%, respectively, versus 14.2% in the control group (both P < 0.05). Additionally, ROS levels in the patient groups were significantly elevated compared with the control group (mean ± s.d.: 4.4 ± 0.3 for oligospermia patients and 5.2 ± 0.6 for cryptorchidism patients, compared with 2.8 ± 0.9 in the control group, both P < 0.05). However, there were no significant differences in TAC levels among the groups.37 These findings highlight the critical impact of oxidative stress-induced sperm DNA damage on the pathogenesis of infertility.

DNA oxidation damage observed in sperm includes single-strand and double-strand breaks, DNA fragmentation, the introduction of base sites, modifications to purines, pyrimidines, and deoxyribose, as well as DNA cross-linking. Such damage leads to gene transcription suppression or activation, signal transduction pathway modulation, accelerated telomere wear, replication errors, genomic instability, and guanine-cytosine to thymine-adenine transitions (GC to TA). Additionally, sperm mitochondrial DNA is particularly vulnerable because of its relatively naked form (lacking histone and DNA-binding protein protection), coupled with a low intrinsic repair capacity. This makes mitochondrial DNA highly susceptible to various damaging agents, especially oxidative stress induced by ROS. Throughout the maturation process, sperm are especially prone to different forms of mitochondrial DNA damage, including deletions, mutations, and variations. Studies have indicated a high incidence of mitochondrial DNA damage in samples from infertile males.48,49,50

Lipid peroxidation in the sperm plasma membrane is a significant contributor to male infertility. Byproducts of lipid oxidation include mutagenic and genotoxic molecules, such as malondialdehyde (MDA) and 4-hydroxynonenal. Approximately 50% of the sperm membrane contains the polyunsaturated fatty acid docosahexaenoic acid, with each molecule comprised of six double bonds. Once lipid oxidation commences, free radicals attack the double bonds of unsaturated fatty acids, giving rise to lipid peroxyl radicals. These radicals then react with adjacent lipid molecules, initiating a chain reaction. During lipid peroxidation in sperm, nearly 60% of fatty acids can be lost from the membrane.51,52 Damage to the plasma membrane affects its normal structure and function, thereby impacting fluidity, ionic gradients, receptor-mediated signaling, transport processes, and membrane enzyme activity. Aitken et al.53 studied the semen of 31 infertile men, using fluorescence assays to quantify MDA as a marker of lipid peroxidation. By monitoring motility characteristics and sperm–oocyte fusion capability, they found a negative correlation between sperm–oocyte fusion and MDA concentration.53 de Lamirande and Gagnon54 demonstrated that sperm flagellar beating frequency decreased rapidly following exposure to ROS and was correlated with a swift reduction in intracellular adenosine triphosphate levels. This indicates that oxidative stress diminishes sperm motility and fusion with oocytes.

ROS-induced DNA damage accelerates the apoptotic process in germ cells, contributing to a decline in sperm count associated with male infertility. Elevated ROS levels compromise mitochondrial membrane integrity, leading to caspase activation and ultimately triggering apoptosis. The release of cytochrome c in the apoptotic pathway further elevates ROS levels, promoting DNA damage and fragmentation while possibly extending the apoptotic cycle. Sperm exposed to elevated oxidative stress show increased levels of p53, p21, Bax, caspase-3, and caspase-9, leading to enhanced caspase activation and subsequent apoptosis.55

While ROS production is essential for spermatogenesis, sperm capacitation, and the acrosome reaction, an excess of ROS leads to oxidative stress that damages the cell membrane, DNA, and other cellular components, adversely affecting sperm quality and fertility. To counteract this damage, the body’s antioxidant system maintains a balance between ROS production and elimination, thus protecting sperm. Therefore, the equilibrium between ROS generation and antioxidant defenses is crucial for maintaining male fertility.

GENITOFEMORAL NERVE DAMAGE

Androgens play a crucial role in testicular descent through the release of calcitonin gene-related peptide (CGRP) by the genitofemoral nerve (GFN) as the testes migrate from the inguinal region to the scrotum. CGRP is a widely distributed neuropeptide in both the central and peripheral nervous systems, primarily associated with vasodilation.56,57,58

Research conducted by Goh et al.59 investigated the effects of the antiandrogen drug flutamide on GFN masculinization and testicular descent in mice prior to pregnancy. The findings indicated that pre-pregnancy administration of flutamide effectively blocked androgen action, resulting in inhibited masculinization of the GFN and a significant reduction in CGRP levels.59 These findings support the hypothesis that androgens regulate testicular descent during the inguinal phase through GFN and CGRP as secondary messengers.

Related studies examined the impact of GFN transection on testicular outcomes in a unilateral cryptorchid mouse model. Compared with mice with intact GFN on the affected side, those undergoing GFN transection exhibited a significant increase in the weight of the contralateral testis, along with a marked reduction in MDA levels and apoptosis indices. These results suggested that in cases of unilateral cryptorchidism, the ectopic environment of the undescended testis causes sustained stimulation of the sensory terminals of the affected GFN, leading to decreased CGRP secretion. This, in turn, reduces blood flow to the contralateral testis via sympathetic reflex pathways, resulting in damage to the unaffected testis.60

The function of the GFN is closely linked to testicular development and health, with androgens and their signaling pathways playing critical roles in regulating testicular descent. Dysfunction of the GFN may have detrimental effects on the reproductive system.

ANTISPERM ANTIBODIES (ASA)

ASA are antibodies produced by the immune system that target specific components of sperm.61 Under normal circumstances, germ cells and sperm, which are considered self-antigens, do not elicit an immune response because of the immunological privilege of testes. This privilege is maintained by a combination of Sertoli cells, immune cells in the stroma, the blood–testis barrier (BTB), and the membranes of seminiferous tubules, which collectively protect self-antigens.

Kurpisz et al.62 conducted a study measuring serum antibody levels, using the indirect immunobead test, in prepubescent boys who underwent surgery for cryptorchidism. The results revealed significantly elevated levels of ASA, particularly pronounced in patients with bilateral testicular abnormalities. This finding suggested that ASA may be an important factor contributing to infertility in adulthood.62 Urry et al.63 further demonstrated that 66% of men with cryptorchid-related infertility tested positive for ASA, while the positivity rates in the general infertile population and sperm donors were only 2.6% and 2.8%, respectively. These results underscore the significant role of ASA in the fertility of patients with cryptorchidism.

Although the mechanisms underlying ASA formation remain unclear, most researchers believe that they are closely associated with the disruption of the BTB. In cases of cryptorchidism, these antibodies may interfere with sperm function, adversely affecting motility and the ability to bind with oocytes, ultimately leading to infertility.64,65,66 Therefore, a comprehensive investigation into the mechanisms of ASA formation and their impact on fertility is crucial for improving treatment strategies for patients with cryptorchidism.

TYPE AD SPERMATOGONIA DAMAGE

In the normal testis, the development of germ cells begins as an active process a few months after birth. During the neonatal period, germ cells differentiate into adult-type Ad spermatogonia, which are considered stem cells that support sperm production. Type Ad spermatogonia undergo continuous mitotic proliferation, generating more spermatogonia, while some of these cells further differentiate into more mature spermatocytes, progressing through subsequent stages of spermatogenesis.67

Hadziselimovic68 conducted a study analyzing semen samples from 218 men with cryptorchidism who underwent surgical orchidopexy. The results indicated that 53% of these patients had sperm concentrations below the normal range. Notably, boys with unilateral cryptorchidism who had detectable type Ad spermatogonia at the time of surgery exhibited significantly higher sperm concentrations as the age at surgery decreased. Conversely, those boys who lack type Ad spermatogonia, regardless of the age at surgery, demonstrated consistently low sperm concentrations and an increased risk of infertility.69 These findings suggested a close association between reduced type Ad spermatogonia and male infertility in the context of cryptorchidism.

Further research revealed that cryptorchid patients receiving hormone therapy (such as long-acting luteinizing hormone-releasing hormone [LHRH] analogs like Buserelin and human chorionic gonadotropin) had an Ad/T ratio (the ratio of type Ad spermatogonia per tubule) of 53%, compared with only 18% in those who underwent orchidopexy alone. This difference indicated that improved Leydig cell function following hormone treatment may aid in restoring fertility, and suggested that decreased T levels due to cryptorchidism contribute to the reduction in AD spermatogonia.70 Therefore, early identification and appropriate hormone therapy for men with cryptorchidism hold significant clinical implications for enhancing their fertility potential.

LIMITATIONS AND FUTURE DIRECTIONS

This review has several limitations. First, most studies mentioned in the review are single-center investigations, which will introduce selection bias. Variability in environmental and genetic factors across different regions can limit the generalizability of the results, affecting their applicability to broader populations. The lack of multicenter and large-scale studies restricts the development of universally applicable clinical guidelines, compromising the scientific rigor of clinical decision-making. Additionally, some studies rely on animal models, which may not fully replicate the complex physiological and psychological factors in humans. This limitation may impede a comprehensive understanding of the mechanisms by which cryptorchidism causes infertility. There is also a notable absence of long-term follow-up data regarding fertility outcomes and related indicators after treatment. Insufficient longitudinal studies hinder the assessment of the lasting effects of interventions. The lack of long-term data may obscure potential complications or adverse outcomes, such as late-onset hormonal imbalances or further reproductive decline, complicating risk evaluations. Moreover, potential confounding factors have not been adequately addressed. Environmental influences, such as exposure to chemicals, and lifestyle factors, including variations in habits such as smoking, drinking, and exercise, may be associated with both cryptorchidism and infertility. Failing to consider these factors raises questions about the accuracy of study conclusions, potentially leading to an overestimation or underestimation of the impact of cryptorchidism on infertility.

Given the limitations discussed above, further refinement of research on cryptorchidism, infertility, and potential treatments remains an important direction for the future. Currently, for infants under six months of age, the primary management approach is observation and waiting. However, for children over six months whose testes have not descended into the scrotum, timely surgical intervention is necessary, with most countries recommending orchidopexy before the age of one.15 Based on the discussions in this article, it can be concluded that earlier treatment correlates with a shorter duration of exposure to negative factors and will be more beneficial for the restoration of fertility and subsequent development. Therefore, it is recommended that children whose testes have not descended by six months of age undergo immediate surgical treatment to maximize fertility preservation and reduce the risk of malignancy. However, in practice, many cases of cryptorchidism are not promptly identified, leading to delays in surgery. For patients who undergo surgery after puberty, undescended testes often exhibit poor development, loss of fertility, and even the potential for malignancy, thus necessitating orchiectomy.1 For testes that retain some fertility potential, even after orchidopexy, complete restoration of fertility may not be achievable. Research indicates that hormone therapy following orchidopexy can reduce the rate of testicular atrophy and improve fertility outcomes.13 Consequently, some medical centers utilize hormone therapy to assist patients in recovering their fertility. However, a cross-sectional study by Varela-Cives et al.71 revealed that among cryptorchid patients who underwent surgery after the age of 18 years, those who received combined surgical and hormone treatment had an average testicular volume of 9.2 ml, compared with 8.6 ml for those who underwent surgery alone, with no statistically significant difference.

Thus, preventing testicular atrophy and enhancing fertility in patients who do not receive timely surgical intervention remains a significant challenge in the management of cryptorchidism. Based on the theory of oxidative stress, recent studies suggest that antioxidants, such as Moringa oleifera extracts, curcumin, and tetrahydroquinoline, mitigate testicular damage in animal models of cryptorchidism.72,73,74 These findings indicate a potential avenue for clinical application in the future. Therefore, further investigation into the mechanisms underlying infertility due to cryptorchidism and the exploration of effective treatment strategies will be critical directions for future research.

CONCLUSIONS

An in-depth exploration of factors such as apoptosis, endocrine disruption, oxidative stress, nerve damage, and autoimmune responses related to cryptorchidism allows for a more comprehensive understanding of the association between cryptorchidism and male infertility. This understanding will provide a crucial basis for clinical diagnosis and treatment and also outline important directions for future research. Early intervention and treatment for patients with cryptorchidism are essential for improving their fertility potential and enhancing their quality of life. Therefore, further studies should focus on developing effective therapeutic strategies to mitigate the adverse.

AUTHOR CONTRIBUTIONS

YXL was responsible for writing the manuscript, participating in the design of the study, and contributing to the manuscript. HYZ provided guidance throughout the study, supervised the research group, ensured the accuracy and integrity of the work, and critically revised the manuscript. Both authors read and approved the final manuscript.

COMPETING INTERESTS

Both authors declare no competing interests.