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Cryptorchidism – Aspects Of Pathogenesis

Jorgen Thorup, Dina Cortes

Department of Paediatric Surgery, Rigshospitalet, University of Copenhagen, Denmark.



The aetiology of undescended testes is multifactorial, since disrupted endocrine regulation and several gene defects may cause cryptorchidism. The exact regulation of testicular descent is unknown. The testis descends in two phases. The transabdominal descent depends on insulin-like hormone 3 and the inguinoscrotal descent depends on production of fetal androgens influenced by placental hormonal stimulation. Cryptorchidism is often related to congenital anomalies in the caudal developmental field and the role of placental insufficiency in the pathogenesis of cryptorchidism may be crucial.

Key words: cryptorchidism, testis, germ cells, placenta.



Jorgen Thorup

Department of Paediatric Surgery 4072, Rigshospitalet, University of Copenhagen,

2100 Copenhagen O, 


E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.



The aetiology of undescended testes is multifactorial, since disrupted endocrine regulation and several gene defects may cause cryptorchidism. The exact regulation of testicular descent is unknown. The aim of the present study is to discuss critically current theories on the pathogenesis of undescended testes.

Normal descent of the testis

Normal testicular descent in humans requires the formation of a testis from an indifferent gonad, close to the kidney, at about 7 weeks of gestation. The descent of the testis into the scrotum starts with the trans-abdominal phase. This phase of testicular descent in humans has been described to be completed at the 13th week of gestation (1). There is a cranial suspensory ligament, which connects the diaphragm to the mesonephros and the gonad. In male fetuses the ligament regresses, but in female fetuses it is replaced by the suspensory ligament of the ovary (2). Both genders have also the gubernaculum, which becomes surrounded by peritoneum, except where it is attached to the abdominal wall. In male fetuses the testis and the epididymis glide over the genital ducts, become embedded caudally in the gubernaculum testis, and enter the internal inguinal ring. The gubernaculum temporally increases, but after the testis has reached the scrotum it regresses to the scrotal ligament (2).

In males, the processus vaginalis appears, as a blind pouch of peritoneum lying ventromedially to the gubernaculum and forming the later inguinal canal. The genital branch of the genitofemoral nerve runs parallel to the gubernaculum, which cranially is connected to the caudal epididymis and the testis. Before the testis and epididymis descend through the inguinal canal the absolute and relative mass of the gubernaculum in creases because of an increase in its water content (2,3). The diameter of the gubernaculum reaches its maximum during the seventh month, which induces the widening of the inguinal canal. The intra-abdominal pressure and the shrinkage of the gubernaculum may force the testis through the inguinal canal. After that, the gubernaculum, the testis and the epididymis are covered by peritoneum of the prolonged processus vaginalis (2). The caudal end of the gubernaculum enters the coming scrotum, but is neither firmly attached to any structure nor does it extend to the bottom of the scrotum. After the testis has reached the bottom of the scrotum the distal part of the processus vaginalis is called the tunica vaginalis testis and the peritoneal connection to the peritoneum involutes. The gubernaculum shrinks, becomes more fibrous and persist as the scrotal ligament (2,3). Some authors claim that the primary organ to descent is the epididymis governed by the gubernaculum and requiring androgens (4). It is important to notice that the gubernaculum seems to have part as well inside as outside the processus vaginalis. According a hypothesis on testicular maldescent the gubernaculum gives rise to both smooth and striated muscles. The testis is descended through the processus vaginalis via the propulsive force generated by the muscles. Failure in descent is associated with a diminution in smooth muscle content and a decrease sympathetic tonus that depends on androgens (5).

No gubernaculum has been observed distal to the external inguinal ring before 23rd gestational week. The descent of the testis through the inguinal canal has been described to occur rapid and to be completed by the end of seventh month of gestation (2,3). The inguinoscrotal phase of testicular descent is proposed to be completed by the end of 35th week (1).

Table 1 

16 boys (mean age 1.7  yr) with normal mean S/T > 0.38

FSH t:0 FSH t:30 LH t:0 LH t:30
iU/L iU/L iU/L iU/L

12 patients

Median (range) 


(0.7- 1.9)


 (1.3 - 6.1)


(0.2 - 1.6)


(1.1 - 7.8) 

Patient A 0.4 1.3 0.2 1.2
Patient B 0.4 1.4 0.6 1.0

Patient C

0.4 1.8 0.6 1.0
Patient D 0.4 0.7 0.6 3.6

Results of Gonadotropin Releasing Hormone stimulation test carried out with 100 microgram suprefact and bood samples for Luteinizing Hormone (LH) and Follicle-stimulating Hormone (FSH) with basic levels (t:0) and 30 minutes after stimulation (t:30) in 16 boys with normal germ cell numbers.

12 boys had normal response and one boy (A) had equivocal response.

Patient B,C and D had an insufficient rise in LH and/or FSH

Normal basic level of FSH = 0.4-1.1 iU/l

Normal stimulated level of FSH > 1.3 iU/l and/or 3 x basic level

Normal basic level of LH = 0.2-1.1 iU/l

Normal stimulated level of LH > 1.3 iU/l and/or 3 x basic level

Pathological abnormalities in cryptorchidism

Human cryptorchidism is usually due to abnormalities in the inguinoscrotal phase of testicular descent, whereas the transabdominal phase is more seldom disrupted and only about 5% of operated undescended testes are intra-abdominally placed (6).

Several etiologies for cryptorchidism have been proposed: Abnormal action of the hypothalamic-pituitary axis, abnormal testicular differentiation, deficient androgen production/action, deficient production/action of anti-Mullerian hormone (AMH), and deficient action of insulin-like hormone 3 (INSL3) (6). In addition cryptorchidism occurs in many syndromes, and there is familial occurrence of cryptorchidism (7). Cryptorchidism is associated with an increased GGN repeat length of the androgen receptor gene, which may cause decreased function of the androgen receptor (8). However, mutations for instance in androgen receptor gene or in the gene of 5-alpha-reductase seem to be rare in isolated cryptorchidism (9,10,11).

Also mutations of INSL3 genes have only rarely been associated with human cryptorchidism. Furthermore, only heterozygous changes of these genes are described in human cryptorchidism (12). Mutations in HOXA10 gene seem to be rare among cryptorchid patients (13,14).

Reduced content of calcitonin gene-related peptide (CGRP) is associated with cryptorchidism in rodents. But the human gubernaculum contains only a small amount of muscle cells and therefore it is controversial whether the aforementioned active contraction of the gubernaculums induced by CGRP release from the genitofemoral nerve (GFN) has an important role in the descent of the testis in man (15). Mutation screening in patients with idiopathic cryptorchidism showed no pathogenic sequence changes in the CGRP pathway (16). However, CGRP caused fusion of the processus vaginalis in human inguinal hernia sacs in vitro, and thus the genitofemoral nerve (GFN) and CGRP may control the obliteration of the processus vaginalis after the testis has descended (17,18). Furthermore, the frequency of cryptorchidism is increased to about 20% in boys with spina bifida in general, and to about 35% in cases of high lumbar lesions (19). Therefore, the genitofemoral nerve, which has its central nucleus at the level of L1-L2 of the spinal cord, may play an important role (18,19).

Several studies have shown that in 75% of cryptorchid boys the undescent of the testes is caused by a prepubertal transient hypothalamus-pituitary-gonadal hypofunction (20). However recent research has questioned this hypothesis. In a follow-up study on 38 infertile adult men having been operated for bilateral undescended testes in childhood 61% had actually elevated FSH indicating hypergonadotrop hypogonadism (21). In a study on 122 cryptorchid boys and 699 controls with blood samples 3 months old FSH and LH was significantly increased in cryptorchids versus controls and it was concluded that this gonadotropin drive supported the hypothesis that cryptorchidism is associated with a primary testicular disorder (22). In a recent presentation bilateral cryptorchid boys aged 9 months to 3.7 years with normal or severely decreased testicular germ cell number had a Gonadotropin Releasing Hormone stimulation test carried out with 100 microgram suprefact and blood samples for Luteinizing Hormone (LH) and Follicle-stimulating Hormone (FSH) at t:0 minutes and t:30 minutes. Most patients had normal basic hormonal values and a normal response and only 23% had an insufficient rise in LH and/or FSH. More than 50% of the boys with severely decreased testicular germ cell number had elevated FSH indicating hypergonadotrop hypogonadism (tables 1 and 2) (23).

Table 2

19 boys (median age 2.1 yr) with severely decresed mean S/T < 0.2

FSH t:0 FSH t:30 LH t:0 LH t:30
iU/L iU/L iU/L iU/L

15 patients

median (range)


(0.4 - 4.7)


(2.0 - 10.0) 


(0.2 - 1.5) 


(2.1 - 8.8) 

patient E 0.4 1.0 0.3  1.5
patient F 0.4 0.6 1.0 0.6

 patient G 

 0.6  3.2 0.3 1.1
 patient H  1.5  4.2  0.2  1.1

Results of Gonadotropin Releasing Hormone stimulation test carried out with 100 microgram suprefact and blood samples for Luteinizing Hormone (LH) and Follicle-stimulating Hormone (FSH) with basic levels (t:0) and 30 minutes after stimulation (t:30) in 19 boys with severely decreased germ cell numbers.

15 boys had normal response. Patient E,F,G and H had an insufficient rise in LH and/or FSH

Normal basic level of FSH = 0.4-1.1 iU/l

Normal stimulated level of FSH > 1.3 iU/l and/or 3 x basic level

Normal basic level of LH = 0.2-1.1 iU/l

Normal stimulated level of LH > 1.3 iU/l and/or 3 x basic level


Intra-abdominal testes can also be due to the persistent Müllerian duct syndrome (PMDS), which is caused by genetic abnormality of AMH or its receptor. In PMDS the testes are very mobile and usually located in an ovarian position, but the testis may also be located in an inguinal hernia together with a Fallopian tube, the uterus and eventually the contralateral testis (24,25,26). The gubernaculum has been reported to be feminized in PMDS (24).

In a surgical series of intra-abdominal testes in boys the cranial suspensory ligament was constantly present and varied in size from large to fan-shaped as normal at ovaries, to small and slim. The testes located high in the abdomen had a larger ligament than the testes located near the internal inguinal ring. In no case were structures developed from the Müllerian ducts demonstrable. Based on the concomitant observed abnormalities of the included cryptorchid boys, it was suggested that intra-abdominal testes may result from abnormal differentiation of the midline developmental field, possible at the third to fourth week after fertilization (6, 27). In a developmental field the included body parts develops as co-ordinated units where differentiation of the appropriate structures is closely connected through embryonal interactions and progresses with a predetermined course as to place and time. Interference in the temporal as well as in the spatial normal morphological development may result in structural abnormalities (28).

From a morphological point of view cryptorchidism has also been reported in association to lumbo-sacral vertebral, kidney and urinary tract abnormalities (6,29,30). Cryptorchidism may in such cases be a feature of a caudal developmental field defect (also described as a feature of caudal dysplasia, the caudal regression syndrome and the caudal regression malformation sequence). These disorders involve abnormalities of the structures derived from the caudal region of the embryo, i. e. the urogenital system, caudal spine, the hindgut, the spinal cord and the lower limbs, which are forming during the third and fourth week after fertilization (29,31). The most extreme constellation of these disorders is the ”mermaid-like” habitus called sirenomelia/tritonmelia (the female/male forms). In tritonmelia there is constantly bilateral cryptorchidism, imperforate anus, abnormalities of the kidneys, the lower vertebral column and the legs, which classsically are fused. The hypothesis of vascular steal closely related to placenta pathology as a causative factor to this anomaly has been presented (32). Placental vascular pathology is well known to result in chronic and acute placental insufficiency which represents the cause for interuterine death associated to serious anomalies of the urogenital and intestinal tract (33). A less severe form of caudal developmental field defect is imperforate anus (6,29). Imperforate anus is classified based on the opening of the fistula, to e.g. a recto-prostatic fistula, or the level of the blind rectum. In general the rectum is anteriorly mislocated (34). In a surgical series of boys with imperforate anus malformations and dysplasias of the kidneys, the ureters and the lumbo-sacral spine were present in 65% of the cryptorchid patients, whereas only 12% of the non-cryptorchid patients with imperforate anus exhibited such abnormalities (6,35). Equivalently, the Spanish collaborative study of multiple congenital malformations reported that the strongest association with imperforate anus was the combination of spine defects + renal/urinary tract defects + genital defects, which was about 12 timer higher than in infants with multiple congenital defects but no imperforate anus (30). It was concluded, that the association of imperforate anus + spine defects + renal/urinary tract defects + genital defects was pathogenically related and constitute a developmental field defect (30).

Furthermore, in a series of cryptorchid human fetuses, who died in the third trimester malformations and dysplasias of the kidneys, the ureters and the lumbo-sacral spine were proved in 34%. Equivalent, these abnormalities occured in 18% of cryptorchid patients younger than 3 years of age in a series from a department of pediatric surgery. In general, in more than 90% of the cases, cryptorchidism was on the same side as the urological abnormalities, which may signify a high interdependence between these elements in the association (35). It is accepted, that the risk of genitourinary abnormalities is elevated in cryptorchidism (6, 36). Cryptorchidism is also described with renal or ureteral abnormalities in genetic diseases, for instance Rubinstein-Taybi syndrome, and in chromosone anomalies, for instance trisomy 13 and 18 (35). Moreover, the described association is in line with the increased frequency of cryptorchidism in lumbo-sacral spina bifida (19). In chicken embryos sirenomelia was produced by destruction of the axial portion of the caudal region. If the injury was localized, then the lower limbs, the Wollfian system and the vertebral column were spared, and the chicken achieved defects of the cloacal region only (29). Consequently, abnormalities in the lumbo-sacral spine may play a very important role.

Other enviromental factors may also play an etiological role in cryptorchidism as a caudal developmental field defect. Both isolated cryptorchidism and sirenomalia occur more frequent in infants born to insulin-dependent diabetic mothers (31, 37, 38, and 39). In addition, prenatal phthalate exposure in male offspring was associated with a short anogenital distance (the distance from the centre of the anus to the anterior base of the penis) (40). Moreover, in a series of boys 2-36 months of age the length of the anogenital distance correlated negatively to the frequency of cryptorchidism. No patient exhibited imperforate anus or other abnormalities. Therefore, in boys prenatal phthalate exposure at environmental levels may adversely affect the anogenital distance and the testicular descent (40). Furthermore, in a surgical series of cryptorchid boys whose mothers had smoked heavily during pregnancy (i.e. more than 10 cigarettes daily during pregnancy) there was a very high risk of bilateral cryptorchidism (52%) and a decreased number of germ cells (41). Moreover, in mice embryos threated with nicotine in levels comparable to levels in human amniotic fluid of tobacco-smoking pregnant women, the posterior body region was hypoplastic and similar to the human condition called caudal dysplasia, and excessive apoptosis was observed in the deformed structures (42).

Altered hormone levels in smokers may play a causal role in cryptorchidism. Human chorionic gonadotropin and epidermal growth factor are found at decreased levels in smokers compared to non-smokers (43). Furthermore, it is notable that there was no increased risk of intra-abdominal testes among the boys with heavy smoking mothers during pregnancy. Consequently, heavy smoking during pregnancy induced bilateral cryptorchidism with undescended testes located distal from the abdominal cavity. The current model for testicular descent involves two independent steps, the transabdominal phase and the inguinoscrotal phase (44,45). It is generally agreed that the inguinoscrotal phase is at least partially dependent on fetal testicular testosterone secretion, which in turn, is initiated and maintained by human chorionic gonadotropin produced by the placenta (45).

Life style factors, such as maternal smoking during pregnancy and less significant environmental factors, such as endocrine disrupting chemicals may explain the slight changes in the incidents of cryptorchidism seen in some countries .

Histological abnormalities in cryptorchidism

Cryptorchidism may be associated with reduced number of germ cells, defective or delayed maturation of germ cells, and reduced number and atrophy of Leydig cells (6,46,47,48,49). The percentage of patients having normal age-related number of spermatogonia decreases with increasing age, and less than 10 percent of patients older than two years have normal number of spermatogonia. Lack of germ cells is rare before the age of 15 months, but after that age a progressive number of patients lack germ cells in biopsies taken at time of surgery for cryptorchidism, and for example about 20% of cases at 3 years of age may lack germ cells (6,47). The adverse effects of cryptorchidism on germ cells may be due to the abnormal position of the testis, since an undescended testis has an abnormally high environmental temperature (50), and animal studies showed that heat affects germ cell maturation and Sertoli cells (51). Accordingly, in human studies, intra-abdominal testes exhibit more apoptosis than inguinal testes at the time of surgery (52). However, testicular biopsies taken before the age of one year showed more apoptosis than biopsies taken at an older age, and most germ-cell degeneration in cryptorchidism occurs before the age of 6 months (52). This is in line with the dramatical decrease in the number of germ cells within the first 2 years of life. Furthermore, already at birth the number of germ cells may be reduced in cryptorchidism (47). Therefore, besides deficiency of the germ cells as an acquired defect due to abnormal position, deficiency of the germ cells may be congenital; this supports the hypothesis of cryptorchidism representing testicular disorder of fetal origin. Maternal growth factors may be involved in the inguinoscrotal descent of the testis and may be insufficiently produced in case of placental impairment (53,54). Similarily it is proposed that growth factors may influence the germ cell number in the cryptorchid testis (41, 55).


Testicular descent is thought to proceed in two phases and the descent is usually completed at the time of birth. Animal studies suggest that the first transabdominal phase is dependent on the Leydig cell hormone INSL3. The second inguinoscrotal phase is regulated by androgens. Although several aetiologies for cryptorchidism has been described, causative mutations have been described only seldom and in most cases the aetiology remains unclear. The aetiology of cryptorchidism is possibly multifactorial.

Cryptorchidism is associated with adverse effects on germ cells and changes in sex hormone levels. Cryptorchidism is associated to midline differentiation defects, and to lumbo-sacral vertebral, kidney and urinary tract abnormalities, and may be a caudal developmental field defect. These observations suggest that in cryptorchidism there may be primary developmental disorders including the testis, the midline or the caudal part of the body. The role of placental insufficiency in the pathogenesis of cryptorchidism may be crucial.




  1. Hutson J M, Hasthorpe S., Abnormalities of testicular descent. Cell Tissue Res 2005; 322:155-8.
  2. Barteczko KJ, Jacob M I., The testicular descent in human. Origin, development and fate of the gubernaculum Hunteri, processus vaginalis peritonei, and gonadal ligaments. Adv Anat Embryol Cell Biol 2000; 156: 1-98.
  3. Heyns C F., The gubernaculum during testicular descent in the human fetus. J Anat 1987;153: 93-112.
  4. Hadziselimovic F, Herzog B., The development and descent of the epididymis. Eur J Pediatr 1993; 152: (suppl 2) s6-s9.
  5. Tanyel FC., The descent of the testis and reason for failed descent. Turk J Pediatr 2004; 46: suppl 7-17.
  6. Cortes D., Cryptorchidism – aspects of pathogenesis, histology and treatment. Scand J Urol Nephrol Suppl 1998; 196: 1-54.
  7. Elert A., Jahn K, Heidenreich A, Hofmann R. The familial undescended testis. Klin Padiatr 2003;215: 40-45.
  8. Aschim, E. L., Nordenskjold, A., Giwercman, A., Lundin, K. B., Ruhayel, Y., Haugen, T. B., Grotmol, T., and Giwercman, Y. L., Linkage between cryptorchidism, hypospadias,and GGN repeat length in the androgen receptor gene. J Clin Endocrinol Metab 2004; 89: 5105-5109.
  9. Wiener JS, Marcelli M, Gonzales E T, Jr. Roth D R, Lamb D J., Androgen receptor gene alterations are not associated with isolated cryptorchidism. J Urol 1998;160: 863-865.
  10. Suzuk Y, Sasagawa I, Ashida J, Nakada T, Muroya K., Ogata T., Screening for mutations of the androgen receptor gene in patients with isolated cryptorchidism. Fertil Steril 2001; 76: 834-836.
  11. Suzuki Y, Sasagawa I, Itoh K, Ashida J, Ogata T., 5Alpha-reductase type 2 genes in Japanese males do not appear to be associated with cryptorchidism. Fertil Steril 2002; 78: 330-334.
  12. Bogatcheva NV, Agoulnik A I., INSL3/LGR8 role in testicular descent and cryptorchidism. Reprod Biomed Online 2005; 10: 49-54.
  13. Kolon T F, Wiener J S, Lewitton M, Roth D R, Gonzales E T, Lamb D J ., Analysis of homeobox gene HOXA10 mutations in cryptorchidism. J Urol 1999; 161: 275-280.
  14. Bertini V, Bertelloni S, Valetto A, Lala R, Foresta C, Simi P., Homeobox HOXA10 gene analysis in cryptorchidism. J Pediatr Endocrinol Metab 2004; 17: 41-45.
  15. Costa W S, Sampaio F J, Favorito L A, and Cardoso L E., Testicular migration: remodeling of connective tissue and muscle cells in human gubernaculum testis. J Urol 2002; 167: 2171-2176.
  16. Zuccarello D, Morini E, Douzgou S, Ferlin A, Pizzuti A, Salpietro D C, Foresta C, Dallapiccola B., Preliminary data suggest that mutations in the CgRP pathway are not involved in human sporadic cryptorchidism. J Endocrinol Invest 2004; 27 760-764
  17. Hutson J M, Albano F R, Paxton G, Sugita Y, Connor R, Clarnette T D, Gray A Z, Watts L M, Farmer P J, Hasthorpe S., In vitro fusion of human inguinal hernia with associated epithelial transformation. Cells Tissues Organs 2000; 166: 249-258.
  18. Hutson J M, Hasthorpe S., Testicular descent and cryptorchidism: the state of the art in 2004. J Pediatr Surg 2005; 40: 297-302.
  19. Hutson JM, Beasley SW, Bryan AD., Cryptorchidism in spina bifida and spinal cord transection: a clue to the mechanism of transinguinal descent of the testis. J Pediatr Surg 1988; 23: 275-7.
  20. Hadziselimovic F, Herzog B., Hodendystopie. In: Kinderurologie in Klinik und Praxis. Thüroff J W and Schulte-Wissermann eds. Thieme 2 ed. Stuttgart-New York, 2000; 484-500
  21. Cortes D, Thorup J, Lindenberg S and Visfeldt J., Infertility despite surgery for cryptorchidism in childhood can be classified by patients with normal or elevated follicle-stimulating hormone and identified at orchidopexy. BJU Int. 2003; 91:670-4.
  22. Suomi A M, Main K M, Kaleva M, Schmidt I M, Chellakooty M., Virtanen H E, Boisen K A, Damgaard I N, Kai C M, Skakkebaek N E, Toppari J., Hormonal changes in 3-month-old cryptorchid boys. J Clin Endocrinol Metab 2006; 91: 953-8.
  23. Thorup J, Cortes D, Petersen B L., Gonadotropin releasing hormone test and testicular histology in infants with bilateral cryptorchidism. J Pediatr Urol 2006; 2: 118.
  24. Clarnette T D, Sugita Y, Hutson J M., Genital anomalies in human and animal models reveal the mechanisms and hormones governing testicular descent. Br J Urol 1997; 79: 99-112.
  25. Hadziselimovic F, Huff D., Gonadal differentiation--normal and abnormal testicular development. Adv Exp Med Biol 2002; 511: 15-21; discussion 21-13.
  26. Josso N, Belville C, di Clemente N, Picard J Y., AMH and AMH receptor defects in persistent Mullerian duct syndrome. Hum Reprod Update 2005; 11: 351-356.
  27. Cortes D, Thorup J M, Visfeldt J., The pathogenesis of cryptorchidism and plenogonadal fusion: a new hypothesis. Br J Urol 1996; 77: 285-90.
  28. Opitz JM ., The developmental field concept in clinical genetics. J Pediatr 1982; 101: 805-9.
  29. Duhamel B., From the mermaid to anal imperforation: the syndrome of caudal regression. Arch Dis Child 1961; 36: 152-5.
  30. Martinez-Frias ML, Bermejo E, Rodriguez-Pinilla E., Anal Atresia, Vertebral, Genital, and Urinary tract Anomalies. Amer J of Medical Genetics 2000; 95: 169-173.
  31. Larsen WJ., (Ed.). Human embryology. (2001) 3rd ed. Churchill Livingstone.
  32. Stevenson RE, Jones KL, Phelan MC, Jones MC, Barr M Jr, Clericuzio C, Harley RA and Benirschke K, Vascular steal: thepathogenetic mechanism producing sirenomelia and associated defects of the viscera and soft tissues. Pediatrics 1986;78: 451-7.
  33. Horn LC, Lagner A, Stiehl P, Wittekind C, Faber R. , Identification of the causes of intrauterine death during 310 consecutive autopsies. Eur J Obstet Gynecol Reprod Biol 2004; 113:134-8.
  34. Pena A., Imperforate anus and cloacal malformations pp 473-92. In Pediatric Surgery 3rd edition. Ashcraft KW, Murphy JP, Sharp RJ, Sigalet DL and Snyder CL (Eds). WB Saunders Company. 2000.
  35. Cortes D, Thorup J M, Beck BL, Visfeldt J .Cryptorchidism as a caudal developemtal field defect. APMIS 1998; 106: 953-8.
  36. Briggs LM, Baer A, Critchlow CW., Maternal, delivery, and perinatal characteristics associated with cryptorchidism: a population-based case-control study among birth in Washington state. Epidemiology 2002; 13: 197-204.
  37. Passade E, Lenz W., Syndrome of caudal regression in infants of diabetic mothers: observations of further cases. Pediatrics 1966; 37: 672-5.
  38. Twickler D, Budorick N, Pretorius D, Grafe M, Currarino Gp., Caudal regression versus sirenomelia: sonografic clues. J Ultrasound Med 1993; 12: 323-30.
  39. Ghirri P, Ciulli C, Vuerich M, Cuttano A, Faraoni M, Guerrini L, Spinelli C, Tognetti S, Boldrini A., Incidence at birth and natural history of cryptorchidism: a study of 10,730 consecutive male infants. J Endocrinol Invest 2002; 25: 709-715.
  40. Swan SH, Main KM, Liu f, Stewart SL, Kruse RL, Celafat AM, Mao CS, Redmon JB, Ternand CL, Sullivan S, Teague JL and the study for future families research team. Environ Health Perspect 2005;113: 1056-61.
  41. Thorup J, Cortes D, Petersen BL., The incidence of bilateral cryptorchidism is increased and the fertility potential is reduced in sons born to mothers who have smoked during pregnancy. J Urol 2006; 176: 734-7.
  42. Zhao Z, Reece EA., Nicotine-induced embryonic malformations mediated by apoptosis from increasing intracellular calcium and oxidative stress. Birth Defects Res Dev Reprod Toxicol. 2005;74:383-91.
  43. Lindqvist P, Grennert L, Marsal K., Epidermal growth factor in maternal urine: a predictor of intrauterine growth restiction? Early Hum Dev 1999; 56: 143-50.
  44. Husmann DA, Levy JB., Current concepts in the pathophysiology of testicular undescent. Urology 1995; 46: 267 – 76.
  45. Tomiyama H, Sasaki Y, Huynh J, Yong E, Ting A, Hutson JM., Testicular descent, cryptorchidism and inguinal hernia: the Melbourne perspective. J Ped Urol 2005; 1: 11-25.
  46. Huff D S, Hadziselimovic F, Snyder H M, Blythe B, Ducket J W., Histologic maldevelopment of unilaterally cryptorchid testes and their descended partners. Eur J Pediatr 1993;152: Suppl 2, S11-14.
  47. Cortes D, Thorup J M, Beck B L., Quantitative histology of germ cells in the undescended testes of human fetuses, neonates and infants. J Urol 1995; 154: 1188-1192.
  48. Canavese F, Cortese M G, Magro P, Lonati L, Teruzzi E, de Sanctis C, Lala R., Cryptorchidism: medical and surgical treatment in the 1st year of life. Pediatr Surg Int 1998;14: 2-5.
  49. Huff DS, Fenig D M, Canning D A, Carr M G, Zderic S A, Snyder H M 3rd, Abnormal germ cell development in cryptorchidism. Horm Res 2001; 55: 11-17.
  50. Mieusset R., Fouda P J, Vaysse P, Guitard J, Moscovici J, Juskiewenski S., Increase in testicular temperature in case of cryptorchidism in boys. Fertil Steril 1993; 59: 1319-1321.
  51. Setchell B P. The Parkes Lecture., Heat and the testis. J Reprod Fertil 1998; 114: 179-194.
  52. Ofordeme K G, Aslan A R, Nazir T M, Hayner-Buchan A, Kogan B A., Apoptosis and proliferation in human undescended testes. BJU Int 2005; 96: 634-638.
  53. Cain M P, Kramer S A, Tindall D J, Husmann D A., Alterations in maternal epidermal growth factor (EGF) effect testicular descent and epididymal development. Urology 1994; 43: 375-378.
  54. Cain M P, Kramer S A, Tindall D J, Husmann D A., Epidermal growth factor reverses antiandrogen induced cryptorchidism and epididymal development. J Urol 1994; 152: 770-773; discussion 774-775.
  55. Cortes D, Visfeldt J, Thorup JM., Erythropoietin may reduce the risk of germ cell loss in boys with cryptorchidism. Horm Res 2001; 55:41-5.
  56. Briggs LM, Baer A, Critchlow CW., Maternal, delivery, and perinatal characteristics associated with cryptorchidism: a population-based case-control study among birth in Washington state. Epidemiology 2002; 13: 197-204.