Current evidence shows increasing rates of male reproductive disorders, including cryptorchidism, hypospadias, poor semen quality, decreased testosterone level, and testicular cancer. Therefore, studies on the physiology of the male reproductive system, and the current state of male reproductive health are necessary.

In the first study, we followed the semen quality of young men for 10 years from the age of 19 years to see whether there are age-related changes in semen quality. In the second study, we compared the semen quality of young Finnish and Danish men to clarify possible differences between the countries. In the third study, we compared serum levels of reproductive hormones and insulin-like growth factor (IGF)-I) during puberty between individuals with and without a history of cryptorchidism to see whether a history of orchiopexy or delayed testicular descent is associated with alterations in the levels of these hormones.

The first study showed that sperm concentration and total sperm count do not generally increase after 19 years, indicating that the maximum sperm production capacity is reached already in young adulthood. Sperm motility and morphology continue to improve after the age of 19 years. The second study showed that unlike in earlier studies, sperm concentrations, total sperm counts, and percentages of morphologically normal spermatozoa of Finnish and Danish men were now similar. However, Finnish men had better sperm motility. The third study showed that especially boys with a history of operated bilateral cryptorchidism had higher serum FSH and lower inhibin B levels than non-cryptorchid boys during puberty. These findings indicated an impaired Sertoli cell and/or germ cell function or number. Leydig cell function and the function of the growth hormone-IGF-I system were generally well-preserved during puberty in boys with a history of cryptorchidism.