The influences of environmental factors on the male reproductive system have been much debated over the last 3 decades. We studied the impact of genes and environment on semen quality, sex hormone levels, and sperm chromatin stability by using a twin design. The study population consisted of monozygotic and dizygotic twins from the population-based Danish Twin Registry and a random selection of pairs of singleton brothers from the Danish Civil Registration System. All men were 20 to 45 years of age. The study population comprised 100 monozygotic twin brothers (50 pairs), 102 dizygotic twins (51 pairs), and 102 single-born brothers (51 pairs). A semen sample and blood sample were collected from all participants. Heritability was estimated to account for 20% (95% confidence interval = 0% to 68%) of the variation in sperm density. A higher heritability was found for the hormones reflecting Sertoli cell function (inhibin B, 76% [36% to 84%] and follicle-stimulating hormone, 81% [40% to 88%]) and for percent morphologic normal cells (41% [0% to 60%] and sperm chromatin parameters (mean alphaT, 68% [34% to 81%] and COMP alphaT, 72% [25% to 82%], respectively). Our study indicates a substantial hereditary component in plasma levels of hormones reflecting Sertoli cell function and in sperm cell chromatin stability and morphology. The environmental contribution (including the prenatal environment) appeared to be largest for sperm count.
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