The results of this study clearly indicate that continuous exposure of males starting from low concentrations of dioxin before and after birth due to the mother being exposed during the Seveso accident, and then due to breast-feeding during “neonatal minipuberty,” results in a permanent impairment of the reproductive system (reduction of about 50% of sperm concentration and total sperm count and about 20% of sperm progressive motility, and increase of FSH with decrease of inhibin B) in young adulthood.
This impairment is not seen in males who were born to similarly exposed Seveso mothers who did not breast-feed. In fact, this group of men had sperm counts similar to those of the breast-fed and formula-fed comparison group.
The breast-fed exposed men had lower sperm variables than did the breast-fed comparison and formula-fed exposed men. In the latter case, statistical significance was not reached, probably because of the small number of cases. These observations on semen quality are considerably reinforced by corresponding changes in hormone levels: decreased inhibin B and increased FSH found in the breast-fed exposed group.
We had the rather unique opportunity to discriminate TCDD exposure with respect to background levels. Men exposed to rather high maternal serum TCDD concentrations at birth, such as 54.6 ppt (the 75th percentile of formula-fed exposed children), plus the then background value of 90 ppt of dioxin-like chemicals, did not show differences compared with the breast- and formula-fed comparison group members.
At the age of 4–5 months, formula-fed infants almost double their birth weight and halve the dioxin concentration, to about 27 ppt for the formula-fed exposed group and 5 ppt for the formula-fed comparison group, whereas the breast-fed comparison group approximately doubles the concentration to 20 ppt because of background dioxin exposure. On the other hand, the median of 19 ppt of dioxin in the breast-fed exposed group increases with breast-feeding to about 40 ppt. Our data show that impaired semen quality in adulthood results from exposure in utero
, as well as continuous exposure through breast-feeding during “neonatal minipuberty,” with adverse effects starting with a dioxin exposure range of 19–40 ppt and higher (the concentrations of background dioxin-like chemicals must also be added). Similar doses of TCDD administered to rhesus monkeys during pregnancy and lactation have recently been shown to produce lower semen quality in their young adult offspring (Arima et al. 2009
). It would be interesting to investigate the effect of TCDD administration only during lactation.
Our observations show for the first time that continuous exposure of the developing human male in utero, and even more so during lactation, the “neonatal minipuberty period,” to modest elevations of dioxin above background exposure levels can permanently impair later adult sperm production.
In fact, the only similar data (Guo et al. 2000
) are related to in utero
and nursing exposure of children to high doses of burned PCBs/PCDFs, but not to TCDD. That study demonstrated a reduction in sperm motility, but not density, as well as reduced egg penetration ability. Unfortunately, data concerning the specific concentration of the toxicants in maternal blood at conception were not available.
Mechanism of action
Dioxin and the structurally related PCDF chemicals mediate their toxic effect (but with different potencies) mainly as a ligand to the same aryl hydrocarbon receptor (AHR)/AHR nuclear translocator (ARNT) complex that binds to the xenobiotic responsive element (XRE; also called dioxin-responsive element, DRE) on target DNA. The widely expressed AHR/ARNT, when ligand activated, interacts with several transcription factors, steroids, and growth factors and possibly plays a critical constitutive role, especially in early tissue development. Through targeted gene networks, AHR/ARNT regulates or directly intervenes in reproductive system development (Schultz et al. 2003
As our study shows, the untimely interference of dioxin in utero
and within the first 4–5 months of life (the “neonatal minipuberty”) (Forest et al. 1973
; Quigley 2002
) results in a permanent impairing effect. It is during this time that the human testes almost double their volume (Cortes et al. 1987
), due in great part to Sertoli cell proliferation, which determines spermatogenic potential (Sharpe et al. 2003
). An indication of possible Sertoli cell number (or functionality) reduction is given by the decrease of inhibin B, a marker of their activity, associated with the decrease of sperm concentration, not only in the exposed breast-fed with respect to the comparison breast-fed group, but also in the exposed breast-fed compared with the exposed formula-fed group. The increase of FSH associated with the decrease of inhibin B (as observed here) has been reported to be a very sensitive marker for impaired spermatogenesis in men (von Eckardstein et al. 1999
It is well established that sons born to mothers who smoked heavily in pregnancy have a 20–50% reduction in sperm concentration in adulthood (Jensen et al. 2005
; Jensen et al. 2004
; Ramlau-Hansen et al. 2007
), reduced blood levels of inhibin B, and increased FSH (Storgaard et al. 2003
), compared with unexposed males. No information is provided about smoking during breast-feeding in these studies, but it is likely that exposure to the chemicals in tobacco smoke occurred both prenatally and postnatally, similar to our dioxin study. This is relevant to our data because a common mechanism of the phenomenon we observed and the one due to smoking could be the action on XRE (Kasai et al. 2006
) of some polycyclic aromatic hydrocarbon (PAH) compounds contained in cigarette smoke, using the same AHR as dioxin.
Relevance to general population and individuals
These observed effects of perinatal dioxin exposure on lowering adult sperm counts might also explain, in part, the wider semen quality reduction reported in young men in some industrialized regions, such as in Spain, Denmark, and Germany (Andersen et al. 2000
; López-Teijòn et al. 2008
; Paasch et al. 2008
; Zheng et al. 1997
). The men observed in these studies were in fact born in the 1970s and 1980s to mothers born in the 1950s and 1960s, a period when environmental concentrations of dioxin and dioxin-like chemicals peaked (Hagenmaier and Walczok 1996
). The bioaccumulation of these compounds with age is well demonstrated, and a concurrent effect of maternal smoking could also contribute to this phenomenon.
Because of environmental policies instituted throughout most of Europe and the United States, TCDD and total TEQ concentrations in women 20–40 years of age have recently decreased sharply, to less than 2 ppt and 12 ppt, respectively (Päpke 1998
; Patterson et al. 2008
). Thus, current serum dioxin levels in infants in these areas, even after being increased 2- to 3-fold because of breast-feeding, are far from the concentrations that would yield adverse effects.