Scholarly article on topic 'Environmental pollutants and dysregulation of male puberty—A comparison among species'

Environmental pollutants and dysregulation of male puberty—A comparison among species Academic research paper on "Environmental engineering"

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Abstract of research paper on Environmental engineering, author of scientific article — Ulf Magnusson, Karl Ljungvall

Abstract The scientific literature on altered onset of puberty predominantly involves studies on females. This paper reviews current knowledge on the role of environmental pollutants in dysregulation of male puberty in humans, laboratory rodents and farm animals. The methods used to determine the onset of puberty are well developed in humans and farm animals, and standardized across studies in humans. In laboratory rodents standardized external morphological endpoints are used. There is an increasing weight of evidence from epidemiological studies in humans, as well as from experiments in animals, indicating that environmental pollutants dysregulate puberty in males. Most data are from studies on “classical” persistent environmental pollutants. Assessing the effect of multichemical environmental pollution on dysregulation of puberty in humans is more challenging; further solid epidemiological data would likely contribute most to our understanding, especially if combined with systematically collected field-data from selected wildlife.

Academic research paper on topic "Environmental pollutants and dysregulation of male puberty—A comparison among species"

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Reproductive Toxicology xxx (2013) xxx-xxx

ELSEVIER Review

Environmental pollutants and dysregulation of male puberty—A comparison among species^

Ulf Magnusson3'*, Karl Ljungvallb

a Division of Reproduction, Department of Clinical Sciences, Swedish University of Agricultural Sciences, P.O. Box 7054, SE-750 07 Uppsala, Sweden b Swedish Medical Products Agency, P.O. Box 26, SE-751 03 Uppsala, Sweden

Contents lists available at ScienceDirect

Reproductive Toxicology

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ARTICLE INFO

ABSTRACT

Article history: Received 31 December 2012 Received in revised form 29 July 2013 Accepted 12 August 2013 Available online xxx

Keywords:

Male puberty

Farm animal

Rodent

Wildlife

Chemicals

Environmental pollution

The scientific literature on altered onset of puberty predominantly involves studies on females. This paper reviews current knowledge on the role of environmental pollutants in dysregulation of male puberty in humans, laboratory rodents and farm animals. The methods used to determine the onset of puberty are well developed in humans and farm animals, and standardized across studies in humans. In laboratory rodents standardized external morphological endpoints are used. There is an increasing weight of evidence from epidemiological studies in humans, as well as from experiments in animals, indicating that environmental pollutants dysregulate puberty in males. Most data are from studies on "classical" persistent environmental pollutants. Assessing the effect of multichemical environmental pollution on dysregulation of puberty in humans is more challenging; further solid epidemiological data would likely contribute most to our understanding, especially if combined with systematically collected field-data from selected wildlife.

© 2013 The Authors. Published by Elsevier Inc. All rights reserved.

Contents

1. Introduction....................................................................................................................................................................................................................................................................................00

2. Materials and methods..............................................................................................................................................................................................................................................................00

3. Regulation of normal puberty and possible implications of dysregulated male puberty......................................................................................................................00

4. Endpoints used to determine onset of puberty............................................................................................................................................................................................................00

4.1. Boys......................................................................................................................................................................................................................................................................................00

4.2. Rodents ..............................................................................................................................................................................................................................................................................00

4.3. Farm animals..................................................................................................................................................................................................................................................................00

5. Associations between chemical exposure and onset of puberty........................................................................................................................................................................00

5.1. Polychlorinated biphenyls (PCBs)........................................................................................................................................................................................................................00

5.2. Dichlorodiphenyltrichloroethane (DDT), dichlorodiphenyldichloroethylene (DDE)................................................................................................................00

5.3. Dioxins................................................................................................................................................................................................................................................................................00

5.4. Polybrominated diphenyl ethers (PBDEs) ........................................................................................................................................................................................................00

5.5. Perfluorocarbons..........................................................................................................................................................................................................................................................00

5.6. Phthalates........................................................................................................................................................................................................................................................................00

5.7. Alkylphenols....................................................................................................................................................................................................................................................................00

5.8. Pesticides..........................................................................................................................................................................................................................................................................00

5.9. Lead......................................................................................................................................................................................................................................................................................00

6. Species differences and similarities..................................................................................................................................................................................................................................00

6.1. Differences in types of data and challenges in interpretation..............................................................................................................................................................00

6.1.1. The issue of exposure..............................................................................................................................................................................................................................00

6.1.2. Different endpoints..................................................................................................................................................................................................................................00

* This is an open-access article distributed underthe terms ofthe Creative Commons Attribution License, which permits unrestricted use, distribution and reproduction in any medium, provided the original author and source are credited. * Corresponding author. Tel.: +46 18 672324. E-mail addresses: ulf.magnusson@slu.se (U. Magnusson), karl.ljungvall@mpa.se (K. Ljungvall).

0890-6238/$ - see front matter © 2013 The Authors. Published by Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.reprotox.2013.08.002

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6.2. Are there consistent effects across species?..................................................................................................................................................................................................00

6.2.1. Estrogenic actions....................................................................................................................................................................................................................................00

6.2.2. Antiandrogenic actions..........................................................................................................................................................................................................................00

6.2.3. Other actions........................................................................................................................ 00

7. Conclusions....................................................................................................................................................................................................................................................................................00

Conflicts of interest....................................................................................................................................................................................................................................................................00

References......................................................................................................................................................................................................................................................................................00

1. Introduction

Over the last decades there has been an increasing attention paid to the decreasing age of onset of breast development in girls in North America and Europe [1,2]. The onset of puberty in humans depends on a range of factors such as genetics, nutrition and socioeconomic conditions [3]. However, the presently observed secular trend in decreased age at onset of puberty cannot be a genetic phenomenon since the change has been too rapid. Among environmental factors, changes in socio-economic conditions have been excluded as important factors in the US and in Europe [1,2], whereas altered nutritional status, like the obesity epidemic, may play a significant but not fully explained role in the trend [4,5]. Another possible, and worrying, explanation for this secular trend relates to the impact of chemical pollution of the environment [6-10].

Human data on the connection between reproductive toxicology and environmental pollution are, for obvious reasons, epidemiological rather than experimental. However, causality for the associations or trends in these epidemiological data has often been supported by data from experiments with laboratory animals [reviewed by 11, 12, 6]. A third source of data for the connection between reproductive disorders and environmental pollution is from field studies of wildlife [reviewed by 13, 14, 15]. However, according to our knowledge there are no internationally published data related to wildlife, regarding the dysregulation of puberty by environmental pollutants.

The current literature on altered onset of puberty predominantly involves studies on females. The objectives of this paper were, therefore, to review the current knowledge of the role of environmental pollutants in dysregulation of male puberty, specifically to: (i) review the methodology used to determine the onset of puberty in boys and males of other mammals; (ii) review the epidemiological as well as experimental evidence of a connection between alteration in onset of puberty and chemicals in boys and other male mammals; (iii) comment on differences and similarities among species regarding chemical dysregulation of puberty in the male.

2. Materials and methods

Articles for this review were found in the PubMed database (http://ncbi.nlm.nih.gov/pubmed) by using the search-string Puberty Chemicals Male. In addition the authors used other relevant original or review articles they were aware of. Original articles that either dealt with measuring different stages of puberty or that recorded male fertility parameters at the time around presumed puberty were then scrutinized thoroughly.

3. Regulation of normal puberty and possible implications of dysregulated male puberty

The increase in pulsatile release of gonadotrophin-releasing hormone (GnRH) from the hypothalamus is considered as the onset of puberty in mammals. However, this increase in release of GnRH is preceded by a number of trans-synaptic and glia-neuronal stimulations of the GnRH neuronal network [16].

From a reproductive viewpoint, puberty could generally be described as the activation of the hypothalamic-pituitary-gonadal axis, leading to steroidogenesis, gametogenesis and downstream events. In the male, the key features are the gonadotrophin-induced production of testosterone in the testicular Leydig cells and growth and development of the seminiferous epithelium.

It is known that the age of onset and length of puberty in several species are influenced by a variety of factors such as genetics and nutrition, as well as the physical and social environment [17,18]. Chemical pollutants in the environment may, theoretically, interact at several positions along the neuroendocrine axis described above, as well as with systems regulating the axis' peripheral target organs and growth.

Dysregulated male puberty, in terms of early or delayed onset, may have different implications in different groups of mammals. For instance, in humans, where the time for normal onset of puberty ranges over several years, an early or late puberty might result in mostly psychosocial issues [19]. In farm animals, delayed puberty in males will have negative effects on the reproductive performance of the whole herd and if the species is a seasonal breeder the negative effect may be particularly prominent, i.e., in its extreme there might not be any breeding at all in a certain year. Early puberty may, on the other hand, cause management problems and extra costs, in that males and females must be separated or kept apart at a much earlier age than normal. In wildlife, dysregulated puberty might affect population-dynamics entirely in those species that are seasonal breeders; that is, breeding might be delayed a whole year if puberty is not completed before the breeding season. Furthermore, early and late puberty may disturb the complex hierarchical system among adult males in several species, with negative effects on the population's reproductive output.

4. Endpoints used to determine onset of puberty

There is a considerable discrepancy in the availability of endpoints to determine the onset, stage or completion of male puberty among species. In the human, for instance, there is a well-established staging system, whereas in laboratory rodents some single standardized endpoints are used. Another approach used, for instance, in domestic animals, is to use male fertility variables in longitudinal or cross-sectional studies at the time around puberty as a proxy for onset, stage or completion of puberty.

4.1. Boys

In boys, the well-established staging system for determining pubertal development assesses genital development as well as pubic hair growth [20]. Another common endpoint used to determine puberty in boys is testicular volume [21]. In addition, blood concentration of testosterone, penile length and other reproductive endpoints have been used (see Table 1 for more examples).

4.2. Rodents

In male rats the separation of the preputium from the glans penis (PPS) has been proposed as an external sign of puberty

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Table 1

Summary of studies of pubertal dysregulation in male mammals investigating the possible role of environmental pollutants.

Chemical(s) Species Recorded Type of study Data on Concentration in Effect on male Reference

end-point exposure tissue/dose puberty

Estradiol Rat PPS Experimental, Postnatal day 1, 0.015 |xg/kg to Inhibition of [39]

cross-sectional 3 and 5 15 mg/kg preputial

and separation

longitudinal

PCB, DDE Human Reproductive Epidemiological, Measured Interquartile Lowered LH- and [38]

hormones, cross-sectional prenatally and range:Cord testosterone

testicular size, at time of study blood PCB concentration

Tanner stage ng/mL:1.16-3.16; associated with

DDE ng/mL high prenatal PCB

0.49-1.57; exposure

serum at

examination:

PCB |xg/g lipid:

0.49-1.56; DDE

|xg/g lipid

0.29-1.14

PCB Human Penile length, Epidemiological, Prenatal Mean serum PCB Shorter penis at [33]

sperm quality cross-sectional exposure concentrauion in time of puberty

exposed

mothers:

49.3ng/ml

PCB, DDE Human Tanner stage Epidemiological, Measured Total exposure No effects [34]

longitudinal prenatally and estimates

and during breast Prenatally PCB

cross-sectional feeding 0-3+ppm; DDE

0-4+ ppm

lactational: PCB

0-10+mg DDE

0-15+mg

PCB Human Spermaturia, Epidemiological, Measured Cord tissues PCB No effect [36]

sex hormone cross-sectional prenatally 1.96 vs. 1.82 ng/g

concentrations,

Tanner stage,

testis size

PCBs Human Pubertal Epidemiological, Measured at Sum of marker Delayed genital [35]

development, case-control the time of PCB in serum: deveopmnet and

Testicular study 1.34-2.49 nmol/L pubic hair growth

volume

PCBs Goat Sex hormone Experimental, Fetal and PCB126 PCB 153 altered LH [30]

concentration, cross-sectional lactational 49ng/kg/day; and testosterone

testis PCB153 lconcentrations,

morphology, 98 |xg/kg/day; and increased

sperm quality orally three percentage of

times weekly DNA-damage in

sperms

PCB or estradiol Rat PPS Experimental, Gestational day 50 |xg/kg of Delayed onset of [24]

cross-sectional 16 and 18 estradiol puberty in both

and benzoate, estradiol and PCB

longitudinal 1 mg/kg of exposed males

PCB.mix

DDE Human Testosterone Epidemiological, Measured DDE in maternal No effect [41]

concentrations cross-sectional prenatally serum

<14-20 |xg/g

DDE, Rat PPS Experimental, Gestational day Effects at Delayed onset of [42]

investigated cross-sectional 14-18 200 mg/kg puberty

with dioxin and

longitudinal

Dioxins Human Tanner stage, Epidemiological, Measured Median Higher age at first [45]

testicular cross-sectional prenatally, concentrations ejaculation

volume, first during breast in breast milk

ejaculation feeding and at Prenatal

the time of 28.6 pg/g lipid;

examination in serum at

examination

2.3 pg/g lipid

Dioxin, PCB Human Tanner stage Epidemiological, Measured Median maternal PCB; earlier onset [37]

and testicular cross-sectional, prenatally serum sum PCB: of puberty (by

volume longitudinal 260 ng/g lipid, staging). Dioxin

median maternal and breast feeding

serum total weakly associated

dioxin: 25TEQ/g with delayed

lipid puberty

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Table 1 (Continued)

Chemical(s) Species

Recorded end-point

Type of study

Data on exposure

Concentration in tissue/dose

Effect on male puberty

Reference

Dioxin

Experimental,

cross-sectional

longitudinal

Experimental,

cross-sectional

longitudinal Experimental, cross-sectional and

longitudinal

Gestation, and lactation since the compound is excreted over a long period of time Postnatal day 23-54

Gestational day 6-weaning

TCDD: 0, 2S, 93 or S30 ng/kg feed

0, 2, 30 or б0 mg/kg(day

0,1.7,10.2, 30.б mg/kg/day

Delayed onset of puberty

Delayed onset of puberty.

Antagonist for the androgenreceptor Delayed onset of puberty

PBDE, PCB Rat PPS Experimental, Gestational day PBDE 1 or PBDE: Early onset [40]

cross-sectional 10-18 10 mg/kg/day. of puberty. PCB:

and PCB delayed onset of

longitudinal 30 mg/kg/day puberty

Endosulfan Human Tanner stage, Epidemiological, Measured at Mean total Delayed puberty by [6S]

reproductive case-control the time of endosulfan (ppb) Tanner stage and

hormones study in serum 1.37 vs. testosterone

7.47 concentrations

Nonylphenol Human Body hair, face Epidemiological, Measured at <1.62-178.25 |xg/g No effect [б3]

hair, voice cross-sectional the time of creatinine in

change and study urine

Tanner Stage

Octylphenol Sheep Reproductive Experimental, Prental - s.c injection Birth to weaning [31]

hormones cross-sectional rebubertal 1 m/kg/day at exposure was

concentration, and differnt stages of associated with

Semen longitudinal pregancy and/or Increased number

analysis, sperm lactation of morphologically

morphology, abnormal sperm

motility and cells

fertilization

ability

Lead Human Testicular Epidemiological, Measured at <1-31 |xg/dL Delayed onset of [71]

volume and cross-sectional the time of blood puberty

Tanner stage study

Non-persistant Human Testicular and Epidemiological, Measured Not Recorded Smaller testes, [бб]

pesticides pituitary case-control prenatally Estrogenicity shorter penile

hormones, and length and lower

testicular androgencity in inhibin B

volume, penile serum concentrations,

length, tanner determined

stage in

prepubertal

Prochloraz Rat PPS Experimental, Exposure from 0,31.3, 62.5 or 125 mg/kg and day [б7]

cross-sectional post natal day 125 mg/kg/day caused a delay in

and 23 pubertal onset.

longitudinal Probably blocking

the androgen

receptor

Vinchlozolin Rat PPS Experimental, Postnatal day 0,10,30 or Delayed onset of [23]

cross-sectional 22-56 100 mg/kg/day puberty.

and Antiandrogenic

longitudinal action resulting in

increased LH and

testosterone

Dichlorophenyl Rat PPS Experimental, Postnatal day 0, 50,100 or Delayed onset of [S3]

dicarboximide cross-sectional 23-52 200 mg/kg puberty, inhibition

fungicide and of steroid hormone

Iprodione longitudinal synthesis

(IPRO)

Atrazine and its Rat PPS Experimental, Postnatal day Delayed onset of [68,69]

metabolites cross-sectional 23-53 puberty, possibly

and due to a

longitudinal downregulation of

Atrazine Rat PPS Experimental, Gestational day 0 or Delayed onset of [70]

cross-sectional 15-19, also 100 mg/kg/day puberty. Possibly

and resulting in due to effects on

longitudinal lactational LHRH

exposure

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Table 1 (Continued)

Chemical(s) Species Recorded Type of study Data on Concentration in Effect on male Reference

end-point exposure tissue/dose puberty

Methoxychlor Rat PPS Experimental, From 0, 24, 240 or Delayed onset of [84]

cross-sectional gestational day 1200 ppm in diet puberty

and 15 to post natal

longitudinal day 10

Dibromoacetic Rat PPS Experimental, From 0, 4, 40 or More than 400 ppm [85]

acid in drinking cross-sectional gestational day 400 ppm in caused a delay of

water and 15, into drinking water onset of puberty

longitudinal puberty

DIBP Rat PPS Experimental, Gestational day DIBP at 125, 250, Early onset of [54]

cross-sectional 12-21 500 or puberty with

and 625 mg/(kg day) lowest dose of

longitudinal DBP at DIBP. Delayed

500 mg/(kg day) onset of puberty

with hihgest dose

and with DBP

DEHP Rat PPS Experimental, Postnatal day 0,10,500 or Early onset at [55]

cross-sectional 21-48 750 mg/kg/day 10 mg/kg, small

and effects at

longitudinal 500 mg/kg and

delayed onset of

puberty at

750 mg/kg

DEHP Pig Sex hormone Experimental, Weeks 3-7 of 0 or 300 mg/kg No effect [82]

concentration, cross-sectional life by mouth every

testis and other day

morphology longitudinal

DEHP Rat PPS Experime, Postnatal day 0,10,100,300 or Delayed puberty at [56]

cross-sectional 22 or 23-56, 64 900 mg/kg/day 300 and

and or 98 900 mg/kg/day, no

longitudinal effects at lower

exposure

DEHP Pig Sex hormone Experimental, Weeks 3-7 of 0 or 300 mg/kg Precocious [57]

concentration, cross-sectional life by mouth every development of

testis and other day bulbourethral

morphology, longitudinal glands

accessory sex

morphology

DEHP Pig Sperm Experimental, Weeks 3-7 of 0 or 300 mg/kg No effect [29]

morphology cross-sectional life by mouth every

and other day

longitudinal

PFOA, PFOS Human Testosterone Epidemiological, Measured at >27 ng/mL serum Delayed puberty [51]

concentrations cross-sectional the time of associated with

study PFOS

PFOA Mouse PPS Experimental, Gestational day Perfluorooctanoic Earlier onset of [52]

cross-sectional 1-17. Due to acid (PFOA) at 1, puberty

and the 3,5,10, 20 or

longitudinal toxicokinetics 40 mg/kg

of PFOA, also

during suckling

Abbreviations: DDE: dichlorodiphenyldichloroethylene, DDT: dichlorodiphenyltrichloroethane, DEHP: di-2-ethylhexyl phthalate, DIBP: diisobutyl phthalate, PBDE: poly-brominated diphenyl ethers, PCB: polychlorinated biphenyls, PFOA: perfluorooctanoic acid, PFOS: perfluorooctane sulfonate, and PPS: preputial separation.

[22]. The separation occurs around day 40, but can vary [23,24]. Additionally, descent of the testicles into the scrotum has been used as a marker of progression of sexual maturation and also of the anti-androgenic effects of environmental chemicals [25,26]. Testicular descent occurs around day 25 after birth in the male rat [26].

4.3. Farm animals

In farm animals, puberty is defined as the ability to accomplish reproduction successfully and comprises the following elements in order of appearance: sexual behavior, ejaculation and the presence of ejaculate with enough sperm cells to succeed in fertilization. Even though there are several studies on the effect of environmental pollutants on the reproductive system in farm animals [27], the

above mentioned elements of puberty have rarely been used in a systematic way to assess pubertal development in relation to presumed chemical dysregulation. However, in a set of longitudinal studies in pigs, sexual behavior, ability to ejaculate and the quality of ejaculate were assessed in relation to exposure to the plastic softener di-2-ethylhexyl phthalate [28,29]. Furthermore, male fertility parameters recorded at the time of puberty, such as semen and sperm quality, concentration of reproductive hormones and testis morphology, have been used to study the effect of chemical exposure on pubertal development in goats [30] and sheep [31].

In summary, the approaches used for the study of pubertal development are different among species. In laboratory rodents, single standardized endpoints have frequently been used, whilst in studies on boys and farm animal more complex, composite endpoints have been developed.

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5. Associations between chemical exposure and onset of puberty

(For summary of detailed data please see Table 1).

5.1. Polychlorinated biphenyls (PCBs)

The PCBs are the classical environmentally persistent and bioac-cumulative chemicals [32]. They are very hydrophobic, found in high concentrations in human and wildlife tissues and are globally distributed. PCBs are technical mixtures used for insulation of electrical devices and as sealants in buildings until the 1980s and comprise more then 100 congeners defined by the position and number of chlorine atoms. There are several studies about the association between PCB exposure and onset of puberty from various parts of the world such as Western Europe, Russia, Taiwan and the US [33-38]. Notably, exposure data are extensive, often comprising records that are prenatal, lactational and collected at the time of examination of the boys. However, the effects on puberty are not unambiguous: some studies suggest positive associations between PCB levels and delayed puberty whereas others find no such associations. In rodent models, PCBs have been demonstrated to delay the onset of puberty as indicated by PPS in male rats, this has been suggested to be mediated by an anti-androgenic action [24,39]. Male goats exposed to PCBs in utero and during lactation showed altered testosterone and LH levels around puberty as well as a high proportion of damaged sperm at lower dosages than those causing effects in rats (see Table 1) [30]. PCBs have been suggested to influence thyroid hormone regulation, as well as being estrogenic and anti-androgenic [24,40].

5.2. Dichlorodiphenyltrichloroethane (DDT), dichlorodiphenyldichloroethylene (DDE)

Like the PCBs, DDT/DDE is highly persistant and bioaccumula-tive, and found all over the world in most species [32]. It has a broad range of agricultural as well as non-agricultural uses. The most well-known use is that of controlling the malaria mosquito. The use of DDT was banned in many countries during the 70s and 80s, but it is still used in some countries for indoor spraying against malaria mosquitos. In a study on boys born in the sixties in eastern US, no association could be seen between the onset of puberty and concentrations of DDT in the serum collected from their mothers during pregnancy [41]. Similarly, in a recent study of boys on the Faeroe Islands, no or only weak associations were found between DDE concentrations in blood collected at the time of examination and pubertal development [38]. In contrast, doses of 200 mg per kg of bodyweight of DDE administered to pregnant rats caused a delay in the onset of puberty in male offspring [42]. The latter effect in rats is consistent with the view of DDE and DDT as estrogenic or anti-androgenic as reviewed by Mrema et al., [43]. Furthermore, exposure to persistent organic pesticides at doses of 50-100 mg/kg/day may also affect the levels of thyroxine [43]. However, the doses used in these references probably result in higher internal exposure than has been the case in the epidemi-ological studies in man.

5.3. Dioxins

Dioxins form a group of substances that includes polychlori-nated dibenzo-para-dioxins (PCDDs), polychlorinated dibenzofu-rans (PCDFs), and a subgroup of PCBs with dioxin-like properties [44]. The dioxins show long persistence in the environment and animals. They are highly lipophilic and accumulate in the adipose fraction of organs and tissues. While PCBs were produced for commercial purposes, the other dioxins are unwanted by-products

of combustion processes of chlorine-containing components and are highly toxic. Measures have therefore been taken in several countries in an attempt to reduce dioxin contamination ofthe environment.

In a recent study in a Russian town contaminated with dioxins during past industrial activities, associations were seen between delayed pubertal onset and maternal serum dioxins concentrations, in boys who were breast-fed for 6 months [37]. Boys born in urban areas of the Netherlands showed a delayed age of first ejaculation if they were exposed to high prenatal and current concentrations of dioxins. However, no such association was seen for Tanner stage or testicular volume [45]. In rats, maternal exposure to dioxins caused delayed onset of puberty in male offspring at all investigated dose levels, including 28 TCDD ng/kg feed [46]. The latter dose resulted in blood levels in the dams of 1.34 ± 19 ng/kg. Dioxins are known to interact with the aryl hydrocarbon receptor (AhR), for which there is no known endogenous ligand. Interaction with the AhR may in turn affect other receptor systems, such as the estrogen receptors. However, the exact mechanisms for dioxin teratogenicity or developmental disturbances remain poorly understood [47].

5.4. Polybrominated diphenyl ethers (PBDEs)

Like PCBs, PBDEs are technical mixtures consisting of several congeners that are very persistent in the environment [32]. PBDEs are used as flame-retardants in textiles, furniture, buildings etc. Some congeners bioaccumulate and magnify in the food web and are found globally in humans and wildlife. In rats there are conflicting data regarding the effects of PBDEs on puberty. This may in turn depend on the timing of exposure since short-term exposure during gestation initiated early onset of external signs of puberty, while exposure during lactation or at the time of puberty was associated with delayed onset of puberty [40,48,49]. Additionally, different mixtures of PBDEs may also exert different effects, with PBDE-99 having less pronounced effects on the androgen receptor than DE-71 [40]. There is also evidence of PBDEs having negative effects on the thyroid gland in rats [40].

5.5. Perfluorocarbons

These perfluorated compounds are used as surfactants in different products such as stain repellants, fire-fighting foams and emulsifier in Teflon®. The bioaccumulating potential of these compounds is unclear. They are, however, highly resistant in the environment [50].

A cross-sectional study of boys was performed in an area in the US where water was contaminated with perfluorcarbons. Puberty was determined by blood testosterone level [51] and a relationship was found between reduced odds of reaching puberty and increasing levels of perfluorooctane sulfonate (PFOS) in the blood at the time of examination. In male mice, exposure to perfluorooctanoic acid (PFOA), at dose levels theoretically resulting in approximately 100ng/L in maternal blood, during fetal life have been associated with premature onset of puberty, as indicated by age at preputial separation, despite a lower bodyweight at puberty [52]. The mechanisms for this last effect are unclear, but the authors suggested that the compound affected the secretion of luteinizing hormone.

5.6. Phthalates

Phthalates, or esters of phthalic acid, are widely used as industrial plasticizers. Different phthalates can be found in a variety of commercial products such as vinyl plastic containers, perfumes, detergents, medical devices or building material. Several phthalates have been demonstrated to have adverse effects on the endocrine system in laboratory animals [53].

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Early onset of puberty has been described in rats after continuous exposure to low doses, 10mg/kg/day, of phthalates from fetal life through the suckling period. After exposure to higher doses, 750 mg/kg/day, during the same periods of fetal life and suckling, delayed onset of puberty has been observed. These effects may be mediated by differentiated effects on testicular testosterone synthesis [54,55]. However, the results have later been questioned since the nonmonotonic response to DEHP could not be reproduced in two different strains of rats; only the delay in pubertal onset at the higher doses, 300 and 900mg/kg/day, was seen [56]. In male pigs exposed to di-2-ethylhexyl phthalate [DEHP] during the early postnatal period, there was precocious development of the bul-bourethral glands when compared to untreated siblings. This was linked to increased concentrations of sex hormones in the exposed animals [28,57]. Species differences in response to DEHP may be due to differences in dose rate, or due to differences in absorption and metabolism of the phthalic esters. Generally, phthalates are considered as anti-androgenic or weakly estrogenic in their action [58]. Additionally, phthalates have recently been demonstrated to activate the AhR in a fish embryo model [59].

5.7. Alkylphenols

Alkylphenols are organic compounds used in detergents, paints, herbicides, pesticides, emulsifiers and other kinds of chemicals. Their solubility in water varies from low to high as do the degrees of bioaccumulation, and they are found in several environmental matrices [60,61]. The alkylphenol, nonylphenol, originates from the degradation of nonylphenol ethoxylates, which are widely used as industrial surfactants [62]. Due to its low solubility and high hydrophobicity, nonylphenol accumulates in environmental compartments with high organic content. Due to the harmful effects of degradation products of nonylphenol ethoxylates in the environment, production of such compounds has been banned in the EU.

In studies of boys in Taiwan, where nonylphenol is ubiquitous in food, no association between nonylphenol concentration in urine at the time of study and onset of puberty was seen [63]. In sheep, parenteral exposure of male lambs with octylphenol, twice weekly from birth to weaning, increased the number of morphologically abnormal sperm [31]. Alkylphenols are most often considered estrogenic; however, some isomers have also been noted for their anti-estrogenic actions [58].

5.8. Pesticides

The pesticides are a chemically heterogeneous group. Endosul-fan is an organochlorine insecticide used for food and non-food crops [64]. It is regarded as a persistent organic pollutant and is found world-wide in the environment. The fungicide vinclozolin is used in many countries world-wide on fruits and vegetables and is regarded as moderately persistent [32]. Atrazine is a herbicide no longer approved in the EU, but used in many other countries in the world [32]. It is chemically relatively stable, especially in cold climates, and thus has a high potential for surface- and groundwater contamination.

Pubertal development in schoolboys who lived in an Indian village where endosulfan had been aerially sprayed for more than 20 years was compared with the development in boys from non-sprayed areas [65]. It was shown that development of pubic hair, testes and penis, as well as serum testosterone level, was negatively correlated with aerial exposure to endosulfan. Endosulfan is regarded as weakly estrogenic and potentiates endogenous estrogens [43].

In a study of Danish boys whose mothers had worked in greenhouses and been exposed there to non-persistent pesticides during

early pregnancy, it was found that sons to the exposed mothers had smaller genitals at school age compared to sons of non-exposed mothers [66]. Similarly, the anti-androgenic biocides prochloraz and vinclozolin appear to delay the onset of puberty in rats and also affect the morphology of male genital organs [23,67]. Other substances that delay pubertal manifestations in rats are atrazine and metabolites thereof. The mechanism of action may be by inhibition of the release of luteinizing hormone [68-70]. However, the high doses (tens or hundreds of mg active substance per kilogram of bodyweight) used in the experimental studies are unlikely to be encountered in natural settings.

5.9. Lead

Man-made products where lead has been used include, for instance, batteries, paints, gasoline-fuel and ammunition. In a population of boys from an area of Russia with environmental contamination of lead from large industries, the association between blood concentrations of lead and pubertal development was investigated [71]. It was shown that boys with blood levels of lead >5 |ig/dL had reduced odds for reaching a Tanner genital stage G2 compared to those with lower levels. The authors discussed the possibility of lead disrupting the release of GnRH as mechanism of action for this finding.

6. Species differences and similarities

6.1. Differences in types of data and challenges in interpretation

Considering the wide variety of methods used in different species, are there any advantages of combining epidemiological data from humans, laboratory rodents intentionally exposed to concentrations often higher than those found in a natural setting, and other mammals, such as farm animals or wildlife, to help increase our understanding of the contribution of environmental pollutants to dysregulation of puberty?

6.1.1. The issue of exposure

One challenge in interpreting the epidemiological data from humans, and also frequently from outbred populations of farm animals or wildlife, is the temporal relation between exposure data and effect data measurements (or "records"). It is generally believed that the severity or consequence of exposure to environmental pollutants depends not only on the dose but also on the stage of life the exposure takes place [72]. The effects of exposure during adult life are often transient, or reversible, i.e., they disappear when the exposure ceases. During development, the effects are mostly irreversible, i.e., they last throughout life, even if the exposure occurred in utero or early postnatally. There seem to be critical periods during development when the reproductive system is particular vulnerable to exposure to pollutants [73,74]. This means that data from these periods of exposure are critical for establishing associations between exposure and dysregulated puberty (more critical than the measurements of effects). Some of the epidemiological studies reviewed here lack exposure data from these assumed critical periods of development (see Table 1), making inferences difficult. Nevertheless, in some instances one may assume that there has been a continuous and uniform exposure from the environment over time, making exposure-data gathered at the time of examination of effects relevant also for the entire development period [e.g., 37]. Furthermore, internal exposure may be poorly characterized in laboratory studies, making comparisons with epidemiological data impossible. Yet another overarching issue when discussing exposure is, of course, that real life exposure is multi-chemical in nature. To perform multiple exposure experiments with different combinations of chemicals is very costly and

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methodologically challenging. Real life exposure-studies in farm animals such as sheep [75] or, to increase the sensitivity, in high-trophic level wildlife such as the mink [76,77], would complement the epidemiological studies in humans, with, in addition, fewer ethical constraints.

6.1.2. Different endpoints

The controlled exposure - effect experiments in inbred laboratory rodents are probably too simplistic to allow the effects of chemicals on the onset of puberty in males to be properly dissected, because dysregulation of the events of pubertal maturation may be more complex than can be deduced from one or two morphologic endpoints (see Table 1 and Section 4.2). The experimental studies in farm animals, with their more extended recording of the onset of puberty, are superior in this respect (Section 4.3). Again, the staging system used in epidemiological studies in humans is more developed than the endpoints used in rodents (Section 4.1). In general, detailed comparisons of the effects on dysregulation of puberty are difficult to perform among species due to the differences in endpoints recorded.

6.2. Are there consistent effects across species?

The comparison of effects in different species is complicated by differences in sensitivity to certain chemicals. For example, it has been demonstrated that mink are exceptionally sensitive to different effects of PCBs, compared to rats [78]. The reasons for such differences are not always well understood, but may include differences in the kinetics of absorption, metabolism or excretion, resulting in different internal exposure to the various metabolites of xenobiotic chemicals. This can be illustrated by the kinetics of PFOA, which differ markedly between rats and mice, two species that can be regarded as closely related [52]. Similarly, the plasma half-life of the main metabolite of the phthalate DEHP in the pig appears to be more than twice as long as in the rat [79]. Thus, to evaluate the consistency of chemical's effects on pubertal dysregulation across species one must bear in mind the possible, and largely unknown, differences in internal exposure across different species, and that downstream effects may also vary from species to species [80].

6.2.1. Estrogenic actions

Elevated concentrations of PCB or endosulfan in boys have been linked to delayed appearance of pubertal landmarks [33,35,45,65]. This is generally in accordance with studies that indicate that estro-genic substances inhibit or cause a delay in the appearance of pubertal landmarks in male rats, apparently due to either prenatal or postnatal exposure [24,39]. When directly comparing the effects of estradiol and PCB, it was found that both substances caused a delay in the appearance of PPS in rats [24]. However, there are also studies in humans where exposure to neither increased prenatal and lactational concentrations of estrogenic substances (such as PCB, DDE) nor increased current concentrations of nonylphenol have been associated with an effect on the timing of puberty[31,34,36,63]. Furthermore, elevated concentrations of PCB and DDE have been associated with lowered concentrations of luteinizing hormone and testosterone, without effects on Tanner stage in boys. To further complicate the picture, an association between increased serum concentrations of PCB in women and an earlier onset of puberty in their sons has been reported [37]. In addition, the endocrine disrupting and estrogenic mycotoxin zear-alenon has been associated with idiopathic precocious puberty in boys as indicated by testicular enlargement [81].

6.2.2. Antiandrogenic actions

There are sparse data on the effects of anti-androgenic substances such as PBDE, prochloraz and vinclozolin, in humans. However, no associations with pubertal timing and exposure to PBDE were seen [45]. In studies in rats, it appears that postnatal exposure to the above mentioned anti-androgens may cause a delay in pubertal onset, while gestational exposure may cause the opposite [23,67,4849,40]. The dioxins have been linked to anti-androgenic effects, possibly mediated via the Ah-receptor. There are indications that exposure to dioxins early in life causes delayed onset of puberty both in rats and, to some extent, in boys, although the associations are weak [37,46].

6.2.3. Other actions

Phthalates are generally considered as anti-androgenic in their actions and have been associated with increased concentrations of testosterone, which complicates interpretation of their biological actions [82]. Interestingly, phthalates seem to cause an early onset of markers of sexual development at lower doses in rats [54,55] and in pigs [57], but delay the onset at higher doses, possibly by interfering with androgen synthesis [54,55].

Other chemicals, such as atrazine and dichlorophenyl dicarbox-imide, interfere with the production of reproductive hormones, which seems generally to cause delayed onset of puberty in rats [67-70], although data are lacking in humans. There is scarce and conflicting data for the effects of fluorinated compounds in mice and humans; long term exposure to PFOA may cause early onset of puberty in mice [52] while exposure to PFOS was associated with delayed onset of puberty in boys [51].

7. Conclusions

This review shows that the methods used to determine the onset of puberty are well developed in humans and farm animals, and standardized across several studies in humans. In laboratory rodents a standardized external endpoint - the preputial separation - is mostly utilized. More diversified methodology for determining the onset of male puberty in laboratory rodents would likely enhance our understanding of the mechanisms of chemical dys-regulation. Since the timing of exposure to chemicals is known to be critical for effects on the reproductive system, another methodological improvement in future epidemiological studies would be to pay more attention to the linkage between exposure and effect. This should not only include the external exposure to chemicals but also consideration of the toxicokinetic aspects for the chemicals that are metabolized.

It can be concluded that there is an increasing weight of evidence, from sound epidemiological studies in humans as well as from experiments in animals, to support the claim that environmental pollutants dysregulate puberty in males. However, the data are unevenly distributed with respect to the chemicals concerned. Most data are from studies on the effects of classical persistent environmental pollutants. Potentially, the observed effects are a public health issue in the human population, an economical concern in farm animals and a population health issue in wildlife.

The effects are sometimes consistent within and among species and across different types of studies. When inconsistencies are reported, they may of course be due to different external exposure or be the result of differences in kinetics, leading to different internal exposure to parent compounds or metabolites. These inconsistencies may also be due to differences in the reproductive systems of the exposed species.

Finally, the epidemiological studies in humans indicating associations between delayed puberty and certain environmental pollutants suggest that the environmental concentration of these

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Conflicts of interest

The authors declare they have no conflicts of interests.

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