Page 314 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

Reproductive Toxicity and Endocrine Disruption Chapter | 17 281

VetBooks.ir are potential targets for many reproductive toxicants (i.e., to induce gonadotropin production by the anterior pitui-
tary gland (Senger, 2003). This endocrine milestone is
dioxins). These structures within the brain and proper
brought about by the postnatal developmental changes
function of the hypothalamic pituitary gonadal axis
facilitate development of the reproductive tract and endo- which allow the hypothalamus to overcome the negative
crine regulation of spermatogenesis in the male and the feedback of testicular androgens and estrogens in males
estrous or menstrual cycle in the female. The onset of and which facilitate the ovary’s ability to produce suffi-
puberty and sexual behavior in males and females, the cient estrogens to induce the preovulatory surge of GnRH
ability to achieve erection and ejaculation in males, and in females (Senger, 2003; Evans et al., 2007; Evans and
the normal progression of gestation, parturition and lacta- Ganjam, 2017). Many of the endocrine changes which
tion in females are all affected by the secretions of the come into play with the onset of puberty are also involved
hypothalamus and pituitary gland, as well as interactions in the transition from anestrus to the ovulatory season in
between these structures and the reproductive tract seasonally polyestrous female animals (Evans et al., 2007;
(Ginther, 1992; Senger, 2003; Evans et al., 2007; Evans Evans and Ganjam, 2017).
and Ganjam, 2017).
The hormones involved in the neuroendocrine control The Susceptibility of the Pubertal Process
of reproduction are produced in several regions of the to Reproductive Toxicants
brain. Melatonin is produced in the pineal gland. The
While puberty is often described simply in terms of a sin-
major hormones of reproductive interest which are of
gle, initial reproductive event (e.g., first estrus, ovulation
hypothalamic origin are dopamine, CRF, GnRH and
or ejaculation), the attainment of reproductive compe-
TRH. Oxytocin is released from the posterior pituitary
tency is actually a process which is susceptible to the
(neurohypophysis), and ACTH, FSH, LH, prolactin and
effects of reproductive toxicants. Xenobiotics can inter-
TSH are synthesized and released from the anterior
fere with important physiological and morphological
pituitary (adenohypophysis) (Ginther, 1992; Evans
transformations necessary for the normal stepwise pro-
et al., 2007; Evans and Ganjam, 2017). The production
gression toward reproductive competency. Pre-pubertal
and release of these hormones are regulated by various
follicular development, as well as the onset of the preovu-
positive and negative feedback loops, which are poten-
latory LH surge in the female and the transition in testicu-
tially susceptible to the effects of hormonally active
lar estrogen synthesis from the Sertoli cell to the Leydig
xenobiotics.
cell in the males of many species, in addition to the post-
natal proliferation of Sertoli cells in some mammals, are
Puberty all potentially susceptible to the adverse effects of xeno-
biotics. Pre- or peri-pubertal exposure to hormonally
The Onset of Puberty active xenobiotics, such as anabolic steroids and antian-
Puberty in male and female offspring implies reproductive drogens, can interfere with postnatal reproductive devel-
competence and corresponds to the onset of normal sper- opment and function and can impair an animal’s ability to
matogenesis in the male and reproductive cyclicity in the reach its maximum reproductive potential (Monosson
female. Puberty can be indicated in the female by the age et al., 1999; Evans et al., 2007; Evans, 2011a; Evans and
at first estrus or ovulation or even the age at which preg- Ganjam, 2017).
nancy can be maintained safely (Senger, 2003; Evans
et al., 2007; Evans and Ganjam, 2017). In the male, the Normal Male Reproductive Anatomy
age at the time of preputial separation and the acquisition
of the ability to ejaculate or the age at the first appearance and Physiology
of spermatozoa in the ejaculate or urine, as well as the Reproductive Anatomy of the Male
production of threshold concentrations of fertile sperm in
Anatomical structures associated with reproduction in the
the ejaculate, have all been used as indicators of puberty
male usually include, especially in mammals, paired tes-
(Senger, 2003). Species, nutritional status, environmental
tes (male gonads) positioned outside the abdominal cavity
and social factors, pheromones and photoperiod in short-
in most species; an excurrent duct system (i.e., efferent
or long-day breeders can all influence the age of onset of
ductules, paired epididymidies, vas deferens and urethra);
puberty in animal species (Senger, 2003; Evans et al.,
accessory sex glands (i.e., ampullae, seminal vesicles,
2007; Evans and Ganjam, 2017).
prostate and bulbourethral glands); a scrotum and its asso-
ciated thermoregulatory functions to protect the testes
The Endocrinology of Puberty from mechanical and thermal insult and some form of
From an endocrine perspective, puberty is associated with copulatory organ or penis with a mechanism for protru-
the ability of the hypothalamus to release enough GnRH sion, erection, emission of glandular secretions and