Biomedical Engineering Reference
In-Depth Information
exclusive milk feeding of young, i.e. weaning at a relatively
late age.
An unusual feature of lactation in most nonhuman
primates is the occurrence of a prolonged lactation-induced
anovulatory period, termed lactational amenorrhea in
nonhuman primates that undergo menstrual cycles. It is
well established that the suckling stimulus, rather than milk
production, is the driving force behind lactation's effects
upon the ovary, as the suckling stimulus results in impaired
hypothalamic GnRH release that in turn causes impaired
pulsatile LH release from the pituitary (
Weiss et al., 1976;
McNeilly, 1994
). The only nonhuman primate group in
which lactation-induced anovulation is not routine is the
New World marmosets and tamarins. While central
administration of oxytocin will inhibit LH release in rhesus
macaques (
Luckhaus and Ferrin, 1989
), such administra-
tion increases pituitary luteotropic hormone release in the
marmoset (
O'Byrne et al., 1990
).
followed by declining GTHs. These hormonal changes are
believed to affect disease risks (
Wise, 2006
). The risk
associated with bone loss due to decreasing estrogenic
activity on osteoblasts is well described; however, cardio-
vascular effects continue to be hotly debated.
With increasing numbers of older nonhuman primates
available for study, it is now clear that monkeys and apes
also experience follicular depletion and associated
hormonal alterations (
Hodgen et al., 1977; Graham, 1979;
Tardif, 1985; Tardif and Ziegler, 1992; Shideler et al., 2001;
Schramm et al., 2002; Atsalis and Margulis, 2008b; Videan
et al., 2008
). However, the stage of life at which this occurs
is generally later than that observed in humans.
Atsalis and
Margulis (2008a), in review
ing the data on monkeys and
apes, conclude that “potentially up to 25% of a female's life
can be post-reproductive,” This claim is made in reference
to maximal life span; in comparison, a human female
reaching the maximal life span (now around 120 years) will
spend around 58% of her life in a post-reproductive state.
When compared with average life span (as opposed to
maximal life span), most nonhuman female primates will
die at or before the point at which reproductive senescence
begins. These comparisons have been controversial and
will continue to be refined, given the oft-made claim that
human female reproductive aging is unique and may be
driven by indirect fitness advantages to post-reproductive
women providing resources to grandchildren; i.e. the
grandmother hypothesis (
Hill and Hurtado, 1991; Peccei,
2001
).
Reproductive Senescence
Nonhuman primates, in common with many other
mammals, display an inverted-U shaped pattern relating
female fertility parameters to age (e.g.
Caro et al., 1995;
Smucny et al., 2004
). Anovulation, insufficient luteolysis,
and impairment of gestational and lactational processes are
all more common at the beginning and end of reproductive
life (
Atsalis and Margulis, 2008a
).
Reproductive senescence will be used herein to describe
the process through which the hypothalamic-pituitary-
gonadal axis ages, resulting ultimately in cessation of
function.
Walker and Herndon (2008)
provides an excellent
overview of what is known about reproductive senescence
and menopause in nonhuman primates, with discussion of
controversies stemming from differing uses of the term
“menopause.”
Wise (2006)
provides a thoughtful perspec-
tive, comparing what is known about reproductive aging in
rodents with that in women. Recent findings on nonhuman
primate reproductive senescence, along with commentary,
are also found in
Atsalis and Margulis (2008a)
. Female
reproductive senescence differs among mammalian taxo-
nomic groups. In nonhuman primates, the loss of the
follicular pool is the primary event shaping the end of
reproductive life, whereas in rodents, striking variation is
seen in the size of the follicular pool remaining at the end of
reproductive life as well as at maximum life span (
Wise,
2006
).
Within nonhuman primates, human females are unusual
in experiencing follicular depletion relatively early in the
maximum life span, resulting in an extended period of
altered hormonal environments. These alterations stem
from the declining negative feedback signals from the
ovary (reduced circulating estrogens, P
4
, and inhibin),
resulting in elevated GTH concentrations for a time,
Male
The male primate reproductive system, like that of other
animals, functions to produce sperm capable of fertilizing
an ovum and to package and deliver those sperm to the
female reproductive tract. In this review, this process is
broken down into: (1) spermatogenesis; (2) sperm matu-
ration; (3) structure and function of epididymal and seminal
fluid; (4) copulation and ejaculation; and (5) environmental
effects. As with other areas of primate reproduction dis-
cussed in this chapter, the majority of information available
has come from studies on a limited number of species
e
most notably rhesus macaques (Macaca mulatta),
cynomolgus macaques (Macaca fascicularis), baboons
(Papio anubis), squirrel monkeys (Saimiri sp.), and
common marmosets (Callithrix jacchus). For more detailed
reviews on the endocrine regulation of male reproduction,
see
Graham (1981), Wickings et al. (1986), McLachlan
et al. (2002)
, and
Saltzman et al. (2011)
.
Spermatogenesis
The process of spermatogenesis involves the multiplication
and
proliferation
of
spermatogonial
stem cells,
