Biomedical Engineering Reference
In-Depth Information
Finally, a number of environmental factors are known to
modulate the timing of puberty in humans and nonhuman
primates. Social influences can advance or delay puberty, as
described below. In seasonally breeding species, aspects of
pubertal maturation may be gated by seasonal cues such as
photoperiod. Rhesus males show seasonal increases in
sexual behavior during the second and third year prior to
the rise in plasma T. This species has shown a rise in both
LH and T during the third year of life, with rapid decreases
in the fall months, which coincides with the breeding
season (
Mann et al., 1989
). In the seasonal Japanese
macaque (M. fuscata), the process of maturation occurred
over a 2-year period, with full maturity achieved at
Ovarian Cycles
As in other mammals, the development of oocytes to the
point at which they undergo either ovulation or atresia
proceeds from the development of primordial follicles in
which the oocyte is associated with supportive layers of
granulosa cells. These primordial follicles develop into
early antral follicles through the growth of the oocyte,
formation of a zona pellucida, proliferation of granulosa
cells followed by formation of the antral cavity, and
development of the thecal cell layer. This early stage of
development occurs in a continuous stream largely inde-
pendent of gonadotropin stimulation.
Maturation of early follicles to the preovulatory stages is
under the control of LH and FSH and includes expansion of
the antral cavity, secretion of follicular fluid into the antrum,
expression of LH receptors by the granulosa cells, and
increasing secretion of estrogens and inhibin B. Estrogen
production is a result of interaction between the granulosa
and thecal cells, whereby thecal cells convert C21 steroids
to C19 steroids under the influence of LH and granulosa
cells subsequently aromatize these androgens to estrogens
under the influence of FSH. Steroidogenesis is also affected
by numerous paracrine factors, including insulin-like
growth factor (IGF), activin, and inhibin (
Zeleznik and
Pohl, 2006
). The majority of preovulatory follicles will
undergo atresia, a process of degeneration and resorption.
Only those follicles that are at the appropriate phase of
development in late follicular phase can proceed to ovulate;
the appropriate phase of development includes significant
increases in FSH and LH receptor content and an increas-
ingly dense capillary network that allows the follicle to
continue to develop in the face of decreasing FSH in the late
follicular phase. Eventually, sustained high concentrations
of estrogens generate positive feedback that causes surges in
GnRH, FSH, and most notably, LH. The LH surge stimu-
lates final preovulatory maturation including completion of
the first meiotic division in the oocyte, initiation of
progesterone secretion by the thecal layer, and increased
expression of collagenases, prostaglandins, vascular endo-
thelial growth factor (VEGF), and matrix metal-
loproteinases (MMPs) leading to the rupture of the follicle
and its ejection into the oviduct.
Following ovulation, the granulosa cells remaining in the
ovary respond to the LH surge through a process called
luteinization, resulting in the formation of the corpus luteum
(CL), made up of luteal cells that secrete steroid hormones
(progesterone and estrogens) and peptide hormones (relaxin,
oxytocin, and inhibin A) (
Zeleznik and Pohl, 2006
). In
humans and macaques, the CL has an intrinsic lifespan of
14
e
16 days in nonconceptive cycles. As opposed to most
other mammals, the regression of the CL in nonhuman
primates is not the result of endogenous prostaglandins but
rather appears to be the result of luteal cells undergoing
6.5
years of age. It was concluded that based on testis size,
plasma T, and seminiferous epithelium, gonadal activity
developed rapidly during a short period of time and
although spermatogenesis started during the mating season
at 4 years of age, full sexual maturation was attained
2 years later. Similarly, in squirrel monkeys, the onset of
ovulatory cyclicity in young females and the first T surge in
young males are restricted to the breeding season,
presumably in response to photoperiodic cues (
Coe et al.,
1981
). Thus, seasonality “imposes a quantum effect” on
pubertal timing such that gonadarche is more closely
dependent on the number of breeding seasons elapsed since
an individual's birth than on age per se (
Plant and Witchel,
2006
). Importantly, seasonally related cues do not neces-
sarily govern maturation of the neural processes underlying
pubertal reactivation of the GnRH neurons but instead may
play a permissive role in the expression of gonadarche
following this reactivation (
Plant and Witchel, 2006
).
Female
The female nonhuman primate reproductive system func-
tions basically as do those of other placental mammals.
However, nonhuman primates are characterized by slow
reproductive processes and low fecundity, and their
reproductive processes, from folliculogenesis through
gestation and lactation, all reflect the slow, prolonged
nature of the nonhuman primate reproductive cycle. The
following section provides an overview of: (1) ovarian
cycles, (2) gestation, (3) lactation, and (4) reproductive
senescence. 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 detailed recent reviews on female
nonhuman primate reproductive physiology, see the arti-
cles by
Zeleznik and Pohl (2006)
and
Saltzman et al.
(2011)
.
Table 8.1
provides basic female reproductive
parameters for species of
importance in biomedical
research.
