Biomedical Engineering Reference
In-Depth Information
apoptosis associated with decreases in LH responsiveness
(
Brannian and Stouffer, 1991; Nakano, 1997
).
NewWorld monkeys differ from Old World monkeys in
that atretic follicles also undergo luteinization, forming
accessory CLs or interstitial glands. These glands are
steroidogenic and may contribute to the extremely high
concentration of circulating progesterone characteristic of
New World monkeys (
Saltzman et al., 2011
).
Anthropoid nonhuman primates are unusual among
mammals in undergoing a menstrual cycle in which the
endometrial layer of the uterus is sloughed off in a cyclical
fashion associated with the fall in progesterone and estrogen
at the end of the cycle's luteal phase. The possible adaptive
significance of menstruation is controversial (
Profet, 1993;
Strassman, 1996; Finn, 1998
), but there is general agree-
ment as to the proximate causes. In the mid to late follicular
phase, the endometrium, under the influence of estrogens
from the ovary, undergoes edema; proliferation of stromal
cells; angiogenesis; and increases in the size, number, and
tortuosity of endometrial glands. During the luteal phase,
progesterone, acting in concert with estrogen, causes further
cell proliferation, edema, increased capillary permeability,
and coiling of the spiral arterioles. At the end of the luteal
phase, with declining concentrations of progesterone and
estrogen, lysosomal membranes in the endometrium break
down, releasing lytic enzymes; spiral arterioles constrict,
causing ischemia; and vascular injury and plasminogen
activators are released. As opposed to other mammals, in
which the endometrium is then resorbed, in Old World
monkeys the majority of the endometrial lining and blood
from the ruptured arterioles are expelled through the vagina.
There are some reports of menstruation occurring in New
World monkeys, but the reports are not consistent.
Saltzman et al. (2011)
provide a description of cyclical
changes in the oviduct, cervix, and vagina resulting from
cyclical changes in exposure to estrogens. In addition to
these changes, some nonhuman primates displayed marked
changes in external genitalia associated with the ovarian
endocrine cycle. In species displaying such changes,
estrogen generally stimulates swelling (tumescence) and
reddening of the sexual skin. The swelling peaks during the
periovulatory phase. Progesterone antagonizes these effects
during the luteal phase so that detumescence occurs shortly
after ovulation. Species displaying such swellings include
chimpanzees, baboons, and mangabeys. Rhesus macaques
display a change in coloration associated with breeding
cycle phase but without notable swelling.
parturition, and maternal physiology are found in the main
section “Pregnancy management.”
One feature that differs dramatically between
mammalian taxa is the form of placentation. Nonhuman
primates display hemochorial placentation, in which the
fetal trophoblast layer (the chorion) is in direct contact with
the maternal blood supply. Although it has long been
proposed that hemochorial placentation evolved from an
epitheliochorial form in which the chorion is not in direct
contact with the maternal blood supply, recent phylogenetic
analyses suggest that hemochorial placentation is likely the
ancestral form in mammals (
Wildman et al., 2006
).
Monkeys exhibit a superficial implantation in which the
trophoblast adheres to the uterine wall without complete
endometrial penetration whereas in apes, the entire blas-
tocyst penetrates the endometrial epithelium and invades
the uterine vasculature (
Luckett, 1974; Mossman, 1987;
Lee and DeMayo, 2004
).
The placenta, in addition to providing the interface for
transfer of nutrients from the mother to the fetus, is
a complex endocrine organ that produces both steroids
(e.g. estrogen, progesterone) and peptide hormones
(e.g. chorionic gonadotropin, chorionic somatomamma-
tropin, corticotrophin-releasing hormone, leptin).
Saltzman
et al. (2011)
provide a more detailed discussion of the role
of each of these hormones in nonhuman primate pregnancy.
One feature of note that is unusual in nonhuman primates is
the interaction of the mother, placenta, and fetus to generate
placental estrogen synthesis (
Albrecht and Pepe, 1999
). In
nonhuman primates, placental estrogen synthesis is
dependent upon precursors supplied by the fetal adrenal
gland. Nonhuman primate fetal adrenal glands are unique
in the presence of a fetal adrenal zone that synthesizes
dihydroepiandrosterone-sulfate (DHEA-S), which is then
used by the syncytiotrophoblast as a substrate for estradiol
synthesis. This fetal adrenal zone involutes after birth,
disappearing before adulthood (
McNulty et al., 1981
).
Lactation
Nonhuman primates are similar to other mammals in the
processes leading to milk synthesis and secretion. An
excellent overview of lactational physiology is provided by
Neville (2001)
. The continued synthesis of milk is depen-
dent upon the presence of the hormone prolactin while
contraction of myoepithelial cells (leading to the “let-
down” of milk into the nipple where it can be accessed by
the infant) is dependent upon the hormone oxytocin.
Compared with other mammals, nonhuman primates
produce milk that has low caloric density (e.g. high water
content) and relatively low protein content (
Oftedal, 1984;
Milligan et al., 2008
). These features form part of a lacta-
tion strategy that involves frequent nursing throughout the
day and night combined with a relatively long period of
Pregnancy
Numerous reviews have been published on the physiology
of mammalian pregnancy (
Albrecht and Pepe, 1990, 1999;
Ogren and Talamantes, 1994; Petraglia et al., 1996
).
Additional details about nonhuman primate pregnancy,
