Biomedical Engineering Reference
In-Depth Information
recombination of genetic material during meiotic division
of spermatocytes, and differentiation and maturation of
spermatids into testicular sperm. An excellent overview of
this process is provided by
Sharpe (1994)
, and detailed
discussions of recent theories about the molecular and
cellular control of this process are provided by
Sofikitis
et al. (2008)
and
Cheng et al. (2010)
. Detailed investiga-
tions of primate spermatogenesis have been conducted in
the rhesus macaque (
Clermont and LeBlond, 1959;
Arsenieva et al., 1961; Conaway and Sade, 1965; Fawcett
et al., 1970; de Barr, 1973; Rooij et al., 1986
), the cyn-
omolgus macaque (
Dang, 1970; Kluin et al., 1983; Fouquet
and Dadoune, 1986
), the stump-tailed macaque (Macaca
arctoides,
Clermont and Antar, 1973
), the African green (or
vervet) monkey (Chlorocebus aethiops,
Clermont, 1969
),
C. sabolus (
Barr, 1973
), the baboon (
Barr, 1973;
Chowdhury and Steinberger, 1976; Chowdhury and
Marshall, 1980; Afzelius et al., 1982
), and the common
marmoset (
Weinbauer et al. 2001
).
Clermont and LeBlond
(1959)
described 12 stages in the cycle of the rhesus
seminiferous epithelium. Steps were defined by changes in
the nucleus and acrosomal structures, and it was noted that
each stage appeared in sequence with time over a particular
area in a given seminiferous tubule. Depending on the
species, tubule cycle durations range from 9.5 to 14.4 days,
and the total duration of spermatogenesis ranges from 36 to
48 days (
Table 8.2
).
A tubular cross-section may contain either a single germ
cell association (single stage tubule) or different germ cell
associations (multi-stage tubule). It has been suggested that
variation in clonal size might lead to multi-stage organi-
zation (
Zhengwei et al., 1997; Wistuba et al., 2003;
Luetjens et al., 2005
). Rodents and prosimian primates
display a single-stage tubule structure while New World
monkeys and hominoids (apes and humans) display
a multi-stage tubular structure. Old World monkeys appear
intermediate, with baboons and mandrills displaying a mix
of single- and multi-stage tubules while macaques and
vervets display predominantly single-stage tubules
(
Wistuba et al., 2003; Luetjens et al., 2005
). Previous
proposals that the multi-stage structure is associated with
low spermatogenic
the conclusion that mitotic growth of the A-pale sperma-
tagonial population is gonadotropin independent, but this
constitutive proliferation is amplified by exposure to LH
and/or FSH. These results indicate that in nonhuman
primates, FSH levels determine the number of germ cells in
the testis.
LH supports spermatogenesis indirectly by controlling
T production by the Leydig cells within the testis. Testos-
terone supports spermatogenesis directly, with androgen
receptors present on the Sertoli cells. In addition, aroma-
tized T may mediate spermatogenesis via estrogen recep-
tors present on spermatocytes, spermatids, and Leydig and
Sertoli cells (
Shaha, 2008
). LH stimulation of the Leydig
cells within the testis is responsible for maintaining high
concentrations of T, the androgen essential for spermato-
genesis. The mechanism by which LH controls androgen
secretion has been studied (
Arslan et al., 1986
), and it was
noted that chronic gonadotropin exposure (hCG) resulted in
the activation of the stimulatory response required for
T production. It was proposed that this activation occurred
via enhancement of LH/CG receptor availability on Leydig
cells (
Wickings et al., 1986
).
Testosterone is capable of stimulating spermatogenesis
in rhesus, cynomolgus, and bonnet macaques, but T stim-
ulation alone does not appear to be sufficient to produce
normal spermatogenesis (
Wickings et al., 1986
). Studies of
hypogonadic rhesus macaques that were supplemented
with T or FSH or both concluded that the differentiation of
A-pale into B spermatogonia may be driven by T or FSH
alone but that differentiation is amplified by the presence of
both (
Marshall et al., 2005
).
Sperm Maturation
The primary functions of the epididymis are maturation and
storage of spermatozoa. Because spermatozoa are largely
synthetically inactive, this maturation process involves the
interaction of sperm cells with proteins synthesized from
the epididymis in a region-dependent manner. An excellent
review of the role of the epididymal microenvironment in
sperm maturation, concentrating on rodents models and
humans, is provided in the article by
Cornwall (2009)
.
Sperm maturation is defined by the ability to undergo
capacitation and the acrosome reaction. There is a wide
disparity among mammals in the role of each epididymal
section in these maturation processes; therefore the acro-
somal response of epidydimal sperm has been examined in
both macaques and marmosets to determine the value of
these animals as models of human epididymal sperm
maturation processes. Both macaques and marmosets
display an acrosome maturation profile similar to that of
humans, with increasing in vitro acrosomal response when
moving from the caput to the caudal epididymis (
Moore,
1981; Moore et al., 1984; Yeung et al., 1996
). In addition to
efficiency have been disproved
(
Luetjens et al., 2005
).
As in other mammals, the process of nonhuman primate
spermatogenesis is governed by the Sertoli cells, the only
somatic cells present in the seminiferous tubules (
Sofikitis
et al., 2008
). Spermatogenesis is supported directly by
FSH, with FSH receptors present on the Sertoli cells.
Manipulation of FSH concentration in macaques will
directly affect germ cell number and seminiferous tubule
size (
Wickings and Nieschlag, 1980a,b; Moudgal, 1981
;
Madhaw
Raj et al., 1982; van Alphen et al., 1988
). Studies
in hypogonadotropic rhesus macaques (
Marshall et al.,
2005
) and common marmosets (
Sharpe et al., 2003
) support
