Page 481 - The Toxicology of Fishes

The Endocrine System 461

(IGFs), are likely involved in intraovarian regulation of steroidogenesis and follicular growth, as well
as pituitary feedback of gonadotropin secretion, although their precise functions in teleosts have not
been elucidated.
Spermatogenesis can be divided into three major phases: mitotic proliferation of the spermatogonia
from stem cells, meiosis of spermatocytes, and the transformation of spermatids into flagellated sper-
matozoa (spermiogenesis) (Schulz and Miura, 2002). Sertoli cells are in close contact with the germ
cells and provide nutrients and stimuli such as growth factors that regulate their development. The
function of Sertoli cells is in turn regulated by FSH and steroid hormones secreted by Leydig cells.
Leydig cells produce testosterone and 11-ketotestosterone, as well as trace amounts of 17β-estradiol
during spermatogenesis. Androgens regulate all stages of spermatogenesis, probably by controlling the
production of IGFs and activin B by Sertoli cells (Schulz and Miura, 2002). Regulation of androgen
production is largely under the control of LH when the germ cells are undergoing meiosis. The identi-
fication of a nuclear estrogen receptor in croaker testes suggests that estrogens have important functions
in the gonads of male fish (Loomis and Thomas, 1999). One function of estrogens in the testis may be
to stimulate stem cell renewal divisions (Schulz and Miura, 2002).
The final stages of gamete maturation and release in teleosts are controlled by LH and involve the
synthesis of maturation-inducing steroids (MISs), C-21 progestins (Miura et al., 1992; Nagahama,
2000; Nagahama et al., 1993; Thomas, 1994). The MISs have been positively identified as 17,20β-
dihydroxy-4-pregnen-3-one (17,20β-P) in amago salmon (Nagahama and Adachi, 1985) and as 17,20β,
21-trihydroxy-4-pregnen-3-one (20β-S) in Atlantic croaker (Trant and Thomas, 1989). The major MIS
for salmonids and cyprinids is 17,20β-P (Scott et al., 1987), whereas 20β-S has been shown to be the
predominant MIS in sciaenids and some other perciform fishes (Thomas, 1994). In addition, other
C-21 progestins and 11-deoxycorticosteroids identified in several flatfishes may function as MISs in
these species (Scott et al., 1987). In females, a surge in LH secretion initiates the process of oocyte
maturation (OM), comprised of two gonadotropin-dependent stages (Patiño and Thomas, 1990a). The
development of oocyte maturational competence (i.e., the ability to respond to the MIS and complete
maturation) occurs during the initial MIS-independent phase (Patiño et al., 2001). This includes
upregulation of the MIS receptor (Thomas et al., 2001) and the gap junctions and their associated
protein, connexin. This phase is followed by the resumption of meiosis, migration of the nucleus
(germinal vesicle) to the animal pole (GVM), dissolution of the nucleus (germinal vesicle breakdown
[GVBD]), and changes in the ooplasm, including lipid coalescence, hydration, and formation of an oil
droplet (in fish that release pelagic eggs) during the MIS-dependent phase (GVBD phase) (Patiño et
al., 2001). The MIS induces GVBD via a cell-surface-initiated, nongenomic mechanism by binding to
specific membrane progestin receptors (mPRs). Ligand binding to these receptors results in a decrease
in cAMP levels that can be blocked by pertussis toxin, an inhibitor of inhibitory G-protein (G )
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activation, indicating that the receptor activates a G (Pace and Thomas, 2005; Thomas et al., 2002;
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Yoshikuni and Nagahama, 1994). The decrease in cAMP levels releases the oocyte from meiotic arrest
via a signal transduction pathway that causes the production of maturation-promoting factor (MPF),
which consists of two components: a catalytic cdc2 kinase and cyclin B (Nagahama et al., 1993).
Ovulation occurs soon after the completion of OM via a genomic steroid mechanism. The MIS activates
a nuclear progestin receptor (nPR) in the ovarian follicle wall (Goetz et al., 1991; Pinter and Thomas,
1997, 1999), causing activation of protein kinase C and the synthesis of arachidonic acid and prostag-
landins, which in turn cause contractions of smooth muscle in the follicle wall to expel the oocyte
(Goetz et al., 1991; Patiño et al., 2003).
Recent studies have shown that MISs cause maturation of spermatozoa by nongenomic and genomic
steroid mechanisms, in Atlantic croaker and salmonids, respectively (Schulz and Miura, 2002; Thomas
et al., 2004). Additional research will be required to reconcile these two models of sperm activation
in teleosts; however, it is probable that both steroid mechanisms may operate in the same species, as
both the nuclear and the membrane progestin receptors have been characterized in the testes of spotted
seatrout and Atlantic croaker (Pinter and Thomas, 1997; Thomas, 2000c; Thomas et al., 1998, 2005).
Increased sperm motility after gonadotropin or progestin treatment has been reported in salmonids,
and this is associated with MIS-induced increases in the pH of the seminal fluid by a genomic
mechanism, presumably by binding to the nPR (Miura et al., 1992). It has been demonstrated that