ISSN 00623604, Russian Journal of Developmental Biology, 2010, Vol. 41, No. 1, pp. 1–12. © Pleiades Publishing, Inc., 2010.
Original Russian Text © M.N. Skoblina, 2010, published in Ontogenez, 2010, Vol. 41, No. 1, pp. 5–18.
More than 100 years ago, Fulton had shown that a
number of marine teleost fishes produce transparent
pelagic eggs. Before spawning, the oocytes in the ovary are
not transparent, but simultaneously upon gaining trans
parence, the eggs sharply increase in volume and their
specific density decreases, allowing them to float in sea
water of normal density; i.e., an egg becomes pelagic
(Fulton, 1891). Further, the author revealed that similar
changes also occur at the maturation of sinking (benthic)
eggs, but in this case much less water is absorbed (Fulton,
1898). For a long time these data did not attract attention.
However, later on, other researchers also observed hydra
tion of oocytes in some teleost fishes described by Fulton.
In addition, hydration of certain other teleost fishes was
described. It was found that changes in the volumes of
oocytes are inconsiderable in freshwater fish and in brack
ishwater spawners (Hisose, 1976; Iwamatsu, 1978; Wal
lace and Selman, 1978, 1979; Craik and Harvey, 1984,
1986; Selman et al., 1993). In fishes spawning in seawater,
egg volume increases by severalfold, sometimes by hun
dreds of times (Wallace and Selman, 1981; Craik and
Harvey, 1984, 1986; Watanabe and Kuo, 1986; LaFleur
and Thomas, 1991).
The increase in water content serves as a primary
cause for the volume and weight gain in a follicle (Greeley
et al., 1991). It is believed that hydration of oocytes in
teleost fish during maturation is a unique phenomenon
among vertebrates (Wallace and Selman, 1978).
The hydration of oocytes is assessed by measuring
the increase in their volume (Greeley et al., 1986, 1991;
Finn et al., 2002a; Chen et al., 2003), their wet weight
(Greeley et al., 1991; Milla et al., 2006), or water content
(Craik and Harvey, 1987; Finn et al., 2002a; Chen et al.,
2003). Below we discuss some specific features of the pro
cess of hydration of oocytes in teleost fishes. In the litera
ture, two reasons for the water uptake by oocytes are con
sidered: accumulation of peptides and free amino acids
(FAA) arising during the hydrolysis of the oocytes’ yolk
proteins, and accumulation of inorganic ions.
THE ROLES OF FAA AND PROTEINS
It is well known that oocyte grows within the follicle
predominantly due to accumulation of vitellogenin, a
protein synthesized by liver. After entry into the oocyte,
vitellogenin undergoes partial processing and accumu
lates in the yolk granules. In many teleost fishes, second
processing of the yolk proteins by the proteolytic lysoso
mal enzymes during the maturation of oocytes (usually,
breakdown of oocyte nucleus–germinal vesicle (GVBD),
serves as the criterion for maturation) takes place. The
abovementioned enzymes are activated during the matu
ration process at the appropriate time (Carnevali et al.,
2006). The mechanism of selective processing of certain
proteins is not known yet (Patino and Sullivan, 2002).
Not much is known about the proteases participat
ing in the processing of the yolk proteins during oocyte
maturation in the fish producing pelagic and benthic eggs.
In gilthead sea bream (
) cathepsin L partic
ipates in the proteolysis of yolk proteins (Carnevali et al.,
1999); in zebra fish (
), cathepsins B and L
(Carnevali et al., 2006); in barfin flounder (
) (Matsubara et al., 2003) and in common killifish
) (Raldua et al., 2006), cathepsin B.
It is not clear why different proteases participate in the
abovementioned process in different fish species.
The localization of cathepsins was determined in the
oocytes of common killifish. All cathepsins (in the large
vitellogenic follicles of killifish, cathepsins L, B, and F are
identified by immune cytochemical analysis) surround
yolk globules. During the maturation of oocytes in vitro
Hydration of Oocytes in Teleost Fishes
M. N. Skoblina
Kol’tsov Institute of Biology of Development, Russian Academy of Sciences, ul. Vavilova 26, Moscow, 119334 Russia
Received February 2, 2009; in final form April 23, 2009
—Data on the hydration of oocytes in teleost fishes during maturation stimulated by gonadotropic
or steroid hormones in vivo and in vitro are reviewed. The cause of hydration, its dynamics, and some mech
anisms ensuring uptake of water and ions by the oocyte are considered.
: oocyte, follicle, maturation, hydration, ovulation, gonadotropic hormones, steroids, benthic eggs,
pelagic eggs, gap junctions, yolk proteins proteolysis, inorganic ions, aquaporines, membrane receptor of
progestagenes, nuclear receptor of progestagenes, teleost fishes.