7the oolemma, is incorporated into the oocyte by endocytosis (Davail et al. 1998 ).
Proteolytic cleavage of vitellogenin then leads to the generation of yolk proteins that
are stored in yolk globules or platelets (depending on the species), throughout the
ooplasm. At the time of yolk protein deposition, a significant amount of lipid accumu-
lation also takes place in the growing oocyte of many species. The nucleus and the
cytoplasm move to one pole of the oocyte called the animal pole, while the yolk
becomes localized to the vegetal pole. In reptiles and birds, the sequestered cytoplasm
at the animal pole forms a disklike structure called the blastodisk (or germinal disk)
that contains a large nucleus, the germinal vesicle. The ancestral vitellogenin-encod-
ing genes were lost during mammalian evolution in all but the egg-laying monotremes
(because the development of placentation replaced yolk-dependent nourishment of
the embryo; Brawand et al. 2008 ). Hence, in mammals, oocyte growth is due mostly
to the accumulation of cellular organelles and lipid droplets in the ooplasm.
Concomitant with the onset of oocyte growth, the follicle also begins to develop.
In mammals, granulosa cells assume a more cuboidal shape around the oocyte, dur-
ing the transition from primordial follicle to primary follicle. Then these cells begin
to rapidly proliferate and, as a result, enclose the oocyte in several layers creating a
secondary follicle. This is followed by the formation of the antrum, a fluid-filled
cavity, as the follicle turns into a tertiary (or antral) follicle. The oocyte takes up an
acentric position because of the antral cavity, and the granulosa cells form two dis-
tinct cell populations: cumulus granulosa cells that immediately surround the oocyte
and mural granulosa cells that cover the inner surface of the follicular wall.
Toward the end of oocyte growth, an acellular investment forms around the gamete.
This structure is variously called the chorion in fish, vitelline envelope in anuran amphib-
ians, the inner perivitelline layer in birds, and zona pellucida in mammals (Bi et al.
2002 ). It is made primarily of glycoproteins, and, although its composition differs from
species to species, similarities in its structure across all vertebrate species examined so
far point at a common evolutionary origin (Prasad et al. 2000 ). This extracellular matrix
plays a vital role in sperm-egg recognition, determination of the sperm entry point (in
certain species), the permanent block to polyspermy, and protection of the developing
embryo. Fish oocytes have a special structure called the micropyle, located in the extra-
cellular matrix, which is a pore that provides easy access for the sperm during fertiliza-
tion. In most fish species, there is a single micropyle at the animal pole of the oocyte. In
rare cases (such as in sturgeon and paddlefish), there are several micropyles that are also
restricted to the animal pole region. The surface of the chorion in many fish species is
constructed to guide the sperm toward the micropyle (Iwamatsu 2000 ).
1.2.4 Oocyte Maturation
Fully grown oocytes in the ovarian follicles are still diploid as they are arrested at
prophase of the first meiotic division. The arrest is maintained even as the oocyte
grows and its volume increases significantly. When fully grown, it is regarded
as meiotically competent and is able to respond to a cue to resume meiosis. The cue
1 Egg Activation at Fertilization