6
membrane; this tight physical contact between the oocyte and the granulosa cells is
lost just before ovulation. In Xenopus, the earliest follicles contain an oocyte sur-
rounded by a single layer of squamous follicular cells (Dumont 1972 ). As the oocyte
begins to grow, the follicular cells become more cuboidal. Right before ovulation
the follicular cells become flat and most cytoplasmic projections disappear. In the
zebrafish ovary, the layer of prefollicle cells initially envelopes the entire nest of
oocytes. Later, after germ cells cellularize, individual oocytes become surrounded
by the somatic cells whose shape undergoes changes similar to those seen in the
Xenopus ovary (Selman et al. 1993 ). Some follicular cells in the fish ovary have
specialized function: a single cell called the micropylar cell participates in the for-
mation of the micropyle that will serve as an entry point for sperm at fertilization
(Hart 1990 ).
Follicle assembly in mammals has multiple steps. Somatic cells first envelop
clusters of oocytes; in these polyovular follicles, oocytes are still connected by
intercellular bridges (Weakley 1967 ). The intercellular bridges then disappear and
the somatic cells end up encapsulating individual oocytes. The follicle at this point
is called a primordial follicle, and it consists of a small oocyte surrounded by a
single layer of flattened granulosa cells. The granulosa cells and their associated
basement membrane completely envelop the oocyte. At the same time, a heteroge-
neous layer of thecal cells is added over the basement membrane, thereby creating
the basic structure of the ovarian follicle (Tokarz 1978 ).
1.2.3 Oocyte Growth
Females of most vertebrate animals are born with primary oocytes in their ovaries;
the oocytes remain dormant until the animal reaches puberty. Some increase in the
size of the oocyte may occur in the prepubertal period; in fish and amphibians, the
growth at this time is due to ribosomal RNA production by the nucleoli, lipid depo-
sition, and the synthesis of a large amount of glycoproteins incorporated into the
cortical alveoli (Wallace and Selman 1990 ). The major size increase, however, takes
place after puberty, when gonadotropins produced in the pituitary gland induce
oocyte growth. In nonmammalian vertebrates the oocyte grows mainly due to the
accumulation of yolk (also known as vitellus). In salmonids, increased follicle-stim-
ulating hormone (FSH) levels have been shown to stimulate estrogen production in
the follicles, which in turn induce hepatic vitellogenesis (Specker and Sullivan
1994 ). In chickens, it is also the increased FSH level that causes vitellogenin pro-
duction in the liver (Schoenwolf 1997 ).
During vitellogenesis a large amount of yolk is accumulated in the oocyte. The
function of yolk is to provide nutrition for the developing embryo. Yolk is composed
mainly of lipids and proteins (mostly lipoproteins and phosphoproteins). Vitellogenin,
a glycolipophosphoprotein expressed in the females of nearly all oviparous species, is
the precursor of the yolk proteins (Robinson 2008 ). After being synthesized in the
liver, it is transported to the ovaries through the blood stream. At the ovaries, vitellogenin
leaves the blood vessels, crosses the follicular wall, and, after binding its receptor on
Z. Machaty et al.