181perivitelline space found animally (Motosugi et al. 2006 ). The animal pole exclu-
sion zone for sperm entry represents inherent polarity of the mouse egg derived
from the oocyte. During oocyte maturation, which occurs in late stages of oogene-
sis, the oocyte nucleus (also called the germinal vesicle) migrates to the oocyte
periphery and completes meiosis I, extruding the first polar body. This defines the
animal pole in the mouse oocyte and future egg and confers it with distinct proper-
ties, such as the fertilization exclusion zone (Motosugi et al. 2006 ). The animalward
migration of the GV and polar body extrusion processes in oocyte maturation are
conserved, except that AV polarity in frogs and fish has already been defined much
earlier in oogenesis.
In zebrafish, the egg is fertilized through a micropyle (Fig. 5.1a), which is gen-
erated during oogenesis by a single specialized follicle cell evident in stage III
oocytes at the animal pole (Fig. 5.2c). The micropyle is an opening in the vitelline
membrane that allows only a single sperm to fertilize the egg (monospermy) in
zebrafish (Amanze and Iyengar 1990 ). In the absence of AV polarity establish-
ment in zebrafish bucky ball (buc) mutant eggs, multiple ectopic micropyles form,
resulting in polyspermy (Marlow and Mullins 2008 ). Interestingly, Buc functions
in the oocyte and thus non-cell autonomously acts in micropyle formation, sug-
gesting cross talk between the oocyte and surrounding follicle cells in micropyle
biogenesis (Heim et al. 2014 ).
Not all fish use a single micropyle as a mechanism to block polyspermy.
Sturgeons, for example, can have several micropyles at the animal pole (Cherr and
Yanagimachi 2014 ). To prevent polyspermy, sturgeons rely on the cortical granule
reaction, typical of most animals. Cortical granules are specialized vesicles stored
beneath the oocyte cortex that are exocytosed after fertilization (Fig. 5.2c). They
modify the vitelline membrane and extracellular space in a way that precludes the
entry of additional sperm at this stage. Interestingly, zebrafish sperm lack an acro-
some, whereas sturgeon sperm possess one (Cherr and Clark 1985 ; Cherr and
Yanagimachi 2014 ). The acrosome is a large vesicle in the sperm head of many
animals that is secreted upon binding to the egg. The acrosome contains enzymes
that modify the vitelline envelope allowing the sperm to penetrate it and fertilize
the egg. The zebrafish sperm need not have an acrosome, since the single micro-
pyle in the zebrafish egg generates a hole in the vitelline envelope. In the sturgeon,
the micropyle may not allow the sperm to directly contact the egg membrane, and
therefore sturgeon sperm cells necessitate an acrosome reaction for fertilization
(Cherr and Yanagimachi 2014 ; Morisawa 1999 ; Morisawa and Cherr 2002 ).
Overall, these differences in gamete morphology and strategies for blocking poly-
spermy in fish are relevant to AV polarity establishment. Zebrafish buc mutant
females produce eggs with multiple micropyles that are no longer restricted to a
single pole. The relationship between Buc function (see sections below) and the
multiple micropyles found in other fish is an exciting evo-devo question that awaits
future studies.
5 Localization in Oogenesis of Maternal Regulators of Embryonic Development