2
genetic recombination, i.e., the reshuffling of the genetic information content of the
DNA. This is believed to be the evolutionary advantage of sex: increased adaptation
to a changing environment and the possibility to reduce the effect of deleterious
mutations (Lodé 2012 ). In fact, among the more than 35 different forms of repro-
duction, it is the bisexual mode through fertilization that accomplishes this feat
most effectively (Tarín and Cano 2000 ). Initially, sex cells were of equal size; devel-
opment then led to the formation of two types of gametes that show stark dimor-
phism: a small, mobile gamete and a large, nonmoving one.
Fertilization is the union of the male and female gametes, a process that creates
a bridge between two generations (Hardy 2002 ). Gamete fusion leads to the forma-
tion of a single cell with remarkable developmental potential: the zygote is able to
give rise to all cell types of a new organism. The sperm delivers half of the chromo-
somes to restore the diploid configuration in the resulting embryo. In most species
it also brings the centriole necessary for the first mitotic cell division and provides
the stimulus to initiate embryo development. The stimulus arrives in the form of a
calcium (Ca2+) signal; in response the egg undergoes a series of changes that are
collectively known as activation. The events include cortical granule exocytosis to
prevent polyspermy, the completion of meiosis, and formation of the female and
male pronuclei. The pronuclei migrate toward each other, and the genetic material
they carry join to create the blueprint for the newly formed embryo.
In this chapter, we will briefly describe the formation of the primordial germ
cells and the transformations they go through until they become mature eggs. We
will touch upon the most important steps of fertilization, the chain of events that
culminate in the creation of the zygote. The major emphasis will be on egg activa-
tion, the process that guarantees that the developmental program of the egg is suc-
cessfully stimulated and embryo development is initiated. We will discuss in detail
the machinery that is responsible for the meiotic arrest of the egg prior to fertiliza-
tion and the signal transduction mechanism that stimulates this machinery to induce
the resumption of meiosis and entry into the first mitotic cell cycle.
1.2 Oogenesis
Oogenesis is the process by which an egg is created with the ability to undergo fertil-
ization. The definition of an egg requires clarification. In the strictest sense, an oocyte
becomes an egg when it completes both rounds of meiotic divisions (Bi et al. 2002 ).
Oocytes of a few species, such as the sea urchin, do finish meiosis by the time of
ovulation; these are truly eggs as they await fertilization. In most animals, however,
the sperm fertilizes an oocyte that has finished maturation and is arrested at a certain
stage of the meiotic cell division; meiosis is completed and the second polar body is
extruded only after gamete fusion. Nevertheless, many times these are also called
eggs. In this review we will also use the looser terminology and refer to the female
gamete as an “oocyte” when it is at a stage prior to maturation and as an “egg” once
it has gone through the process of maturation and is released from the ovary.
Z. Machaty et al.