Fertilization

Fertilization
Fertilization, the process by which male and female gametes fuse, occurs in the
ampullary region of the uterine tube. This is the widest part of the tube and
38 Part One: General Embryology
is close to the ovary (Fig. 2.4). Spermatozoa may remain viable in the female
reproductive tract for several days.
Only 1% of sperm deposited in the vagina enter the cervix, where they
may survive for many hours. Movement of sperm from the cervix to the oviduct
is accomplished primarily by their own propulsion, although they may be assisted
by movements of fluids created by uterine cilia. The trip from cervix
to oviduct requires a minimum of 2 to 7 hours, and after reaching the isthmus,
sperm become less motile and cease their migration. At ovulation, sperm
again become motile, perhaps because of chemoattractants produced by cumulus
cells surrounding the egg, and swim to the ampulla where fertilization
usually occurs. Spermatozoa are not able to fertilize the oocyte immediately
upon arrival in the female genital tract but must undergo (a) capacitation and
(b) the acrosome reaction to acquire this capability.
Capacitation is a period of conditioning in the female reproductive tract
that in the human lasts approximately 7 hours. Much of this conditioning,
which occurs in the uterine tube, entails epithelial interactions between the
sperm and mucosal surface of the tube. During this time a glycoprotein coat
and seminal plasma proteins are removed from the plasma membrane that
overlies the acrosomal region of the spermatozoa. Only capacitated sperm can
pass through the corona cells and undergo the acrosome reaction.
The acrosome reaction, which occurs after binding to the zona pellucida,
is induced by zona proteins. This reaction culminates in the release of enzymes
needed to penetrate the zona pellucida, including acrosin and trypsin-like substances
(Fig. 2.5).
The phases of fertilization include phase 1, penetration of the corona radiata;
phase 2, penetration of the zona pellucida; and phase 3, fusion of the
oocyte and sperm cell membranes.
PHASE 1: PENETRATION OF THE CORONA RADIATA
Of the 200 to 300 million spermatozoa deposited in the female genital tract,
only 300 to 500 reach the site of fertilization. Only one of these fertilizes the
egg. It is thought that the others aid the fertilizing sperm in penetrating the
barriers protecting the female gamete. Capacitated sperm pass freely through
corona cells (Fig. 2.5).
PHASE 2: PENETRATION OF THE ZONA PELLUCIDA
The zona is a glycoprotein shell surrounding the egg that facilitates and maintains
sperm binding and induces the acrosome reaction. Both binding and the
acrosome reaction are mediated by the ligand ZP3, a zona protein. Release
of acrosomal enzymes (acrosin) allows sperm to penetrate the zona, thereby
coming in contact with the plasma membrane of the oocyte (Fig. 2.5). Permeability
of the zona pellucida changes when the head of the sperm comes
in contact with the oocyte surface. This contact results in release of lysosomal
Chapter 2: First Week of Development: Ovulation to Implantation 39
enzymes from cortical granules lining the plasma membrane of the oocyte.
In turn, these enzymes alter properties of the zona pellucida (zona reaction)
to prevent sperm penetration and inactivate species-specific receptor sites for
spermatozoa on the zona surface. Other spermatozoa have been found embedded
in the zona pellucida, but only one seems to be able to penetrate the oocyte

PHASE 3: FUSION OF THE OOCYTE AND
SPERM CELL MEMBRANES
The initial adhesion of sperm to the oocyte is mediated in part by the interaction
of integrins on the oocyte and their ligands, disintegrins, on sperm. After
adhesion, the plasma membranes of the sperm and egg fuse (Fig. 2.5). Because
the plasma membrane covering the acrosomal head cap disappears during the
acrosome reaction, actual fusion is accomplished between the oocyte membrane
and the membrane that covers the posterior region of the sperm head
(Fig. 2.5). In the human, both the head and tail of the spermatozoon enter the
cytoplasm of the oocyte, but the plasma membrane is left behind on the oocyte
surface. As soon as the spermatozoon has entered the oocyte, the egg responds
in three ways:
1. Cortical and zona reactions. As a result of the release of cortical oocyte
granules, which contain lysosomal enzymes, (a) the oocyte membrane
becomes impenetrable to other spermatozoa, and (b) the zona pellucida
alters its structure and composition to prevent sperm binding and
penetration. These reactions prevent polyspermy (penetration of more
than one spermatozoon into the oocyte).
2. Resumption of the second meiotic division. The oocyte finishes its second
meiotic division immediately after entry of the spermatozoon. One
of the daughter cells, which receives hardly any cytoplasm, is known as
the second polar body; the other daughter cell is the definitive oocyte.
Its chromosomes (22+X) arrange themselves in a vesicular nucleus
known as the female pronucleus (Figs. 2.6 and 2.7).
3. Metabolic activation of the egg. The activating factor is probably carried
by the spermatozoon. Postfusion activation may be considered to
encompass the initial cellular and molecular events associated with early
embryogenesis.
The spermatozoon, meanwhile, moves forward until it lies close to the
female pronucleus. Its nucleus becomes swollen and forms the male pronucleus
(Fig. 2.6); the tail detaches and degenerates. Morphologically, the male
and female pronuclei are indistinguishable, and eventually, they come into
close contact and lose their nuclear envelopes (Fig. 2.7A). During growth of
male and female pronuclei (both haploid), each pronucleus must replicate its
DNA. If it does not, each cell of the two-cell zygote has only half of the normal
amount of DNA. Immediately after DNA synthesis, chromosomes organize on
the spindle in preparation for a normal mitotic division. The 23 maternal and
23 paternal (double) chromosomes split longitudinally at the centromere, and
sister chromatids move to opposite poles, providing each cell of the zygote
with the normal diploid number of chromosomes and DNA (Fig. 2.6, D and
E ). As sister chromatids move to opposite poles, a deep furrow appears on the
surface of the cell, gradually dividing the cytoplasm into two parts (Figs. 2.6F
and 2.7B).
The main results of fertilization are as follows:

Restoration of the diploid number of chromosomes, half from the father
and half from the mother. Hence, the zygote contains a new combination
of chromosomes different from both parents.

Determination of the sex of the new individual. An X-carrying sperm
produces a female (XX) embryo, and a Y-carrying sperm produces a male
(XY) embryo. Hence, the chromosomal sex of the embryo is determined
at fertilization.

Initiation of cleavage. Without fertilization, the oocyte usually degenerates
24 hours after ovulation.