Fertilisation
With meiosis
and sexual reproduction an organism is able to reproduce and create genetically
individual offspring. Here we discuss what happens when the gametes (ovum and
spermatozoon) meet, combine their genetic material and begin the formation of
an embryo.
Spermatozoa
in the female genital tract become prepared for fertilisation with a process
called capacitation. Spermatozoa stored in the epididymis pass through
the ductus deferens during ejaculation and mix with secretions from the seminal
vesicles, prostate and bulbourethral glands (Figure 10.1) as they are released
into the vagina. With this, and a possible cue from the female environment, the
outer surface of the acrosome becomes modified by the removal of glycoproteins
and proteins. This is the final maturation step of the spermatozoa.
The
spermatozoa become hyperactive and make their way through the cervix, uterus
and uterine tube to find the ovum.
Ovulation
With
ovulation the secondary oocyte (or ovum) is expelled from the follicle on the
ovary surface. Fimbriae at the opening of the uterine tube collect it and pass
it into the uterine tube (Figure 10.2). The ovum is moved towards the ampulla
of the uterine tube where it has roughly 24 hours to meet with a spermatozoon
to become fertilised.
The oocyte
is surrounded by cumulus cells (also termed the corona radiata) from the
follicle and spermatozoa must break through this outer layer to reach the
oocyte itself (Figure 10.3). When a spermatozoon succeeds in this it encounters
the zona pellucida surrounding the plasma membrane of the oocyte and
insulating it from the external environment. The spermatozoon binds to the zona
pellucida and is triggered to begin the acrosome reaction.
The
acrosomal cap of the head of the sperm breaks down, releasing enzymes that
dissolve the zona pellucida locally allowing the spermatozoon to enter the
oocyte (Figure 10.4).
Once through
the zona pellucida the membranes of the egg and sperm meet and fuse. The
contents of the sperm are now within the egg, as its plasma membrane is left
behind and lost (Figure 10.4). Cortical granules containing enzymes are
released from the egg, causing the binding proteins of the entire zona
pellucida to become altered, preventing further sperm from binding.
With the
zona pellucida, and the acrosome and cortical reactions fertilisation by
multiple sperm (dispermy or polyspermy) is prevented. This is a very important
process in mammalian reproduction as hundreds of sperm reach the egg at the
same time and dispermy would create an embryo with three haploid sets of
chromosomes (triploidy) that would be extremely unlikely to survive.
Meiosis II
The
secondary oocyte was paused partway through meiosis II (see Chapter 9). With
the fusion of the spermatozoon cell membrane the oocyte is triggered to
continue meiosis.
The two
cells that result from this division are the definitive oocyte and the
second polar body. The second polar body receives little cytoplasm, allowing
the definitive oocyte to maintain its size.
The
fertilised oocyte contains the DNA of the spermatozoon and the DNA of the
oocyte. In principle it contains a diploid set of chromosomes.
Although the
DNA has not been reorganised yet, fertilisation has formed a genetically unique
individual. This cell can be called a zygote (see Chapter 11).
The
spermatozoon’s nucleus becomes the male pronucleus, and aligns with the
female pronucleus. Each pronucleus is haploid at this stage. The two pronuclei
lose their nuclear membranes and their DNA is duplicated. This takes around 18
hours.
The DNA
condenses into chromosomes, and paternal and maternal chromosomes become
aligned together on the equator of the cell. Sister chromatids from each
chromosome are pulled towards either end of the cell, as observed during
anaphase in the mitosis chapter (see Figure 6.4).
Mitosis
continues and the cell is split in two.
With fertilisation
the diploid number of chromosomes has been restored by combining chromosomes
from the father and the mother.
The
spermatozoon will bring either an X or Y sex chromosome to the oocyte’s X sex
chromosome. The spermatozoon determines the sex of the embryo by producing
either an XY (male) or XX (female) pair of sex chromosomes.
Fertilisation
occurs during an 18–24 hour period shortly after ovulation in humans. It is
impossible to determine an exact time of fertilisation, and very difficult to
determine on which day fertilisa- tion occurred.
Embryologically
we talk about developmental processes occurring a number of days after
fertilisation. For example, we say that the first somites form at 20 days.
These are the timings that we use in this book, and that appear at the top of
each chapter.
Clinically,
however, gestation is timed from a more evident event: the last menstrual
period (LMP). As ovulation occurs fairly reliably 2 weeks after menstruation,
and fertilisation occurs within 24 hours of ovulation, it is easier and more
reliable to note the date of the LMP for a patient and from this record weeks
of pregnancy and the predicted date of birth.
It is
important to be aware that there is a 2‐week difference between embryological
and clinical timings (see Figure 5.2). If this textbook notes that the first
somites occur at 20 days (around 3 weeks after fertilisation), this occurs at 5
weeks clinically.
Clinical relevance
An extra‐uterine
pregnancy (or ectopic pregnancy) can occur because of the movement of the
ovum from the ovaries to the uterus. A fertilised ovum may implant into the
uterine tube, the cervix, the ovary or the abdomen. Tubal pregnancies within
the uterine tube are the most common type. Typically, an ectopic pregnancy is
not viable and in extreme cases can lead to the death of the mother.
For in
vitro fertilisation techniques, sperm must be artificially induced to
begin capacitation. With capacitation the sperm is prim some reaction when it
meets the ovum.
