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.
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.
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.
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.