Conception | Embryonic and Fetal Periods
Objective 10
Explain the process of conception, including coitus, sperm capacitation, slow block to polyspermy, and fertilization. Describe embryonic events from fertilization to gastrulation and the development of extra-embryonic membranes. Describe fetal events from gastrulation to organogenesis.
When ejaculation occurs, about 300 million spermatozoa are released into the female reproductive tract. These sperm must negotiate the route from the vagina to the cervix, through the uterus, and into both uterine tubes (they don’t know which one might contain the secondary oocyte). Only a few hundred, at most, will make it into each uterine tube.
Sperm Capacitation
In the uterine tube, a process called sperm capacitation takes place. The word “capacitation” means “to make capable” or “enable.” In this case, those few spermatozoa that have made it this far are made capable of penetrating a secondary oocyte. Their flagella beat more strongly and their acrosomes lose cholesterol, proteins, and glycoproteins, and activate profilactin (a precursor protein) to become actin, all in preparation to fuse with the oocyte plasma membrane.
Fertilization (conception) occurs when a single spermatozoon penetrates the secondary oocyte, causing it to complete meiosis II and become an ovum. This usually occurs in the first third of the uterine tube, near the ovary. Spermatozoa can survive in the female reproductive tract up to seven days after ejaculation. The unfertilized secondary oocyte survives up to two days after ovulation. This gives a theoretical window of about five days during the female reproductive cycle when conception can occur. As you can see from the graph in the image, that window is actually a bit wider, about nine days. The probability of conception from a single act of intercourse without birth control ranges from zero during menstruation to a peak of about 40% the day before ovulation (Day 13), back to zero the second day after ovulation (Day 16).
Blocks to Polyspermy
Note the first sentence of the previous paragraph, “Fertilization occurs when a single spermatozoon penetrates the secondary oocyte…” This is important. Remember that the sperm is haploid and the ovum is haploid, and together they form a diploid zygote. What would happen if two or more sperm were able to penetrate and fertilize the secondary oocyte? This is called polyspermy and, instead of haploid (23C) plus haploid (23C) equals diploid (23C + 23C = 46C), the resulting zygote would be haploid times three equals triploid (69C) or haploid times four equals tetraploid (92C), etc. In other words, there would be three or four or more copies of each DNA molecule in the nucleus of the new organism. Unfortunately, any ploidy number greater than diploid (ie, triploidy, tetraploidy, etc) is incompatible with human life. To prevent this, the ovum and sperm must work together to prevent polyspermy.
There are at least two different methods to prevent pol