Fertilisation

Fertilisation is the process of the sperm from the father penetrating the recently released oocyte from the mother, allowing the genetic material from each parent to begin the creation of a new human being. Typically this occurs in the ampulla of the fallopian tube, and many consider it to be the beginning of life.


Before fertilisation can happen the sperm has to reach the oocyte, which proves to be a challenging task. The journey to the fallopian tube may seem small to you and I, but to a microscopic sperm it’s a dangerous marathon. There are a couple of obstacles they need to conquer; the acidic environment of the vagina, which is counteracted by alkali secretions in the seminal fluid, and the thick mucus barrier which plugs the cervix, which is easier to penetrate when the woman is at her most fertile (around ovulation). Only the most motile sperm survive this arduous journey, with a maximum of 200 ever reaching the oocyte out of a potential 500 million!

Fertilisation

A = penetration of sperm and formation of female pronucleus, B = formation of male pronucleus, C = fusion of pronuclei, D = formation of zygote


Upon ejaculation, semen containing the sperm is deposited in the posterior fornix of the vagina. Before the sperm can mobilise they must undergo capacitation in which they lose cholesterol to shed weight and receive an influx of Ca2+ ions to increase energy supply. Once ready they head off towards the fallopian tube, guided by chemicals released from the oocyte in a phenomenon known as chemotaxis.


Fertilisation is a complex sequence of events and has several phases:

  1. Once the sperm finds the oocyte it begins burrowing through the outer corona radiata using an enzyme called hyaluronidase released from the acrosome of the sperm, a process known as the acrosome reaction.
  2. Once through that layer, the sperm penetrates the inner zona pellucida using a proteolytic enzyme called acrosin, also release from the acrosome. As soon as the first sperm breaches this layer the oocyte releases cortical granules which blocks entry of other sperm (polyspermy) by changing the properties of the zona pellucida. This is known as the cortical (zona) reaction.Penetration
  3. Once through this second layer, the plasma membrane of the sperm fuses with the plasma membrane of the oocyte, allowing the head and tail of the sperm to enter the cytoplasm of the oocyte.
  4. The oocyte completes the second meiotic division it once started many years ago when the ovaries were developing in the mother. This produces a second polar body which migrates into the plasma membrane, and the oocyte is now termed a mature oocyte. The nucleus of the oocyte forms the female pronucleus.
  5. The nucleus of the sperm expands to create the male pronucleus, and the tail of the sperm degenerates. The DNA contained within the male and female pronuclei replicates in preparation for the first cleavage division.
  6. The pronuclear membranes surrounding the pronuclei break down, allowing the chromosomes to condense and mingle for the first time. The mature oocyte is now termed a zygote which contains the diploid number of chromosomes (46). The chromosomes arrange themselves across the zygote and the first division occurs, approximately 30 hours after fertilisation has begun.

An animation of the events occurring during fertilisation

The process of fertilisation allows the diverse genetic variation that is displayed in humans. This is because each zygote is genetically unique as it contains a combination of chromosomes from each parent; meiosis and crossing over of chromosomes aid genetic variation too. The zygote now continues dividing (but not enlarging due to the persisting zona pellucida) as it slowly floats towards the uterus, assisted by peristaltic contractions of the fallopian tube. After the cells, now called blastomeres, have divided into 16 cells (4th division) and undergone compaction (forming a compact ball of cells) the zygote is now called a morula.