Fertilization takes place when the spermatozoon has successfully entered the ovum and the two sets of genetic material carried by the gametes, fuse together, resulting in the zygote, (a single diploid cell). This usually takes place in the ampulla of one of the fallopian tubes. Successful fertilisation is enabled by three processes which also act as controls to ensure species-specificity. The first is that of chemotaxis which directs the movement of the sperm towards the ovum. Secondly there is an adhesive compatability between the sperm and the egg. With the sperm adhered to the ovum, the third process of acrosomal reaction takes place; the front part of the spermatozoon head is capped by an acrosome which contains digestive enzymes to break down the zona pellucida and allow its entry.[3]The entry of the sperm causes calcium to be released which blocks entry to other sperm cells. A parallel reaction takes place in the ovum called the zona reaction. This sees the release of cortical granules that release enzymes which digest sperm receptor proteins, thus preventing polyspermy. The granules also fuse with the plasma membrane and modify the zona pellucida in such a way as to prevent further sperm entry.
The zygote contains the combined genetic material carried by both the male and female gametes which consists of the 23 chromosomes from the nucleus of the ovum and the 23 chromosomes from the nucleus of the sperm. The 46 chromosomes undergo changes prior to the mitotic division which leads to the formation of the embryo having two cells.