Fertilisation and implantation

The essentials

During coitus, semen is released into the vagina (insemination). Sperm swim through the cervix and uterus into the fallopian tube. The ovum released from the ovary is transported to the ampullary region of the fallopian tube, where fertilisation usually takes place. Fertilisation can occur only if the ovum and sperm reach the ampulla at the same time.

When a sperm contacts the zona pellucida of the ovum, changes in the membrane block the entry of additional sperm, ensuring that only one sperm fertilises an ovum. Secretions of the acrosome help the sperm enter the cytoplasm. Sperm entry induces completion of meiosis II in the secondary oocyte, forming a second polar body and a haploid ovum (ootid). The haploid nuclei of sperm and ovum fuse to form a diploid zygote.

The sex of the baby is decided at this point: the ovum always carries an X chromosome, while a sperm carries X or Y. XX gives a female and XY gives a male, so the father's sperm determines the sex.

The zygote divides by mitosis (cleavage) as it moves towards the uterus, forming 2, 4, 8 and 16 cells called blastomeres. An embryo with 8–16 blastomeres is a morula, which becomes a blastocyst. The outer layer, the trophoblast, attaches to the endometrium, and the inner cell mass differentiates into the embryo. The uterine cells divide rapidly and cover the blastocyst, which becomes embedded in the endometrium. This is implantation, which leads to pregnancy.

Key points

• Site of fertilisation: ampulla (ampullary-isthmic junction region) of the fallopian tube.
• Inner cell mass contains stem cells that can form all tissues and organs.
• Trophoblast helps in attachment and later forms part of the placenta.

Common mistakes

• Implantation is by the blastocyst, not by the morula.