Question

Briefly explain the events of fertilisation and implantation in an adult human female.

Answer 1

During coitus, the semen is released into the vagina, passing through the cervix of the uterus and reaching the ampullary-isthmic junction of the fallopian tube.

• The ovum is also released into the junction for fertilisation.
• The fusion of the sperm and ovum is known as fertilisation.
• During fertilisation, the sperm induces changes in the zona pellucida and blocks the entry of other sperms. This ensures that only one sperm fertilises an ovum.
• The enzymatic secretions of the acrosomes help the sperm enter the cytoplasm of the ovum.
• This completes the meiotic division of the secondary oocyte, resulting in the formation of a haploid ovum (ootid) and a secondary polar body.
• Then, the haploid sperm nucleus fuses with the haploid nucleus of the ovum to form a diploid zygote.
• Mitosis starts as the zygote moves through the isthmus of the oviduct (cleavage), forming 2, 4, 8, and 16 daughter cells called blastomeres.
• The 8-16 cell embryo is called a morula, which continues to divide to form the blastocyst. The morula moves further into the uterus.
• The cells in the blastocyst are arranged into an outer trophoblast and an inner cell mass.
• The trophoblast gets attached to the uterine endometrium and the process is called implantation. This leads to pregnancy.
• The inner cell mass gets differentiated to form the embryo.

Answer 2

The events of fertilization and implantation in an adult human female occur in the following steps:

1. Fertilisation: When a male’s sperm cell effectively unites with a female’s egg cell (ovum), fertilisation takes place. During sexual activity, the sperm enters the female reproductive system and passes via the uterus and cervix before arriving at the fallopian tube, where the egg is waiting. When fertilisation is successful, the sperm enters the egg, and their genetic material comes together to produce a zygote. Because it is diploid, the zygote has genetic material from both parents.
2. Formation of the Zygote: After fertilization, the zygote begins to divide by mitosis, forming a ball of cells known as the blastocyst. As it continues to divide, the blastocyst moves down the fallopian tube toward the uterus.
3. Implantation: The blastocyst must implant in the thicker endometrial lining of the uterus. About six to seven days after fertilisation, implantation occurs. After the blastocyst implants in the uterine lining, it begins to receive nourishment and oxygen from the mother via the placenta.
4. Pregnancy Confirmation: Once implantation is successful, the embryo begins secreting human chorionic gonadotropin (hCG), a hormone that maintains progesterone production, which helps sustain the pregnancy and prevents menstruation.