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 to occur.
• The process of fusion of the sperm and the 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 causes the completion of 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) and forms 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.