Fertilization and Implantation

The precise moment when a fertilised ovum, embryo, foetus, or baby is referred to as a “human being” is debatable and varies between individuals, religious groups, legal systems, and nations. This study session will not address this difficult concept, but will instead focus on theoretical knowledge of how the ovum is fertilised and implanted in the uterus, as well as how the foetus is nourished as it develops during the nine months of gestation. This study session will also assist you in recognising foetal development factors that may lead to complications during the antenatal period.

What is Fertilisation?

The fusion of the sperm nucleus and the ovum nucleus is defined as fertilisation. Sperm must be deposited in the vagina no earlier than three days before ovulation or no later than one day after ovulation for fertilisation to occur. This brief ‘window of opportunity’ exists because sperm and ovum have finite lives and will both die if they do not meet and fuse during this time period. The ovum is picked up by the fimbriae of the fallopian tube on the same side of the body as the ovary that released the ovum after ovulation. The ovum will only be alive and fully functional in the fallopian tube for about 12 to 24 hours.

Sperm can survive in the female reproductive system for up to 72 hours, but the vast majority die before this time.

When sperm enters the vagina, it travels through the cervix, into the uterus, and then up into the fallopian tubes. The ovum is fertilised in the fallopian tube. Sperm movement is aided on this long journey by muscular contraction of the uterine and fallopian tube walls. Sperm can travel several millimetres (mm) per second, so it only takes about 15 minutes for them to enter the fallopian tubes, but millions die along the way.

Only one sperm will successfully fertilise the ovum by penetrating its cell membrane and depositing the male genetic material into the female cell, where the two nuclei fuse. The fertilised ovum (zygote) immediately becomes resistant to penetration by any subsequent sperm. Following fertilisation, the zygote remains in the fallopian tube for approximately 72 hours, during which time it develops rapidly.

What is Implantation?

The blastocyst enters the uterus five to seven days after fertilisation and embeds itself in the thickened endometrium (lining of the uterus). This is known as implantation, and if the embryo survives, it marks the start of a pregnancy. The embryo, however, may not implant or survive for more than a few days. It is shed from the uterus as the endometrium degrades and passes through the vagina in the menstrual fluid in this case. The loss of a very early pregnancy in this manner is very common, and the woman is unaware that she was pregnant at all.

Implantation may also occur in the fallopian tube on occasion, which is known as an ectopic pregnancy and is extremely dangerous for the woman. As the embryo grows, the tube may rupture, causing severe pain and blood loss into the abdominal cavity.

Functions of Fertilisation

Fertilization’s function is to produce a diploid (2N) zygote. The ovum is activated by sperm fertilisation, which occurs in the distal third of the fallopian tube. Once the sperm has entered the ovum, immediate changes occur to prevent further sperm penetration. These precise changes are discussed in the biochemical section of this article. After the sperm has penetrated the ovum, the embryo’s sex is determined by the presence or absence of a Y chromosome, which contains the SRY gene, also known as the testis-determining factor and sex-determining region Y.

Acrosomal reactions

During sperm incapacitation, acrosomal reactions occur, resulting in the release of chemicals known as sperm lysins, which are present in the acrosome.

The plasma membranes of the secondary oocyte and the sperm are fused together by acrosomal reactions, allowing the contents of the sperms to enter. When the plasma membrane of the sperm binds to that of the secondary oocyte, the oocyte’s plasma membrane depolarizes. Polyspermy is thus avoided.

Calcium ions are important in the acrosomal reaction. The most important factors in acrosomal reactions are optimal pH, temperature, and calcium and magnesium concentration.

Karyogamy

The secondary oocyte completes the suspended second meiotic division after the sperm enters. This results in the formation of a haploid ovum and a second polar body.

The male pronucleus is the head of the sperm that detaches from the entire sperm and contains the nucleus. The tail and second polar body both degenerate. Female pronuclei are the nucleus of the ovum.

The nuclear membranes of the male and female pronuclei degenerate as their pronuclei fuse. Karyogamy is the fusion of the chromosomes of male and female gametes. The ovum has now been fertilised and is referred to as a zygote.

Conclusion

The fusion of the sperm nucleus and the ovum nucleus is defined as fertilisation. However the blastocyst enters the uterus five to seven days after fertilisation and embeds itself in the thickened endometrium, This is known as implantation. The precise moment when a fertilised ovum, embryo, foetus, or baby is referred to as a “human being” is debatable and varies between individuals, religious groups, legal systems, and nations. The fusion of the sperm nucleus and the ovum nucleus is defined as fertilisation. The ovum is picked up by the fimbriae of the fallopian tube on the same side of the body as the ovary that released the ovum after ovulation. The ovum is fertilised in the fallopian tube.