Fertilisation and Implantation – Class 12 | Chapter – 3 | Biology Short Notes Series PDF

Fertilisation and Implantation: Fertilisation and Implantation are the two major processes in reproduction in human beings. The zygotes fuse and are later implanted on the walls of the uterus for further growth. During fertilization, the sperm and egg unite in one of the fallopian tubes to form a zygote. Then the zygote travels down the fallopian tube, where it becomes a morula. Once it reaches the uterus, the morula becomes a blastocyst. The blastocyst then burrows into the uterine lining a process called implantation.

Fertilisation and Implantation

Fertilisation is defined as the fusion of the sperm nucleus with the ovum nucleus. For fertilisation to occur, sperm must be deposited in the vagina no earlier than three days before ovulation or within one day after ovulation. This short ‘window of opportunity’ is because the sperm and the ovum have only limited lives, and they both soon die if they do not meet and fuse in this period.

Following ovulation, 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. The ovum will remain in the fallopian tube, alive and fully functioning, for only about 12 to 24 hours. Sperm can live longer in the female reproductive system, up to 72 hours, but most die before this time.

Implantation: Between five to seven days after fertilisation, the blastocyst reaches the uterus and embeds itself in the thickened endometrium (lining of the uterus). This process is called implantation, and if the embryo survives it is the beginning of a pregnancy. However, the embryo may not implant, or it may not survive for more than a few days. In this case, it is shed from the uterus as the endometrium breaks down, and it passes out of the vagina in the menstrual fluid. The loss of a very early pregnancy in this way is very common, and the woman does not even know that she was momentarily pregnant.

Steps of Fertilisation and Implantation

  • Sperm Capacitation: It is the physiological maturation of the sperms, which helps them to penetrate the egg. The sperm, after reaching the ovary in the ampullary-isthmic junction, comes in contact with the zona-pellucida layer of the ovum and blocks the entry of the additional sperms; thus, only one sperm fertilizes the ovum.
  • Acrosomal Reaction: The sperms incapacitation undergo acrosomal reactions and release certain chemicals known as sperm lysins present in the acrosome. Due to the acrosomal reactions, the plasma membrane of the secondary oocyte and the sperm are fused together so that the contents of the sperms can enter. When the plasma membrane of the sperm binds with that of the secondary oocyte, the plasma membrane of the oocyte depolarizes. This prevents polyspermy. Calcium ions play a significant role in the acrosomal reaction. The main factors essential for acrosomal reactions are optimum pH, temperature and calcium and magnesium concentration.
  • Cortical Reaction: Soon after the fusion of the plasma membranes, the oocyte shows cortical reactions. Cortical granules found under the plasma membrane of the oocyte, which fuses with the plasma membrane and releases cortical enzymes between the zona pellucida and plasma membrane. The zona pellucida is hardened by the cortical enzymes that prevent polyspermy.
  • Sperm Entry: A projection known as the cone of reception is formed by the secondary oocyte at the point of sperm contact. This cone of reception receives the sperm.
  • Karyogamy: It is a process in which the male and female gametes fuse together hair the meals acrosome from the sperm gets attached, and the also called as male pronucleus which then gets in contact with the female pronuclei and forms the zygote. Here the egg is already fertilized.
  • Activation of Eggs: The entry of sperm triggers the metabolism in the zygote. Consequently, protein synthesis and cellular respiration increase.

Human developmental stages after fertilisation

  • Fertilisation: fusion of ovum and sperm on day one
  • Morula: solid ball of cells after three days
  • Blastocyst: hollow ball of cells after five days
  • Trophoblast: forms early embryo, fetal membranes and placenta after five to seven days
  • Embryo: the developing human from fertilisation to the eighth week of pregnancy
  • Fetus: the developing human from nine weeks of pregnancy to birth at around 40 weeks
  • Neonate: newborn baby from birth to 28 days old
  • Infant: baby or young child aged less than one year.

Biology Quiz & Notes Physics Quiz & Notes Chemistry Quiz & Notes

Follow on Facebook

By Team Learning Mantras