Fertilisation in Humans
Fertilisation in humans:
- Fertilisation is the process of fusion between a sperm and an ovum (egg), resulting in the formation of a diploid zygote.
- Semen is released into the vagina during copulation, and the motile sperms swim through the cervix, uterus, and reach the ampullary region of the fallopian tube.
- The ovum is released by the ovary and also transported to the ampullary region, where fertilisation takes place.
- Fusion of Sperm and Egg: The sperm comes in contact with the zona pellucida layer of the ovum and induces changes in the membrane that block the entry of additional sperms. Only one sperm can fertilise an ovum.
Ovum surrounded by few sperms
- Sperm Entry: The secretions of the acrosome help the sperm penetrate the zona pellucida and the plasma membrane of the ovum, allowing it to enter the cytoplasm.
- Completion of Meiotic Division: The entry of the sperm induces the completion of the meiotic division of the secondary oocyte, resulting in the formation of a second polar body and a haploid ovum (ootid).
- Formation of Zygote: The haploid nucleus of the sperm and the haploid nucleus of the ovum fuse together, forming a diploid zygote.
Diagrammatic representation of fertilisation and zygote formation
- Sex Determination:
- The sex of the baby is determined at fertilisation.
- Female gametes (ova) carry the X chromosome, while male gametes (sperms) can carry either an X or Y chromosome.
- If an X-carrying sperm fertilises the ovum, the zygote will have XX chromosomes and develop into a female. If a Y-carrying sperm fertilises the ovum, the zygote will have XY chromosomes and develop into a male.
Representation of sex determination