Humans are sexually reproducing animals who give birth to young ones. But before the main event of fertilization, there is the development of gametes in the male sex organ and female sex organ which results in the formation of sperm and egg respectively.
All the events which kick in after puberty are all targeted for the only purpose of life that is to reproduce and leave your progeny behind to do the same and continue the species. There are remarkable differences between the reproductive events in the male and female for example sperm formation counter news even an old man but formation of ovum ceases women around the age of 50 years. Secondary sex characteristics appear after puberty and include several trades like the presence of thick facial hair, low pitch voice, etc., in males and will develop mammary glands, high pitched voice, etc., in females.
Fertilization mainly refers to the event of fusion of a sperm and an egg cell to form a single cell called a zygote which gets implanted into the uterus of the mother and after 40 weeks becomes a baby with a heartbeat, limbs, eyes made up of millions of cells. A complete human. The fertilization process is broken down into several stages, and any one of them may lead to failure. The fusion of male and female gametes normally occurs after sexual intercourse in species that undergo internal fertilization, such as humans. Artificial insemination and in vitro fertilization, on the other hand, have made it possible to get pregnant without having sexual contact. This method may be used voluntarily by homosexual couples and single parents or as a result of infertility.
7 events of fertilization
- Sperm Capacitation
The normal male ejaculates about 200 – 300 million sperms during coitus. For normal fertility circumstances, about 60% of the sperms ejaculated by the male must have normal shape size and at least 40% of them must show vigorous motility. Fertilization is complex and hard for newly ejaculated sperm. Instead, they must first go through a series of changes known as capacitation. Capacitation occurs when sperm spend a period of time in the female reproductive tract, as they do during gamete transport. The time needed varies depending on the species, but it normally takes few hours. Capacitation is linked to the elimination of adherent seminal plasma proteins as well as lipid and protein reorganization in the plasma membrane. It also appears to include an increase in cyclic AMP and a decrease in intracellular pH, as well as an influx of extracellular calcium. Capacitation causes sperm to become hyperactivated, which manifests itself in hyperactive motility, among others. Capacitation, on the other hand, helps to destabilize the sperm membrane in order to prepare it for the acrosome reaction which we’ll discuss in the next step.
- Sperm-Zona Pellucida Binding
Zona Pellucida refers to the thick transparent membrane surrounding the egg.
During fertilization the sperm comes in contact with the zona pellucida of the ovum and induces changes in the membrane which blocks the entry of the others sperms. Thus, ensuring that only one sperm can be fertilizing an egg. Sperm binding to the zona pellucida is a receptor-ligand interaction that is highly species-specific.
- Acrosome reaction
The secretions of the acrosome help the sperm to enter into the cytoplasm of the egg through zona pellucida and the plasma membrane. Acrosomes to the largely modified lysosome loaded with zona-digesting enzymes that are found around the anterior portion of the sperm’s head, right where it is required. More and more of the plasma membrane and acrosomal components are lost as the acrosome reaction progresses and the sperm moves through the zona pellucida.
- Penetration of the zona pellucida
The sperm’s flagellating tail’s constant propulsive force, combined with acrosomal enzymes, allows the sperm to produce a tract through the zona pellucida. The sperm will traverse the zona pellucida thanks to two factors: motility and zona-digesting enzymes.
- Sperm-oocyte binding
If a sperm has passed through the zona pellucida, it binds to and fuses with the oocyte’s plasma membrane.
- Egg activation
The egg is in a dormant state prior to fertilization, stuck in metaphase of the second meiotic division. When a sperm binds to an egg, the egg goes through a series of biochemical and physical changes known as egg activation. entering of sperm induces the completion of meiotic division of the secondary oocyte. The secondary meiotic division results in the formation of a secondary polar body and the haploid ovum.
- The zona reaction
The zona reaction is a change in the structure of the zona pellucida catalyzed by cortical granule proteases. The zona reaction is important since it is the primary barrier to polyspermy in most mammals. A hardening process occurs in the zona pellucida which blocks the entry of other runner-up sperms into the egg. As well as the sperm receptors on the zona pellucida are destroyed so no new sperm can bind to the egg.
- Post-fertilization events
The sperm head is inserted into the egg cytoplasm after fertilising sperm fuse with the oocyte. In a process known as decondensation, the sperm’s nuclear envelope disperses and the chromatin loosens from its tightly packed state. Pronuclei are formed when chromatin from both the sperm and the egg is encapsulated in a nuclear membrane. A haploid genome is found in each pronucleus. They move together, their membranes break down, and the two genomes condense into chromosomes, reuniting the organism as a diploid.
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