Fertilization Educational Materials

Fertilization: Fusion of pronuclei of sperm and ovum takes place, thus diploid zygote (2n) is formed, is called fertilization. Syngamy: The union of male and female gamete is called syngamy, whereas intermixing of their cytoplasm is called plasmogamy. Karyogamy: The fusion of pronuclei of sperm and ovum is called karyogamy. The intermixing of their chromosomes is called amphimixis. Site of Fertilization: Internal Fertilization: Example: Aschehelmenthes, Reptiles, Birds, and Mammals. Ovum is fertilized within the body of female. It takes place usually within the ampullae of fallopian tube. External Fertilization: External fertilization takes place outside the body of female, i.e., in water. Example: In most of the invertebrate, some protochordate, amphibia, and most of the fishes. Type of Fertilization: There are two types of fertilization: Self Fertilization Cross Fertilization Fertilization: Self Fertilization: This process takes place in the body of a single animal, i.e., fusion of male and female gametes by male and female organs of the same animal. This is called self-fertilization. This is possible only in bisexual/hermaphrodite/monoecious animals. Cross Fertilization: Fertilization takes place between two different male and female animals of the same species. This is called cross-fertilization. This process is found in all unisexual/dioecious animals. Cross-fertilization is also found in most of the bisexual or hermaphrodite animals because in these animals, male genital organs develop first. This condition is called protandrous conditions. In some of the species, female organs develop first; this condition is called protogynous conditions.   Example: Spong and protochordate (Herdmamia). Mechanism of Fertilization: We can understand the fertilization process occurs in the following steps: Arrival of Spermatozoa: During copulation, the male discharges seminal fluid in the female vagina close to the cervix. This is called insemination. From the vagina, the sperm reaches the ampulla partly by their own effort and partially by the action of the uterus. Fertilization: 4. Sperm Efferent: The sperm present in the seminal fluid travel a long way through the uterus into the fallopian tube by the swimming in the fluid medium. A single ejaculates of semen into the vagina contain about 200-400 million sperm in some 8-4 ml of fluid secretion. But only about 100 sperm reaches the fallopian tube. 5. Role of Uterus in Sperm Transport: Certain activities of the female reproductive tract help in the transportation of the sperm in the site of fertilization. Extension of the cervix into the vagina, the semen can move from the vagina and contraction of the uterus and fallopian tube in the sperm upward. Although the sperm survives in the female reproductive tract for 1-3/4 days, it can fertilize the oocyte in the 12-24 hours following ovulation. 6. Arrival of Egg: The egg released from the mature graffian follicle of an ovary and into the fallopian tube by movement of fimbriae and their cilia. It is in secondary oocyte stage with the 2nd meiotic division in progress. The oocyte have their egg and their life span is 2 days but can be fertilized only during the first 24 hours of this period. 7. Agglutination: The egg produces jelly-like substance called fertilizin which has a no. of spermophilic site on its surface and sperm produces a 7. Agglutination: The egg produces jelly-like substance called fertilizin which has a number of spermophilic sites on its surface and sperm produces a chemical substance called antiftilizin. Adhesion of spermatozoa to the surface of the egg is about by linking of fertilizin molecules with antiftilizin molecules which establish the initial bond. Diagram: The diagram illustrates the mechanism of fertilization, showing: Fertilizin molecules on the egg surface. Antiftilizin molecules on the sperm. The interaction between fertilizin and antiftilizin molecules, leading to the adhesion of the sperm to the egg surface. The structure of the egg, including the corona radiata, zona pellucida, egg membrane, cortical granules, nucleus, parental chromosomes, centriole, polar body, chromosomal material, spindle, synkaryon or syngamy nucleus, cytoplasm. Penetration of Sperm: The sperm generally comes in contact with ovum in the animal pole. Ovulation in the human female occurs at secondary oocyte stage which contain zona pellucida and corona radiata.   Penetration of a sperm is a chemical mechanism in which the acrosome of the sperm release spermlysin which dissolve the egg membrane totally and make the path for the penetration of sperm. These spermlysin contain the enzyme hyaluronidase which held the granulosa cell of corona radiata together. These enzymes dissolve the zona pellucida and sperm unable to reach the plasma membrane of the oocyte. Activation of Ovum: Immediately after the fusion of sperm and egg plasma membrane, the egg show a cortical reaction to further to check the entry of more sperm. In this reaction, the cortical granules present beneath the egg plasma membrane fuse with and release their content (enzymes) between the plasma membrane and zona pellucida. These enzymes harden the zona pellucida junction as the slow back to polyembryony. Only sperm nucleus and middle piece enter the ovum. Tail is lost. Karyogamy (Fusion of genetic nucleus): The sperm entry stimulates the secondary oocyte to undergo meiosis II division which produces the ovum and second polar body. The centriole of the middle piece of sperm form a spindle. The nuclear membrane of germ nuclei degenerate. The two sets of chromosome initially lie at two poles of the spindle but later these sets of chromosome mix up and the process is called karyogamy. The fertilized egg is called zygote (2n). Significance of Fertilization: It stimulates the secondary oocyte to undergo second maturation division to release second polar body and to form haploid ovum. It restores the diploid no. of chromosome in the zygote.   It combines the character of two parents and introduces variation. The centriole of sperm forms the spindle to initiate the cleavage of zygote. It makes the egg metabolically more active.