Fertilization

Fertilization is the union of two haploid gametes, sperm, and ovum, to restore a diploid individual in the form of the zygote. The male and female gametes are produced from the founder or germ cells of the testis and ovary through a process called spermatogenesis in males and oogenesis in females. The spermatogenesis process starts after the onset of puberty due to the influence of the male sex hormone testosterone, which produces millions of sperm in a continuous manner. In contrast, only an ovum is produced in each reproductive cycle that varies from 18 to 24 days (on average 21 days, known as the estrous cycle) in different farm animals and 28 days in humans (menstrual cycle). Multiple ovulations occur simultaneously in every estrous cycle, particularly for multiparous animals like goats, sheep, pigs, cats, and dogs. Spermatozoa require maturational changes in the head of sperm, known as acrosomal reaction (6-8 hours) that occurs in the female reproductive tract to establish an association between spermatozoa and egg, enabling to attach and penetrate sperm into the ovum. In the fertilization process, the fusion of the membranes of two gametes is the most significant event that allows forming of a channel for transferring genetic materials from one cell to the other. Millions of sperms take part in this journey. Those move forward from the place of semen deposition (vagina) to the site of fertilization. However, several hundred (500-800) could reach the target site, and a single spermatozoon gets a chance to penetrate the ovum in the fertilization process. Fertilization usually occurs in the junction between the ampulla and isthmus of the fallopian tube that connects an ovary to the uterus. After fertilization, the zygote enters a series of mitotic divisions that results in cellular differentiation and embryonic development. [Mohammad Shamsul Alam Bhuiyan]