Fertilization is the joining and union of an egg and sperm, whose result is a zygote. This starts the pre-natal development. The most likely location of fertilization occurs are the ampulla of the uterine tube. Fertilization can be broken down into three phases. Phase one consists of sperm penetration of the corona radiata. This is aided by action of sperm and uterine tube mucosa. In phase two the sperm binding and penetrating of the zona pellicuda occurs. The sperm binds because of the interaction of glucosyltransferase and ZP3 receptors in the zona pellicuda. Then the process of capacitation occurs as this is necessary for acrosome reaction to occur. Capacitation is the functional maturation of the sperm. The glycoprotein layer is removed so that certain cell receptors may be available. The acrosomal cap is also changed so that acrosomal reaction can occur. Acrosome reaction consists of the fusion of the outer acrosomal membrane and the sperm cell membrane. Acrosomal enzymes are released and this helps in the penetration of the zona pellicuda. Once the penetration is completed, cortical reaction occurs and this ensures no other sperm can penetrate. In the last step, phase three, the sperm and oocyte membranes fuse. The sperm, except its cell membrane enters the oocyte. The mitochondria and the tail are degenerated. The oocyte completes meiosis two and is now a mature oocyte (female pronucleas) and fuses with the sperm that is now called male pronucleas. It is now a zygote, which contains maternal and paternal chromosomes.
Often time couples may have difficulty conceiving. After one year of unprotected sex, the couple should consult and see if anatomical or chromosomal deformities are the cause. You should test the male first because it is easier to perform physical exams. Anatomically in males there can be deformities such as the epididymis and/or vas deferens are blocked and sperm cannot be ejaculated. You would also check sperm number and its viability as well its motility. Also want to check the viscosity (thickness) and turbidity (cloudiness). In the female, you want to check if the female is actually a female. You want to make sure the fallopian tubes are not damaged or blocked. The age of the female is important as well. If she is over 35 the quality of the eggs can be affected.