In Antiquity, Aristotle depicted the formation of new individuals occurring through fusion of male and female fluids, with form and function emerging gradually, in a mode called by him as epigenetic. In vitro fertilization (IVF) is a process by which egg cells are fertilized by sperm outside the womb, in vitro. The sperm plasma then fuses with the egg's plasma membrane and their nuclei fuse, triggering the sperm head to disconnect from its flagellum as the egg travels down the fallopian tube to reach the uterus. Upon encountering the secondary oocyte, the acrosome of the sperm produces enzymes which allow it to burrow through the outer shell called the zona pellucida of the egg. Various exceptions to this sequence are possible, including artificial insemination, in vitro fertilization, external ejaculation without copulation, or copulation shortly after ovulation. The most common sequence begins with ejaculation during copulation, follows with ovulation, and finishes with fertilization. The process of fertilization involves a sperm fusing with an ovum. Scientists discovered the dynamics of human fertilization in the nineteenth century. The result of this union leads to the production of a fertilized egg called a zygote, initiating embryonic development. As the semen mixes with the vaginal secretions, the pH settles at a point that is not harmful for sperm, and this new environment is the trigger to activate sperm flagella and increase sperms’ motility.Human fertilization is the union of a human egg and sperm, occurring primarily in the ampulla of the fallopian tube. However, this environment can also be toxic to sperm, though the semen (a basic fluid) can buffer the vaginal acidity to preserve sperm cells. This lowers vaginal pH (to as low as 3.8), creating an acidic environment hostile to pathogens (like the ones that cause sexually transmitted infections). LH triggers the ovaries to release the egg, while higher blood estrogen levels stimulate the vaginal membrane to secrete glycogen, which is then metabolized to lactate. Part of this preparation involves elevated levels of estrogen and luteinizing hormone (LH). During ovulation, ovaries release an egg into one of the fallopian tubes, and the egg proceeds down the tube toward the uterus, which is being prepared for possible implantation. Egg and sperm travel in opposite directions to meet in (most often) the fallopian tubes.
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