Fertilization is the process by which male and female gametes are fused together, initiating the development of a new organism.
The male gamete or ’sperm’, and the female gamete, ’egg’ or ’ovum’ are specialized sex cells, which fuse together to begin the formation of a zygote during a process called sexual reproduction.
Types of Fertilization
Fertilization in Animals
The fertilization process in animals can occur either internally or externally, a difference which is largely determined by the method of birth. Animals which use viviparous and ovoviviparous reproduction (embryos develop within the animal’s body), and oviparous animals which lay hard shelled eggs, use internal fertilization.
Internal fertilization involves the union of sperm and eggs within the body of the (usually female) parent. In order for internal fertilization to occur, the male must implant his sperm into the female reproductive tracts. Implantation can be achieved by either: copulation, in which sperm transfer is performed by insertion of the penis or other male intromittent organ and ejaculation into the vagina, or cloaca: or by a cloacal kiss, in which two birds press their cloacae together and sperm transfer takes place. Some animals, such as mollusks, arachnids, salamanders and certain insects, transfer a spermatophore, a bundle or capsule containing sperm, which is stored within the cloaca until oviposition takes place.
Animals which are oviparous, though produce eggs that are lacking, or have thin egg membranes, reproduce by external fertilization. External fertilization is a reproductive strategy involving the joining of gametes outside of the body, either in a spawning event, where gametes from both sexes are rapidly released into an aquatic environment, or may occur when eggs are laid by a female on a substrate, and are subsequently fertilized by a male. External fertilization holds certain benefits, such as reducing the chance of contracting sexually transmitted diseases, protection from violent behavior between organisms, and increasing the genetic variation within a population.
Fertilization in Plants
Fertilization in plants occurs after pollination and germination. Pollination occurs through the transfer of pollen – which is the male microgametes of seed plants, producing the sperm – from one plant to the stigma (the female reproductive organ) of another. The pollen grain takes up water and germination occurs.
The germinated pollen grain sprouts a pollen tube, which grows and penetrates the ovule (the egg structure of the plant) through a pore called a mycropyle. The sperm are then transferred through the pollen tube from the pollen.
In flowering plants, a secondary fertilization event takes place. Two sperm are transferred from each pollen grain, one of which fertilizes the egg cell to form a diploid zygote. The nucleus of the second sperm cell fuses with two haploid nuclei contained within a second female gamete called the central cell. This second fertilization forms a triploid cell, which subsequently swells and develops a fruiting body.
The process of fertilization, which involves the cross-fertilization between gametes from two different individuals, male and female, is called allogamy. Autogamy, also known as self-fertilization, occurs when two gametes from one individual fuse; this occurs in hermaphrodites, such as flatworms and certain plants.
There are three stages to fertilization which ensure that the appropriate egg and sperm are able to find each other and to warrant that only one sperm enters the egg: chemotaxis, sperm activation/acrosomal reaction and sperm/egg adhesion.
Ovulation must occur before fertilization can happen; in humans, ovulation occurs once a month during the menstrual cycle. This cycle releases an egg cell from the ovaries and the first stage of fertilization begins. In other animals, ovulation can occur in cycles of different length, or is triggered by the occurrence of sexual intercourse.
In mammals, after ejaculation, the sperm locates the oocyte (the immature egg), through changes in temperature and chemical gradients. Sperm chemotaxis, a type of interaction in which sperm cells are guided to the oocyte to the hormone progesterone, which is secreted by the oocyte, and sperm thermotaxis, which involves the response to changes in temperature, ensure that the sperm are able to locate the oocyte (usually within the ampulla of the fallopian tube. While the sperm is in the reproductive tract, it undergoes capacitation, which increases its movement ability and destabilizes its membrane, preparing it for the acrosome reaction.
Once the sperm locates the oocyte, it binds to the zona pellucida, which is a thick layer of jelly-like, extra-cellular matrix consisting of glycoproteins, surrounding the egg. A specialized molecule on the surface of the sperm binds to a ZP3 glycoprotein in the zona pellucida, triggering the acrosome reaction. The acrosome reaction releases hyaluronidase, which digests the hyaluronic acid around the oocyte, allowing the sperm to pass through.
Upon successful implantation of a sperm, the cortical granules within the oocyte fuse with the plasma membrane of the cell, and are expelled into the zona pellucida, causing the surface to become hard and impenetrable. This process is called the cortical reaction and is responsible for ensuring that only one sperm cell can enter and fertilize the egg.
Once the sperm has successfully penetrated the oocyte, the outer coating and the tail of the sperm disintegrates. The oocyte undergoes meiosis to produce the haploid ovum. The two haploid cells, each containing 23 chromosomes, undergo fusion of their genetic material, ultimately creating a diploid cell containing 46 chromosomes, called a zygote. The zygote then begins mitosis, the repeated cellular division necessary for the growth of an organism, forming a blastocyst, which is implanted into the wall of the uterus, beginning the pregnancy.
In humans, the pregnancy can be detected within a few days of fertilization. The most obvious sign of early pregnancy in humans is a missed period. Bleeding and cramping may occur as a result of the implantation process and the basal body temperature, which displays a slight increase at ovulation, remains elevated. As the body begins to prepare for parenthood, the breasts begin to grow, becoming tender; changes in dietary preferences may occur as a mechanism for obtaining the correct nutrients and rapid hormonal changes can lead to sickness, fatigue and mood changes.