Fertilization – Definition, Types, Process, and Significance

Fertilization is the process in sexual reproduction in which compatible haploid gametes unite, combining genetic material and producing a zygote that begins a new developmental program. The details differ among organisms: animals may use internal or external fertilization, while flowering plants undergo double fertilization, producing both a zygote and endosperm.

What Is Fertilization?

Fertilization is the process of sexual reproduction in which two gametes fuse together, resulting in the formation of a zygote. It brings the genetic materials of two gametes into a single cell.

Gametes are reproductive cells. In organisms having a diploid life cycle, these generally contain one haploid set of chromosomes (n). During fertilization, the haploid chromosome set carried by one gamete comes together with that of the other gamete. The diploid chromosome number (2n) is thus restored in the newly formed zygote.

The actual process of fertilization is not same in all groups of organisms. In different taxa, gametes may differ in their structure and also in the way they reach, recognize and fuse with each other. These cellular events are therefore different in animals, plants and other sexually reproducing organisms, while the union of compatible gametes resulting in zygote formation forms the basic process of fertilization.

Fertilization vs Pollination, Insemination, and Implantation

Pollination, insemination and implantation are not fertilization. These are separate events of reproduction. Pollination and insemination take place before possible fertilization, while implantation occurs later after fertilization in mammals.

Classification of Fertilization

Fertilization may be classified on the basis of place where gametes unite and also according to the source of gametes.

A. Based on the place of fertilization

1. External Fertilization– This type takes place outside the body. Male and female gametes are released into the external medium, mostly water where fusion occurs. Aquatic animals commonly show external fertilization. Many fishes, amphibians and aquatic invertebrates are the examples.
2. Internal Fertilization– Fusion occurs inside the body, usually in the female reproductive tract. The sperm is transferred into the female body and reaches the egg for fusion. Reptiles, birds and mammals commonly have internal fertilization, some fishes and invertebrates also show this type.

B. Based on the source of gametes

1. Self-fertilization– The male and female gametes taking part in fertilization are produced by the same individual. It is found in some hermaphroditic animals. Self-compatible plants can also undergo self-fertilization.
2. Cross-fertilization– Gametes are from two different individuals of the same species in this type. It is common in many animals. In plants also, the male gamete from one individual fertilizes the egg belonging to another individual.

The Cellular Process of Fertilization in Animals

The cellular events of fertilization are different among animal groups. However, some basic events such as gamete recognition, sperm-egg fusion, egg activation and joining of parental genetic material are common. The following are the major steps-

1. Sperm-Egg Encounter and Recognition– The sperm first reaches the egg and comes in contact with its outer coverings. Recognition between sperm and egg takes place through molecules present on their surfaces and extracellular coats. In many animals these interactions also provide species-specific recognition. Mammalian sperm undergo capacitation before becoming fully capable of fertilizing the egg. This process is not present as the same mechanism in all animals.
2. Acrosome Reaction– In many animals, the sperm undergoes acrosome reaction before fusion with the egg. The acrosome present over the sperm head releases its contents and sperm surface is also changed during this reaction. It helps the sperm to interact with and pass through the extracellular covering of the egg. The exact trigger and place of acrosome reaction differs among animal groups.
3. Penetration of Egg Coverings– The sperm passes through the extracellular layers surrounding the egg. These coverings are not same in all animals. Mammalian eggs are surrounded by the zona pellucida, whereas many other animals possess a vitelline envelope and other egg coats. After crossing these layers, sperm reaches the egg plasma membrane.
4. Fusion of Sperm and Egg Membranes– The plasma membrane of sperm comes in contact with the egg membrane and both membranes fuse. Sperm components are then delivered into the egg cytoplasm. In mammals, the sperm protein IZUMO1 and its egg receptor JUNO are essential for sperm-egg recognition associated with this stage. Other proteins are also involved in the fusion process.
5. Activation of the Egg– Entry and fusion of sperm activates the egg. A rapid increase or repeated oscillations of intracellular Ca²⁺ occurs in many animals. During this process several metabolic and cellular activities of the egg are started. In mammals, sperm-derived phospholipase C zeta (PLCζ) has an important role in producing these Ca²⁺ oscillations.
6. Cortical Reaction and Prevention of Polyspermy– Cortical granules present below the egg plasma membrane release their contents after egg activation. This is called the cortical reaction. The extracellular egg covering is modified and further sperm entry is restricted. Some externally fertilizing animals also have rapid membrane changes associated with prevention of polyspermy, but such mechanisms are not same in all animal groups.
7. Formation of Male and Female Pronuclei– The sperm nucleus undergoes changes within the egg and forms the male pronucleus. The female genetic material forms the female pronucleus, after completion of meiotic events where these are still unfinished at fertilization. Both pronuclei move within the egg. Their chromosomes are brought together during preparation for the first mitotic division.
8. Zygote Formation and Beginning of Cleavage– Fertilization produces the zygote containing genetic material derived from both gametes. The first mitotic division is then initiated. This begins the process of cleavage and further embryonic development.

What Happens Immediately After Human Fertilization?

After entry of sperm, the secondary oocyte becomes activated. Several changes now occur inside the fertilized egg before its first division. The major events are-

• Oocyte Activation– Sperm fusion causes repeated increase of Ca²⁺ in the oocyte cytoplasm. The oocyte which was arrested at metaphase II becomes activated.
• Cortical Reaction– Cortical granules lying just below the oocyte membrane release their contents. The zona pellucida is modified. Further entry of sperm is prevented, reducing polyspermy.
• Completion of Meiosis II– Human secondary oocyte remains arrested at metaphase II until fertilization. It now completes the second meiotic division. The second polar body is extruded.
• Pronuclei Formation– The sperm nucleus inside the egg decondenses and forms the male pronucleus. Maternal chromosomes form the female pronucleus. Two pronuclei are now present in the fertilized egg.
• Changes in Pronuclei– Both pronuclei increase in size and move closer to each other. DNA is replicated in the maternal and paternal chromosome sets. They remain separate during this period and the pronuclear membranes later disappear.
• First Cleavage– Maternal and paternal chromosomes are arranged for the first mitotic division. The zygote divides into two cells called blastomeres. Further divisions continue and this is known as cleavage.

Fertilization in Flowering Plants

Fertilization in flowering plants occurs inside the embryo sac present in the ovule. Before fertilization, pollen grain germinates on the stigma and produces a pollen tube. Two male gametes are carried through this tube. In flowering plants, two fusion events take place during fertilization, which is called double fertilization.

The process takes place in different steps, which are as follows-

Step 1- Germination of pollen grain

After a compatible pollen grain reaches the stigma, it absorbs water and germinates. A pollen tube is formed.

The pollen tube grows through the stigma and style towards the ovary. Two male gametes are present in the pollen tube.

Step 2- Entry of pollen tube into ovule

The pollen tube reaches the ovule and commonly enters through the micropyle. It now grows towards the embryo sac.

One of the synergids receives the pollen tube. Synergids also take part in guiding the pollen tube towards the egg apparatus.

Step 3- Release of male gametes

The tip of pollen tube bursts after reaching the receptive synergid. Two male gametes are released.

The receptive synergid degenerates during this process. The male gametes are now present close to the egg cell and central cell.

Step 4- Syngamy

One male gamete fuses with the egg cell. A diploid zygote (2n) is formed.

This fusion is called syngamy. The zygote later develops into embryo.

Step 5- Triple fusion

The second male gamete fuses with the central cell containing the two polar nuclei or secondary nucleus. In the common type of embryo sac, a triploid primary endosperm nucleus (3n) is formed.

This process is called triple fusion.

Step 6- Double fertilization

Syngamy and triple fusion occur in the same embryo sac. These two fusion events together make double fertilization.

One fusion produces the zygote. The other leads to formation of the endosperm.

Step 7- Changes after fertilization

The zygote develops into the embryo. The primary endosperm nucleus forms the endosperm, which provides food to the developing embryo.

The ovule later develops into seed. Ovary generally forms the fruit after fertilization.

Biological Significance of Fertilization

Some of the important biological significance of fertilization are as follows-

• Restoration of chromosome number- Gametes contain the haploid chromosome number (n). Fusion of two gametes again brings the two chromosome sets together and diploid condition (2n) is restored in the zygote.
• Zygote formation- Fertilization results in formation of the zygote. It is the first cell of the new organism, from which further development starts.
• Combination of parental genetic material- Genetic materials coming through the two gametes become present in the same cell. The zygote receives chromosomes from both parents.
• Activation of egg- In many animals, the egg remains arrested before fertilization. Sperm fusion activates the egg and different metabolic activities are started. Cell division also follows after this activation.
• Maintenance of chromosome number- Meiosis reduces chromosome number to half during formation of gametes, whereas fertilization restores it again. In this way the characteristic chromosome number of a species is maintained from one sexual generation to the next.
• Genetic variation- Different male and female gametes can unite during fertilization. New combinations of maternal and paternal chromosomes are produced in offspring. Along with recombination and independent assortment during meiosis, it adds to genetic variation.
• Beginning of embryonic development- After formation of zygote, cleavage divisions begin. The zygote gives rise to two cells, then further divisions continue and the embryo develops.

Examples of Fertilization

Some of the examples of fertilization are given below-

1. Fertilization in Sea Urchin

• An example of external fertilization is the fertilization in sea urchin, where eggs and sperms are released into seawater.
• The gametes meet in the surrounding water and sperm reaches the egg.
• Fusion of sperm and egg takes place outside the body.
• Sea urchin fertilization has been commonly used for studying sperm-egg recognition, acrosome reaction and egg activation.

2. Fertilization in Frog

• Another example of external fertilization is found in frogs.
• During reproduction, the male holds the female by amplexus and eggs are released into water.
• Sperms are released over these eggs and fertilization takes place outside the female body.
• A large number of eggs are generally released during this process.

3. Fertilization in Humans

• Humans show internal fertilization, where fusion of sperm and secondary oocyte takes place inside the female reproductive tract.
• The sperm reaches the secondary oocyte and one sperm finally fuses with it.
• After fertilization, male and female genetic materials are present in the same fertilized cell and a zygote is formed.
• Fertilization is followed by cleavage and further embryonic development.

4. Fertilization in Flowering Plants

• A special example of fertilization is found in flowering plants, called double fertilization.
• Two male gametes reach the embryo sac through the pollen tube.
• One male gamete fuses with the egg cell, forming the zygote, whereas the second male gamete fuses with the central cell.
• The two fusion events occur in the same embryo sac and the second fusion results in formation of the endosperm.

References

1. Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. (2002). Molecular biology of the cell (4th ed.). Garland Science. https://www.ncbi.nlm.nih.gov/books/NBK21054/
2. Baroux, C., Spillane, C., & Grossniklaus, U. (2002). Evolutionary origins of the endosperm in flowering plants. Genome Biology, 3(9), reviews1026.1–reviews1026.5. https://doi.org/10.1186/gb-2002-3-9-reviews1026
3. Bleckmann, A., Alter, S., & Dresselhaus, T. (2014). The beginning of a seed: Regulatory mechanisms of double fertilization. Frontiers in Plant Science, 5, 452. https://doi.org/10.3389/fpls.2014.00452
4. Campos, G., Sciorio, R., & Esteves, S. C. (2023). Total fertilization failure after ICSI: Insights into pathophysiology, diagnosis, and management through artificial oocyte activation. Human Reproduction Update, 29(4), 369–394. https://doi.org/10.1093/humupd/dmad007
5. Ciapa, B., & Chiri, S. (2000). Egg activation: Upstream of the fertilization calcium signal. Biology of the Cell, 92(3–4), 215–233. https://doi.org/10.1016/S0248-4900(00)01065-0
6. Clark, M. A., Douglas, M., & Choi, J. (2018). Biology 2e. OpenStax. https://openstax.org/books/biology-2e/pages/1-introduction
7. Coticchio, G., Barrie, A., Lagalla, C., Borini, A., Fishel, S., Griffin, D., & Campbell, A. (2021). Plasticity of the human preimplantation embryo: Developmental dogmas, variations on themes and self-correction. Human Reproduction Update, 27(5), 848–865. https://doi.org/10.1093/humupd/dmab016
8. Dresselhaus, T., Sprunck, S., & Wessel, G. M. (2016). Fertilization mechanisms in flowering plants. Current Biology, 26(3), R125–R139. https://doi.org/10.1016/j.cub.2015.12.032
9. Gilbert, S. F. (2000). Developmental biology (6th ed.). Sinauer Associates. https://www.ncbi.nlm.nih.gov/books/NBK9983/
10. Hillers, K. J., Jantsch, V., Martinez-Perez, E., & Yanowitz, J. L. (2017). Meiosis. In WormBook: The online review of C. elegans biology. WormBook. https://doi.org/10.1895/wormbook.1.178.1
11. Johnson, M. A., Harper, J. F., & Palanivelu, R. (2019). A fruitful journey: Pollen tube navigation from germination to fertilization. Annual Review of Plant Biology, 70, 809–837. https://doi.org/10.1146/annurev-arplant-050718-100133
12. Li, H.-J., & Yang, W.-C. (2020). Central cell in flowering plants: Specification, signaling, and evolution. Frontiers in Plant Science, 11, 590307. https://doi.org/10.3389/fpls.2020.590307
13. Miao, Y.-L., & Williams, C. J. (2012). Calcium signaling in mammalian egg activation and embryo development: The influence of subcellular localization. Molecular Reproduction and Development, 79(11), 742–756. https://doi.org/10.1002/mrd.22078
14. Ohto, U., Ishida, H., Krayukhina, E., Uchiyama, S., Inoue, N., & Shimizu, T. (2016). Structure of IZUMO1–JUNO reveals sperm–oocyte recognition during mammalian fertilization. Nature, 534(7608), 566–569. https://doi.org/10.1038/nature18596
15. Rickard, J. P., Pool, K. R., Druart, X., & de Graaf, S. P. (2019). The fate of spermatozoa in the female reproductive tract: A comparative review. Theriogenology, 137, 104–112. https://doi.org/10.1016/j.theriogenology.2019.05.044
16. Tosti, E., & Ménézo, Y. (2016). Gamete activation: Basic knowledge and clinical applications. Human Reproduction Update, 22(4), 420–439. https://doi.org/10.1093/humupd/dmw014
17. Willmer, P. (2011). Pollination and floral ecology. Princeton University Press. https://doi.org/10.23943/princeton/9780691128610.001.0001
18. Zheng, S., Wang, F., Liu, Z., Zhang, H., Zhang, L., & Chen, D. (2024). The role of female and male genes in regulating pollen tube guidance in flowering plants. Genes, 15(11), 1367. https://doi.org/10.3390/genes15111367
19. Zhong, S., Lan, Z., & Qu, L.-J. (2025). Ingenious male–female communication ensures successful double fertilization in angiosperms. Annual Review of Plant Biology, 76, 401–431. https://doi.org/10.1146/annurev-arplant-083123-071512