A zygote (from Greek ζυγωτός zygōtos “joined” or “yoked”, from ζυγοῦν zygoun “to join” or “to yoke”), is an eukaryotic cell formed by a fertilization event between two gametes. The zygote’s genome is a combination of the DNA in each gamete and contains all of the genetic information necessary to form a new individual. In multicellular organisms, the zygote is the earliest developmental stage. In single-celled organisms, the zygote can divide asexually by mitosis to produce identical offspring.
Oscar Hertwig and Richard Hertwig made some of the first discoveries on animal zygote formation.
In human fertilization, a released ovum (a haploid secondary oocyte with replicate chromosome copies) and a haploid sperm cell (male gamete)—combine to form a single 2n diploid cell called the zygote. Once the single sperm enters the oocyte, it completes the division of the second meiosis forming a haploid daughter with only 23 chromosomes, almost all of the cytoplasm, and the sperm in its own pronucleus. The other product of meiosis is the second polar body with only chromosomes but no ability to replicate or survive. In the fertilized daughter, DNA is then replicated in the two separate pronuclei derived from the sperm and ovum, making the zygote’s chromosome number temporarily 4n diploid. After approximately 30 hours from the time of fertilization, a fusion of the pronuclei and immediate mitotic division produce two 2n diploid daughter cells called blastomeres.
Between the stages of fertilization and implantation, the developing human is a preimplantation conceptus. There is some dispute about whether this conceptus should no longer be referred to as an embryo, but should now be referred to as a proembryo, which is terminology that traditionally has been used to refer to plant life. Some ethicist and legal scholars make the argument that it is incorrect to call the conceptus an embryo, because it will later differentiate into both intraembryonic and extraembryonic tissues, and can even split to produce multiple embryos (identical twins), while others have pointed out that, as so-called extraembryonic tissues are really parts of the embryo’s body that are no longer utilized after birth, much as milk teeth fall out after childhood, and as the process of the embryo splitting to form identical twins leaves the original intact, while generating a new embryo, rendering it no different from the process of cloning an adult human. However, the National Institutes of Health has made the determination that the traditional classification of preimplantation embryo is still correct.
After fertilization, the conceptus travels down the oviduct towards the uterus while continuing to divide mitotically without actually increasing in size, in a process called cleavage. After four divisions, the conceptus consists of 16 blastomeres, and it is known as the morula. Through the processes of compaction, cell division, and blastulation, the conceptus takes the form of the blastocyst by the fifth day of development, just as it approaches the site of implantation. When the blastocyst hatches from the zona pellucida, it can implant in the endometrial lining of the uterus and begin the embryonic stage of development.
The human zygote has been genetically edited in experiments designed to cure inherited diseases.