While answering the question what is a zygote, one can state that the zygote is a eukaryotic cell formed by the fertilization of two gametes. Thus, an egg cell or ovum fertilized by a sperm cell is a zygote. Zygote formation in human or another multicellular organism is the first developmental stage in the process of reproducing a complete individual. This happens as a given zygote is totipotent i.e. it has the potential to completely create a new offspring owing to all the necessary genetic information for the making of a complete organism.
It contains the genetic material of both the parents in cases of sexual reproduction except in cases of single-celled organisms where the zygote divides asexually by mitosis to produce identical offspring. Although usually diploid in nature, it can be a polyploid in certain cases of plant reproduction where the gametes are unreduced meiotically.
Zygote meaning as stated above is a reference to a cell right after fertilization of the gametes. This cell or zygote will further divide into a mass of cells known as blastocyst which will transform into an embryo which will further transform into a fetus and over a period of months the fetus will transform into an offspring. Generally, the division of the zygote occurs mitotically, but there are certain exceptions in fungi where the division can occur meiotically depending on the life cycle. During this mitotic division, the zygote does not change in size but splits into different cells via cleavage. Initially, the cleavage of the zygote leads to the two-cell stage, then to the four-cell stage. During these stages, they are known as blastomeres. These blastomeres are divided into a large number of cells then it is known as a blastocyst. This is clearly shown in the zygote picture moving further.
Taking a well-known example of zygote formation in human it is understandable that the sperm cell has to penetrate the egg cell for fertilisation. The fertilisation event that leads to zygote formation can happen through either sexual intercourse or medically assisted fertilisation such as in-vitro fertilisation (IVF). Then the DNA in the two haploid nuclei which come from the parents now known as pronuclei, undergo replication temporarily taking the number of chromosomes to 4n diploid in a zygote. Over a period of 30 hours from fertilization during zygote development the fusion of the two pronuclei takes place and there is an immediate mitotic division of the zygote into two 2n diploid cells known as blastomeres.
The zygote picture is clearly given below along with the cycle of stages from zygote development to blastocyst formation:
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There is an interesting fact about zygote meaning. The etymology of zygote meaning has its roots in the Greek language where it means ‘to join’. This zygote meaning correctly describes the zygote formation after the fertilization event and the fusion of the pronuclei during zygote development.
Events Following Zygote Formation in Human and Zygote Development
Scientific studies employ the use of zygotes of different species to understand the entire process of regeneration and reproduction. A very common zygote example is that of a rat that is used in scientific research. The zygote example of rats helps in understanding the process of zygote formation in human and zygote development because these processes are quite similar in both.
The cleavage of the zygote occurs between the events of fertilisation and implantation. While the zygote moves down the uterus for implantation it continues to divide mitotically into different cells leading to the formation of blastomeres and then to blastocyst, as is clear from the zygote picture given above. The period of zygote formation and zygote development with continuous division lasts for four days. This is known as the germinal period and lasts up to fourteen days. After fourteen days, the embryonic period begins. The embryonic period lasts from two weeks to eight weeks after conception where the developing organism is known as an embryo. Following the embryonic period is the fetal period where the embryo now transformed into fetus develops until the birth of the child.
Thus, this entire cycle of events takes about nine months in humans for reproduction from the starting point of conception. As fascinating and highly complex, it is also a marvel of the evolutionary processes and development of organisms from a single cell - the zygote.
It is quite well known that there is considerable scientific development with regards to curing disease. In order to cure inherited diseases, there has been a successful attempt by scientists to genetically edit human zygote. The zygote formation in human is a vital stage for such genetic editing because even though zygote contains all the genetic information for developing a complete organism, the process of expression of these genes does not start until the formation of blastocyst. Therefore, the curing of inheritable diseases is possible at the zygote stage.