What is Zygote?- Definition, Formation, Development of Zygote

A zygote is the first diploid cell that is formed by the fusion of male and female gametes resulting in the formation of an embryo. This is formed by the fusion of two gametes. The two gametes which are responsible for the formation of zygote are male and female gametes specially known as sperm and ovum. When both fuse together, then it forms the zygote of the diploid cell. The zygote turns into an embryo at its development stage. The development of zygote occurs in the very first week of the fertilization process.

Key Terms: Zygote, Stages for Development of Zygote, Reproduction, Zygote Production

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What is a Zygote? 

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Human formation depends on the biological event specially known as 'reproduction'. The reproduction is a process in which fusion of male and female gametes takes place to produce an offspring. When the process occurs, it leads to the formation of a diploid zygote. 

Basically a zygote develops step by step and becomes an individual later. It contains DNA which carries all the genetic information. Zygote is an eukaryotic cell. 

Zygote Production

The reproduction is of two types. Sexual reproduction and asexual reproduction. Sexual reproduction involves two gametes. Zygote is always produced by the fertilization of male and female sex cell. Zygote is a main bridge which leads to the continuation of the species between different organisms. An individual starts its life as a single cell. The zygote undergoes different phases to convert it into a whole individual. It contains the genetic material of both the parents. which is called 'DNA'. The zygote forms because of the fusion of male and female gametes.

During the meiosis cell division, the germ cells develop into the haploid state. Then the two gametes carry a set of chromosomes in zygote which is present in diploid state. The egg (female gamete) or ovum fuses with the male gamete commonly called sperm. 

The second phase of meiosis occurs.In this phase, the chromatids separate. This was the pre fertilization step. Then,post-fertilization occurs. In this step, the zygote exhibits the parental characteristics and turns into a mature progeny or offspring. This can be discussed more In detail. In the female body, a menstrual cycle occurs every month. So the ovulation happens within 14 days after the period.Then the ovum raises towards the fallopian tube. Here, the sperm enters into the tube through the cervix canal while reproduction. But the small projections or linings (cilia) present in the cell wall enable the zygote to form a blastocyst.

When the fusion doesn't take place, then the zygote moves into the uterus which causes the menstruation cycle. 

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How is Zygote formed? 

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Zygote is an eukaryotic cell and unicellular. Talking about the zygote formation, the first form of zygote is pronuclei. It is the result of the fertilization of haploid germ cells from male and female sex cells that fuses together. Before fusion,the male gametes break the nuclear membrane. But it is not the exact time to merge together as it has to undergo the different stages of mitotic division.They are unicellular or single nucleic cells. This step involves the exchange of genetic information (DNA). The pair of chromosomes results in the replication of DNA which is at the spindle apparatus of the equator. This is brought by the spermatozoa and the oocyte. Hence, the zygote is formed at the alignment of chromosomes which occurs from the set of spindles. 

Read about: How does formation of zygote occur internally?

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Stages for the Development of Zygote

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As mentioned earlier zygote is the fusion of the haploid nucleus of sperm and of ovum.

Zygote in human reproduction go through several stages to be fully developed and ready for delivery.

Following are the steps: 

1. Within 24-30 hours of fertilization , zygote starts mitotic division. This repeated mitotic division is called cleavage.
2. Mitotic division increases the number of cells from 2 cell stages to 4,8,16 cell stages, known as blastomeres. 
3. Embryos with 8 -16 blastomeres are known as morula. As mitotic division starts zygote moves from isthmus region to uterus.
4. Morula continues to divide and form blastocyst. 
5. Blastocyst having blastomeres form the outer layer, the arrangement of blastomeres is called trophoblast.Trophoblast gets attached to the endometrium called implantation.
6. Remaining cells are called inner cell masses which give raise to different tissue. 
7. Inner cell mass differentiate into three germ layer

• Ectoderm
• Mesoderm
• Endoderm.

All three layers give rise to all organs in adults

1. After 3 weeks the heart is formed. 
2. In 5th month first movement is observed
3. In the 6th month eyelids and eyelashes are formed.
4. By the 9 th month zygote is fully developed. 

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Things to Remember

• Cells are the basic unit of life. Cells consist of the cytoplasm,cell wall,cell membrane. Further the cell forms into specialized tissues and organs through a process of cell division. 
• Reproduction is a natural and biological process through which an organism can produce the young ones. 
• The offspring starts its life cycle in the form of the zygote. Then it undergoes a cell division process to produce new offspring. 
• When the two gametes of male and female sex cells fuses together then it forms a zygote. 
• Zygote is an eukaryotic and diploid cell. It is converted into an embryo after the post fertilization process. 
• The reproduction is of two types and they are asexual reproduction and sexual reproduction. 
• These two types of reproduction depend on the participation of the two different parents of the same species.