Zygote: The zygote is the first stage of the early embryo, following the fusion of the haploid male and female pronucleus, wherein the chromosome number in the fertilized ovum becomes diploid (2n). Fertilization, which normally takes place in the ampulla of the uterine tube, requires contact of spermatozoa with the zona pellucida of a secondary oocyte, penetration of one or more spermatozoa through the zona pellucida and the ooplasm, swelling of the spermatozoal head, extrusion of the second polar body, formation of the male and female pronuclei, completion of the second meiotic division and commencement of the first mitotic division, or cleavage, of the zygote.

Formation of Zygote

The earliest form of zygote shows pronuclei where haploid germ cells from the male and female fuse with each other, breaking down the nuclear membrane, but they do not merge. Hence, they are single nucleic cells at this point.

The first stage of mitotic division occurs where genetic information is exchanged and chromosomes are paired, resulting in DNA replication at the spindle apparatus of the equator brought about by – the spermatozoa (centrioles) and the oocyte contributing the mitochondria. Thus, the first new cell – the zygote, with a chromosomal makeup of 2n4c is formed due to the alignment of chromosomes from both the sets at the spindle.

Development of Zygote

Step 1: The zygote rapidly divides after the first 12-24 hours of formation

Step 2: The cell mass forms a hollow ball during blastulation

Step 3: Cells start differentiating and form cavities

Step 4: The three germ layers form during gastrulation

Step 5: Formation of primitive streak followed by notochord formation

Step 6: Tubes are formed making a neurula

Step 7: The notochord forms the neural plate

Step 8: The neural plate folds to form a neural tube and neural crest

Step 9: The mesoderm is divided into axial, paraxial, intermediate and lateral plate mesoderm, which gives rise to different organs.

Facts on Zygote

• The zygote is unicellular.
• Medically, a zygote is termed as zygocyte.
• The mitosis of zygote results in the formation of an embryo.
• The process of formation of the zygote is known as fertilization.

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By Team Learning Mantras