Zygote

Zygote Definition

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.

The zygote stage development occurs in the first week of fertilization. The genome of the zygote is the combination of DNA in each gamete and contains all the genetic information required to form an individual.

What is Zygote?

A zygote is a fertilized ovum. The germ cells undergo meiosis to reach a haploid state(n) as part of spermatogenesis and oogenesis. The sperm and the unfertilized ovum reinstates a chromosome set in zygote in a diploid state(2n).

After fusing with the sperm, the second stage of meiosis takes place for separation of chromatids. Post-fertilization, mitosis of the zygote is initiated which develops into a mature offspring exhibiting characteristics of its parent.

In the course of the menstrual cycle, ovulation occurs through one of the ovaries after 14 days of the cycle after which the ovum proceeds towards the fallopian tube. Sperm enters through the cervix to reach the fallopian tube where fertilization is facilitated by the lining of the cells. These linings project tiny needle-like structures called the cilia which propel the zygote to the uterus to form a blastocyst (ball of cells) where cells undergo divisions.

If fertilization does not occur, the zygote moves down to the uterus resulting in the onset of menstruation.

Zygote serves as a vital link ensuring species continuation from the present to the next generation. Every sexually reproducing entity initiates life processes with the formation of zygote.

Zygote forms the first stage in the development of a unique entity. It contains all the vital factors essential for the development as they occur as an encoded set of information in the makeup of chromosomes.

The activation of the zygote to produce proteins occurs only after it undergoes a series of divisions into cleavage. The formation of blastomeres serves as building units for the potential organism.

It is a result of the formation of the zygote that the diploid cell is formed containing DNA information. It is during this stage, that the chromosomal gender of the new individual is decided. Zygote formation also brings about cleavage division for the stage of embryogenesis.

See Also: Sexual Reproduction: An Overview

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

The development of zygote takes place in several stages mentioned below:

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

The cell mass forms a hollow ball during blastulation

Cells start differentiating and form cavities

The three germ layers form during gastrulation

Formation of primitive streak followed by notochord formation

Tubes are formed making a neurula

The notochord forms the neural plate

The neural plate folds to form neural tube and neural crest

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

The zygote undergoes rapid mitotic division to make a lot of new cells.

With each round of cell division, the cells of a zygote double in number, i.e., the cell number increases at an exponential rate. Due to rapid division, the cells do not have any time to grow and therefore the 32 cell stage known as morula is the same size as the zygote. The zona pellucida remains intact which also regulates the growth of the morula.

After four days the cells begin to differentiate and develop more specific forms and functions.

It divides into two layers:

  • Trophoblast – This is the outer layer that develops into structures that help in the implantation of the embryo in the mother’s uterus.

  • The inner cell mass – This is the inner layer and comprises of pluripotent cells. They get differentiated into the cells of any body tissue. Here the zona pellucida begins to disappear.

The rest of the fluid-filled cavity is known as the blastocoel.

During the second week, the cells further differentiate into epiblast and hypoblast.

During the third week the following three germ layers are formed that forms one of our organizational tubes.

  • Ectoderm- Epidermis, hair, nails, brain, spinal cord

  • Mesoderm- Muscle, connective tissue, notochord, bone, gonads, kidney, circulatory system

  • Endoderm- Epithelial lining of the digestive tract.

The neural crest is sometimes the fourth germ layer because the cells get differentiated into the parasympathetic and sympathetic nervous system.

After eight weeks all our tubes are in order and our heart has been beating for almost five weeks.

Difference Between Zygote and Embryo

Following are the difference between zygote and embryo:

Zygote

Embryo

Zygote is unicellular.

Embryo is multicellular.

Medically, a zygote is termed as zygocyte.

The embryo is termed as a diploid eukaryote.

The mitosis of zygote results in the formation of embryo.

Embryo is the stage following the zygote stage.

The process of formation of zygote is known as fertilization.

The process of formation of embryo is known as embryogenesis.

Also Read: Fertilization and Implantation

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