zigotos

Zigotos, or zygotes, are the fertilized cells formed when a haploid sperm cell merges with a haploid egg cell. The zygote represents the first stage of development in sexually reproducing organisms and contains a diploid set of chromosomes—one chromosome from each parent—carrying the organism’s complete genetic material. The term zygote comes from the Greek zygotēs, meaning yoked or joined.

Formation and early development: In animals, fertilization typically occurs when a sperm penetrates the egg, resulting in a single diploid cell. The zygote then undergoes rapid mitotic divisions called cleavage, producing a multicellular embryo. Early stages include the morula, a solid ball of cells, and the blastula or blastocyst, a hollow ball with an inner cell mass in mammals. At the earliest stages the cells remain totipotent, capable of giving rise to all cell types of the organism.

In humans, fertilization usually happens in the fallopian tube, and the zygote travels toward the uterus while dividing. Implantation into the uterine lining occurs about six to ten days after fertilization, signaling the start of pregnancy. Zygotes are genetically unique, combining paternal and maternal alleles and contributing to genetic diversity among offspring.

In plants, the zygote forms after pollen fertilizes an ovule and then begins embryonic development within the seed. Across taxa, the zygote is a transient but essential stage that sets the trajectory for subsequent embryogenesis and organismal growth.

Significance: The study of zygotes informs reproductive and developmental biology and underpins assisted reproductive technologies such as in vitro fertilization. Ethical, legal, and regulatory considerations accompany their manipulation in research and clinical contexts.