17.6: Zygote

After ovulation, the secondary oocyte, released into the fallopian tube, remains arrested in the metaphase of meiosis II until fertilization. Completion of meiosis into a full-fledged ovum only occurs upon fertilization. This meiotic process involves an unequal division of the cytoplasm, with the ovum receiving the majority and the second polar body receiving a smaller portion.

The two haploid nuclei originating from the sperm and ovum are known as pronuclei, which undergo decondensation, expansion, and DNA replication in preparation for mitosis. Subsequently, they migrate toward each other, their nuclear envelopes disintegrate, and the male- and female-derived genetic material commingle. This crucial step concludes the fertilization process, resulting in the formation of a single diploid cell, the zygote. The zygote contains the full complement of genetic instructions necessary for the development of a human.

Typically, a woman releases a single oocyte during an ovulation cycle. Nevertheless, two oocytes are released in roughly one percent of cases, and both become fertilized. This leads to the formation of two zygotes that implant and develop, ultimately resulting in the birth of dizygotic (fraternal) twins. Dizygotic twins, resulting from two oocytes fertilized by two different sperm, are no more genetically similar than siblings born at different times. In less frequent instances, a zygote can split into two distinct offspring during early development. This occurrence gives rise to the birth of monozygotic (identical) twins. The zygote can undergo this division as early as the two-cell stage, although it more commonly occurs during the early blastocyst stage when roughly 70 to 100 cells are present. These two scenarios are distinct from one another. Twins resulting from separation at the two-cell stage have individual placentas, whereas twins emerging from separation at the blastocyst stage share both a placenta and a chorionic cavity.³

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