Glossary · Definition · v2026.3
Zygote in Embryology
A zygote is a fertilized egg at the single-cell stage, containing the combined genetic material of both gametes. The zygote undergoes mitotic division to become a multi-cell embryo. Assessment of normal fertilization occurs at this stage.
From Zygote to Embryo
The zygote represents the earliest stage of a new organism. Within hours of fertilization, the zygote begins cell division (cleavage), progressing from a single cell to a multi-cell embryo. By Day 3, a normally developing embryo contains 6 to 8 cells. By Day 5 to 6, it may reach the blastocyst stage. Developmental arrest can occur at any point during this progression.
Biological Constraints
Normal zygote formation requires proper pronuclear development and chromosomal pairing. Abnormal pronuclear patterns indicate failed or abnormal fertilization. The zygote stage is a checkpoint: only normally fertilized eggs proceed to culture. The attrition from mature oocyte to normally fertilized zygote is the first measurable loss in the IVF funnel.
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This content defines terminology for educational orientation. It does not constitute medical advice.
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