Zygote
Also known as: fertilised egg, cleavage, blastomere
The single diploid, totipotent cell formed at fertilisation, which begins cleavage divisions inside the zona pellucida as it is carried along the uterine tube.
Overview
The zygote (≈0.1 mm) still lies within the zona pellucida. About 24–30 hours after fertilisation it undergoes the first cleavage division into two blastomeres. Cleavage divisions are rapid mitoses without growth, so the cells become progressively smaller while the whole embryo stays the same size; they are initially driven by maternal mRNAs and proteins stored in the oocyte, and the embryo's own genome becomes active at about the 4–8-cell stage (day 2–3). Early blastomeres are totipotent — each can form a whole embryo and its extraembryonic tissues.
Meanwhile the embryo is moved towards the uterus by tubal cilia and peristalsis, reaching the 4-cell stage at ≈day 2 and the 8–16-cell stage at ≈day 3.
- Location
Ampulla, then isthmus of the uterine tube.
Function
- Begins embryonic development by cleavage.
- Totipotent precursor of the embryo and all extraembryonic tissues.
Clinical significance
Early pregnancy loss and testing
Many early embryos are chromosomally abnormal (meiotic errors) and fail before or around implantation. In IVF, embryos may be biopsied for preimplantation genetic testing.
Compiled from standard anatomy and physiology references — see the Reproductive System sources. For education only; not medical advice.