Reproductive Science - Clinical Embryology images of human preimplantation embryos

This image shows a human oocyte with one pronucleus visible in the cytoplasm.

When normal fertilisation takes place, an oocyte has two pronuclei visible in the cytoplasm: one come from the sperm that enters the oocyte, and one come from the oocyte in response to the sperm entrance.

However, fertilisation sometimes doesn't go to plan, and only one pronucleus develops. This means that the oocyte is most likely haploid rather than diploid. The single pronucleus most likley originated from the oocyte and has been parthenogenetically activated.

It is possible that pronuclear development is asynchronous and that a second pronucleus may appear in the next few hours. In this case, the oocyte is diploid. However, whilst fertilisation is considered normal (as the oocyte has 2PN), the asynchronicity may indicate the resulting embryo may be less viable and less likely to implant.

A sinlge pronucleus may also form if there are two sets of genetic information (one from the male and one from the female) but only one pronuclear membrane forms around both sets of chromatin, In these instance, the single pronucleus is usually larger.

Healthy live births have been reported after transfer of embryos that develop from 1PN zygotes.