What causes abnormal fertilization?

Only when the egg and sperm meet and combine to form a fertilized egg can they continue to develop into an embryo. However, there is also the problem of abnormal fertilization during the process of egg and sperm combination. When abnormal fertilization occurs, the embryo generally cannot be retained, otherwise even if the embryo grows into a fetus, it will have some congenital diseases. The causes of abnormal ejaculation are also very complicated. Let’s sort out the causes of abnormal fertilization.

1. Evaluation of abnormal fertilization

16-18 hours after in vitro fertilization, the formation of pronuclei and the number of polar bodies in the fertilized eggs were observed under a microscope. Those without two pronuclei were abnormal fertilizations. Under an optical microscope, abnormal fertilization mainly appears in the following forms: no PN, 1PN and multiple PNs.

2. Mechanism of abnormal fertilization

(I) No PN

During in vitro fertilization, approximately 30% of oocytes show no PN. Causes may include:

1: Poor sperm quality, defects in sperm adhesion and penetration mechanisms, and lack of zona pellucida receptors lead to penetration failure.

2: The sperm quality is good, the oocyte cytoplasm is immature or the number of oocyte chromosomes is abnormal.

3: The male and female pronuclei may fuse prematurely, causing the pronucleus to disappear.

4: Late cleavage fertilized egg. This type of embryo has a high rate of chromosomal abnormalities.

(II) 1PN

The incidence of 1PN during in vitro fertilization is 3%-6%. The mechanisms of occurrence may include:

1: 1PN with only female pronucleus, sperm abnormality leads to obstruction of male pronucleus formation.

2: ICSI injection of 1PN with only male pronucleus destroys the spindle undergoing meiosis, leading to obstacles in the removal of the second polar body, etc.

3: Parthenogenetic activation of oocytes.

4: The male and female pronuclei fuse to form one pronucleus. Generally, the single pronucleus of IVF is usually diploid.

5: The development of male and female pronuclei is not synchronous.

(III) Multiple PN

A fertilized egg containing 3 or more pronuclei is called poly-PN, with 3PN being the main component. The main reason is that two or more sperms pass through the zona pellucida and enter the oocyte during fertilization, which is also called polyspermy. Possible mechanisms include:

1: Multiple sperm enter the oocyte. Factors such as immature or overmature oocytes, high sperm concentration, abnormal zona pellucida, high progesterone and high androgen levels in the follicular fluid can all lead to multiple sperm entering the oocyte.

2: Sperm enters abnormal oocyte. Normal sperm enters the polyploid oocyte, resulting in polyspermy.

3: The second polar body is retained in the oocyte and the pronuclear fragments.

For various abnormal fertilization situations, from a safety perspective, priority should be given to transplanting well-developed normally fertilized embryos. However, for patients with a small number of eggs and no normal embryos, choosing to transplant abnormally fertilized embryos still has certain clinical significance.