Question

If in the blastocyst, rupture of zona pellucida layer or zona hatching occurs before time, which among the following could be the possibilities?
1. Implantation occurs in the ampullary region of the fallopian tube
2. Tubular implantation
3. Endometrial implantation

• A. 1 only
  
• B. 2 only
• C. 3 only
  
• D. 1 and 2
  

Answer 1

The correct option is D 1 and 2


Penetration of mature ovum by sperm and fusion of male and female pronuclei forms the zygote. The single-celled zygote divides into a two-celled stage (approximately in one day), four celled stage ( approximately in two days), twelve celled stage (approximately in three days) and sixteen celled stage (approximately in four days). The 8-16 celled stage is called the morula.
Further continuous divisions and rearrangement of the blastomeres in the morula results in the formation of blastocyst. By the blastocyst stage, the embryo reaches 50-150 celled stage and the blastomeres are arranged in the outer and inner layers called trophoblast and inner cell mass respectively. Zona pelucida is intact until this stage and now to commence the implantation, the embryo needs to hatch out from the zona pellucida layer. As the cells in the cytoplasmic mass are dividing and increasing, it creates strain in the wall of the zona pellucida from inside. The pumping of fluid from the uterus into the blastocyst also creates a strain on the wall of the zona pellucida making it thin and finally allowing it to rupture. This allows the blastocyst to escape from the zona pellucida layer and prepares it to get implanted in the endometrium. Through time it matures, becomes competent and reaches the proper location of the uterus for endometrial implantation.

But if this zona pellucida layer ruptures and blastocysts hatch before time like at 8,12 or16 celled stages then it will get implanted in other location and not in the endometrium. This results in ectopic pregnancies.

Ectopic pregnancies are classified on the basis of the site of implantation. Tubular pregnancy results if implantation occurs in the fallopian tube. This occurs if the zygote does not travel but stays at the site of fertilization, fixes to the wall of ampulla and starts growing within it.
Implantation in the ampullary region is the most common among all the abnormal tubular implantations.

Figure: Ectopic pregnancy