Question

Identify the labelled parts 1 - 4 in the given figure of the blastocyst within the uterus.

• A. 1- Blastocyst, 2. Inner cell mass, 3. Corona Radiata, 4.Trophoblast
  
• B. 1- Blastomere, 2.Trophoblast, 3. Corona Radiata, 4.Inner cell mass
  
• C. 1- Blastomere, 2. Trophoblast, 3. Zona pellucida, 4.Inner cell mass
  
• D. 1- Blastocoel, 2.Trophoblast, 3. Zona pellucida, 4.Inner cell mass
  

Answer 1

The correct option is D 1- Blastocoel, 2.Trophoblast, 3. Zona pellucida, 4.Inner cell mass


By the process of cleavage, the single-celled zygote starts dividing as it moves forward towards the uterus. This division results in 2, 4, 8, 16 celled stages of the embryo. An embryo with 8-16 blastomeres is called the morula. Morula reaches the uterus and floats in the uterus. Morula continues to divide by getting nutrients from uterine milk and a cavity is formed within the cells called the blastocoel. This stage of the embryo is called the blastocyst. Blastomeres in the blastocyst are then arranged into an outer layer of cells called trophoblast and an inner group of cells attached to the trophoblast called the inner cell mass. The trophoblast layer gets attached to the endometrium and the inner cell mass gets differentiated into the embryo proper. As a result, blastocyst becomes embedded in the endometrium of the uterus and this is called implantation, which leads to pregnancy.
Zona pellucida is the thick transparent membrane surrounding a mammalian ovum before implantation. The corona radiata is an outer layer of follicular or granulosa cells that form around a developing oocyte in the ovary and remain with it upon ovulation. This layer is present outside zona pellucida.