In human female, the blastocyst:

Explanation

Note: Implantation in endometrial uterine wall takes place at blastocyst stage of embryonic development. Before implantation, the blastomeres of early blastocyst get arranged into an outer layer called trophoblast and an inner group of cells attached to trophoblast called inner cell mass. It is the trophoblast layer through which blastocyst gets attached to the endometrium and the inner cell mass gets differentiated as the embryo. The correct option is: d) Gets implanted in the endometrium by the trophoblast cells. Explanation of each option: a) Forms placenta even before implantation: This statement is not accurate. While the blastocyst is the early stage of embryonic development, it does not form the placenta before implantation. After fertilization, the zygote undergoes several cell divisions and forms a hollow ball of cells called the blastocyst. The blastocyst is not capable of forming the placenta until after it successfully implants into the uterine lining. b) Gets implanted into the uterus 3 days after ovulation: This statement is generally accurate. After fertilization, the blastocyst undergoes development and travels through the fallopian tube for approximately 3-4 days before reaching the uterus. Once in the uterus, the blastocyst then undergoes implantation into the uterine wall. Implantation usually occurs around 6-10 days after ovulation, and the blastocyst adheres to the endometrium (the lining of the uterus) during this process. c) Gets nutrition from uterine endometrial secretion only after implantation: This statement is not accurate. The blastocyst starts obtaining nutrients from the uterine endometrial secretion only after it implants into the uterine wall. Before implantation, the blastocyst relies on its own internal resources to support its early development while it travels from the fallopian tube to the uterus. d) Gets implanted in the endometrium by the trophoblast cells: This statement is correct. Implantation is a crucial process during early pregnancy. After reaching the uterus, the blastocyst undergoes attachment and implantation into the endometrial lining. The trophoblast cells, which are part of the blastocyst, play a significant role in this process. The trophoblast cells are responsible for adhering to the endometrium and initiating the formation of the placenta. In summary, option (d) is correct. The blastocyst gets implanted in the endometrium by the trophoblast cells, leading to the establishment of pregnancy and the eventual development of the placenta, which is vital for supporting the growth and development of the embryo.