Question

In the human female, the blastocyst
(a) Forms placenta even before implantation
(b) Gets implanted into the uterus 3 days after ovulation
(c) Gets nutrition from uterine endometrial secretion only after implantation
(d) Gets implanted in endometrium by the trophoblast cells

Answer 1

Hint: A series of rapid mitotic divisions lead to the formation of a multicellular structure ‘blastocyst.’ Fertilisation is the fusion of male gamete(sperms) and female gamete(ova). This results in the formation of a unicellular diploid body i.e. zygote.

Complete answer:
In the human female, the blastocyst gets implanted in the endometrium by the trophoblast cells. After fertilization, a series of rapid mitotic divisions called cleavage takes place where the cytoplasm of the zygote is divided equally among numerous smaller cells or morulas. In humans, a morula is achieved after the 4th day of fertilization. Morula is succeeded by the blastula stage. The blastula is composed of blastomeres that are arranged in an outer layer called trophoblast and an inner layer called inner cell mass. After around one week after ovulation i.e 7-9 days, the blastula is embedded in the uterine endometrial lining by the trophoblast cells. Before implantation occurs, an embryo’s nutrition is provided by the secreted nutrients of the uterine cavity like iron and fat-soluble vitamins.

Additional Information: -Ovulation is the release of ovum or female gamete from the ovary.
-The fusion of male gamete i.e sperm and female gamete or fertilization occurs in the isthmus portion of the fallopian tube. This fusion forms zygote. It is a diploid unicellular cell that will give rise to a multicellular fetus in the female’s body.
-Fertilisation is followed by cleavage of zygote.
-Cleavage transforms the zygote into a morula. It is characterized by a solid mass of cells with 16-32 cells.
-This will lead to a blastula stage or a hollow mass of cells. A fluid-filled cavity inside the ball of cells is called the blastocoel surrounded by an outer layer ‘trophoblast’ and an inner layer ‘inner cell mass.’
-After around one week after ovulation i.e 7-9 days, the trophoblast layer extends into the endometrial lining of the uterus by breaking down the molecules of the endometrium.
-This is then followed by the gastrulation stage wherein the three germ layers are originated by the rearrangement of cells into endoderm, mesoderm, and ectoderm. And this ball of cells is known as gastrula.
-After implantation, the placenta is formed.
So, the correct answer is ‘ Gets implanted in endometrium by the trophoblast cells.’

Note: The placenta acts as a barrier between the fetus and the mother. Although the blood of the fetus and mother do not mix or blend, the placenta acts as an ultrafilter where soluble nutrients, minerals, hormones, antibodies, can cross over and pass onto the fetus. It also helps in the exchange of gases between the two as well as the elimination of nitrogenous wastes from the fetus.