Read the given passage and answer the following questions:
Spermatogenesis results in the formation of sperms that are transported by the male sex accessory ducts. A normal human sperm is composed of a head, neck, a middle piece and tail. The process of formation of mature female gametes is called oogenesis. The reproductive cycle of female primates is called menstrual cycle. Menstrual cycle starts only after attaining sexual maturation (puberty). During ovulation only one ovum is released per menstrual cycle. The cyclical changes in the ovary and the uterus during menstrual cycle are induced by changes in the levels of pituitary and ovarian hormones. After coitus, sperms are transported to the ampulla, where the sperm fertilises the ovum leading to formation of a diploid zygote. The presence of X or Y chromosome in the sperm determines the sex of the embryo. The zygote undergoes repeated mitotic division to form a blastocyst, which is implanted in the uterus resulting in pregnancy. After nine months of pregnancy, the fully developed foetus is ready for delivery. The process of childbirth is called parturition which is induced by a complex neuroendocrine mechanism involving cortisol, estrogens and oxytocin. Mammary glands differentiate during pregnancy and secrete milk after child-birth. The new-born baby is fed milk by the mother (lactation) during the initial few months of growth.
In the blastocyst, which layer gets attached to the endometrium?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Trophoblast
The zygote divides to form a morula, which becomes a blastocyst as it moves into the uterus. In the blastocyst the cells (blastomeres) form two groups: an outer layer called the trophoblast and an inner group of cells called the inner cell mass. The trophoblast is the layer that attaches to the endometrium. The uterine cells then cover the blastocyst, and it gets embedded in the endometrium. This is implantation.
Option (1) Inner cell mass — Incorrect. The inner cell mass lies inside the blastocyst, attached to the trophoblast. It does not touch the endometrium; it develops into the embryo.
Option (2) Trophoblast — Correct. The trophoblast is the outer layer of the blastocyst. It sticks to the endometrium and starts implantation. Later, chorionic villi grow out from it and help form the placenta.
Option (3) Ectoderm — Incorrect. The ectoderm forms after implantation, when the inner cell mass differentiates into the outer ectoderm and the inner endoderm. It is an embryonic layer, not a layer of the blastocyst.
Option (4) Zona pellucida — Incorrect. The zona pellucida is a layer around the ovum. A sperm makes contact with it at fertilisation. It is not the blastocyst layer that joins the endometrium.
NCERT Extract:
Fertilisation and Implantation – “… The morula continues to divide and transforms into blastocyst (Figure 2.11g) as it moves further into the uterus. The blastomeres in the blastocyst are arranged into an outer layer called trophoblast and an inner group of cells attached to trophoblast called the inner cell mass. The trophoblast layer then gets attached to the endometrium and the inner cell mass gets differentiated as the embryo.”