Read the passage and answer the following question:
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. After attachment, the uterine cells divide rapidly and covers the blastocyst. As a result, the blastocyst becomes embedded in the endometrium of the uterus . This is called implantation and it leads to pregnancy. After implantation, finger-like projections appear on the trophoblast called chorionic villi which are surrounded by the uterine tissue and maternal blood. The chorionic villi and uterine tissue become interdigitated with each other and jointly form a structural and functional unit between developing embryo (foetus) and maternal body called placenta. The placenta facilitate the supply of oxygen and nutrients to the embryo and also removal of carbon dioxide and excretory/waste materials produced by the embryo. The placenta is connected to the embryo through an umbilical cord which helps in the transport of substances to and from the embryo. Placenta also acts as an endocrine tissue and produces several hormones like human chorionic gonadotropin (hCG), human placental lactogen (hPL), estrogens, progestogens, etc.
The signals for parturition originate from:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → The fully developed foetus and the placenta
Parturition is the delivery of the baby at the end of pregnancy. It is controlled by a neuroendocrine mechanism. The first signal comes from the fully developed foetus and the placenta. It causes mild uterine contractions, called the foetal ejection reflex.
Option (1) The maternal pituitary and the ovary — Incorrect. The maternal pituitary releases oxytocin only after the foetal ejection reflex has started, so it responds to the signal and does not start it. The ovary secretes relaxin late in pregnancy, but it is not the source of the signal for birth.
Option (2) The fully developed foetus and the placenta — Correct. When the foetus is fully grown, the foetus and the placenta send the signals that start mild uterine contractions. This is the first step of parturition.
Option (3) The corpus luteum and the endometrium — Incorrect. The corpus luteum makes progesterone, which keeps the endometrium ready and so maintains pregnancy. The endometrium is the lining in which the embryo is implanted. Neither of them gives the signal to end pregnancy.
Option (4) The hypothalamus and the mammary glands — Incorrect. The mammary glands start making milk towards the end of pregnancy, for feeding the baby after birth (lactation). They do not trigger birth, and the book does not name the hypothalamus in this mechanism.
NCERT Extract:
Parturition and Lactation – “… Parturition is induced by a complex neuroendocrine mechanism. The signals for parturition originate from the fully developed foetus and the placenta which induce mild uterine contractions called foetal ejection reflex. This triggers release of oxytocin from the maternal pituitary. Oxytocin acts on the uterine muscle and causes stronger uterine contractions, which in turn stimulates further secretion of oxytocin.”