Q.Which of the following hormones is not secreted by human placenta?

The question tests your understanding of the hormonal feedback loop in the male reproductive system — specifically, the role of inhibin and its target. Many students memorise that "inhibin is from Sertoli cells" but then confuse its effect with that of GnRH or testosterone. Let's break it cleanly.

1. Assertion (A): Inhibin is a glycoprotein hormone. In males, it is secreted by the Sertoli cells of the seminiferous tubules. This is a well-established fact — Sertoli cells support spermatogenesis and also produce inhibin in response to FSH stimulation. So (A) is correct.

2. Reason (R): The statement claims inhibin enhances the secretion of FSH. This is the opposite of the truth. Inhibin acts via negative feedback on the anterior pituitary gland to suppress FSH secretion. When sperm production is adequate, inhibin levels rise and tell the pituitary to reduce FSH output. So (R) is incorrect. …

The question asks us to identify the correct combinations from the given lists, which describe various components and processes in the human reproductive system. To do this, we need to understand the specific roles of each cell type, hormone, and process mentioned, and how they interrelate. We will evaluate each row individually to determine its correctness.

1. Evaluate Combination I: Sertoli cells, Luteinising hormone, Spermiation

   • Sertoli cells are found in the seminiferous tubules of the testes. Their primary role is to provide nourishment to the developing spermatozoa and to secrete various factors like androgen-binding protein (ABP) and inhibin. They are primarily regulated by Follicle-Stimulating Hormone (FSH).
   • Luteinising hormone (LH) in males acts on the Leydig cells (interstitial cells) to stimulate the synthesis and secretion of androgens, primarily testosterone. It does not directly regulate Sertoli cells for spermiation.
   • Spermiation is the process by which mature spermatozoa are released from the Sertoli cells into the lumen of the seminiferous tubules.
   • Conclusion for I: While Sertoli cells are involved in sperm development and spermiation, LH does not directly regulate them for this process. FSH is the hormone that acts on Sertoli cells. Therefore, this combination is incorrect.

2. Evaluate Combination II: Leydig cells, Testosterone, Secondary sexual characters in male

   • Leydig cells (also called interstitial cells) are located in the interstitial spaces between the seminiferous tubules in the testes. They are responsible for synthesizing and secreting androgens.
   • Testosterone is the primary androgen produced by the Leydig cells, under the stimulation of LH.
   • Secondary sexual characters in male include features like the development of facial and body hair, deepening of the voice, increased muscle mass, and development of the male reproductive organs. Testosterone is the key hormone responsible for the development, maintenance, and regulation of these characters.
   • Conclusion for II: Leydig cells produce testosterone, which is responsible for male secondary sexual characteristics. This combination is correct.

3. Evaluate Combination III: Corpus luteum, Oestrogen, Spermiogenesis

   • Corpus luteum is a temporary endocrine structure in the female ovary that forms from the ruptured Graafian follicle after ovulation. Its primary function is to produce large amounts of progesterone, and also some oestrogen, to maintain the uterine lining for potential pregnancy.
   • Oestrogen is primarily produced by the developing ovarian follicles, though the corpus luteum also produces some. …

The question tests a specific fact from the male reproductive system: which hormone is produced by the Sertoli cells? Many students memorise that the testis makes testosterone, but they forget the cellular source — that distinction is the key.

Testosterone is secreted by the interstitial cells of Leydig, not by Sertoli cells. The Sertoli cells, which line the seminiferous tubules and support spermatogenesis, produce a different hormone: inhibin. Inhibin is a glycoprotein that acts on the anterior pituitary to selectively suppress FSH secretion, forming a negative feedback loop that regulates sperm production.

Let’s walk through the options one by one.

1. Testosterone — This is the primary male sex hormone. It is synthesised and secreted by the Leydig cells (interstitial cells) located in the connective tissue between the seminiferous tubules. Sertoli cells do not produce it. So (A) is incorrect.

2. Inhibin — This is the correct answer. Sertoli cells secrete inhibin in response to FSH stimulation. Inhibin then feeds back on the pituitary to reduce FSH release, thereby modulating spermatogenesis. This is a well-established fact in human reproductive physiology.

3. Progesterone — This is a female sex hormone, primarily produced by the corpus luteum in the ovary and by the placenta during pregnancy. In males, progesterone is present in trace amounts as an intermediate in steroidogenesis (e.g., in the adrenal cortex and Leydig cells), but it is not a secretory product of Sertoli cells. So (C) is wrong. …

Concept and intuition:

In a muscle cell (myofibril), the thick filaments (myosin) and thin filaments (actin) are the contractile proteins — they directly generate force by sliding past each other. But contraction must be turned on and off. That’s the job of regulatory proteins: they act like a molecular switch. Tropomyosin is a long, thread-like protein that lies along the actin filament, covering the myosin-binding sites on actin. Troponin is a smaller, calcium-sensitive complex that, when calcium binds, moves tropomyosin out of the way, allowing myosin to grab actin. Without these two, contraction would be constant or impossible. Actin itself is structural/contractile, not regulatory.

Step-by-step reasoning:

1. Identify the contractile proteins.

   Actin and myosin are the two main proteins that directly interact to produce contraction. They are often called contractile or structural proteins, not regulatory. So options (A) and (B) (which include actin) are suspect if they claim actin is regulatory.

2. Define “regulatory protein” in muscle.

   A regulatory protein does not itself generate force; instead, it controls whether contraction can occur. In striated muscle (skeletal and cardiac), the key regulators are troponin and tropomyosin. Tropomyosin blocks the myosin-binding sites on actin in the relaxed state. Troponin binds calcium and shifts tropomyosin, exposing those sites.

3. Evaluate each option.

   • (A) Actin and myosin — Both are contractile, not regulatory. Incorrect.
   • (B) Troponin and actin — Troponin is regulatory, but actin is not. Incorrect.
   • (C) Troponin and tropomyosin — Both are regulatory. Correct. …