With reference to normal human beings, consider the following statements: I. In males, testosterone is secreted into the blood by Leydig cells. II. In females, the cells of the corpus luteum secrete estrogen into the blood. III. The fertilised ovum gets implanted on the wall of uterus at the blastocyst stage. Which of these statements are correct?

Statement I is correct as Leydig cells, located in the interstitium of the testes, synthesize and secrete testosterone into the blood under the influence of luteinizing hormone [t2][t5][t8]. Statement II is correct because the corpus luteum, formed from the ruptured follicle after ovulation, secretes both progesterone and estrogen (estradiol) into the bloodstream to prepare the endometrium for potential pregnancy [t3][t4][t6][t7]. Statement III is correct as the fertilized ovum undergoes cleavage to become a blastocyst, which is the specific developmental stage that implants into the uterine wall approximately 7 days after fertilization [t1][t3]. Since all three statements accurately describe human reproductive physiology, the correct option is 4.

Sources

1. [1] https://www.ncbi.nlm.nih.gov/books/NBK539704/
2. [2] https://pmc.ncbi.nlm.nih.gov/articles/PMC6044347/
3. [3] https://www.sciencedirect.com/topics/biochemistry-genetics-and-molecular-biology/ovary-cycle
4. [4] https://www.ncbi.nlm.nih.gov/books/NBK556007/
5. [5] https://www.sciencedirect.com/topics/medicine-and-dentistry/leydig-cell