Source of PMSG Secretion

PMSG is secreted by:

Correct Answer: Both chorionic girdle and endometrial cups

PMSG (eCG) is secreted by both the chorionic girdle (the initial trophoblast invasion structure) and the endometrial cups (the established secretory structures formed after chorionic girdle invasion). These are sequential stages of the same foetal trophoblast-derived structure in mares.

Sequence of Events

• Day 25: Chorionic girdle begins to form from the trophoblast
• Day 36–38: Chorionic girdle cells invade the endometrium and form endometrial cups
• Day 40: PMSG secretion begins
• Day 60–70: Peak PMSG secretion
• Day 120–150: Endometrial cups degenerate and PMSG disappears

Note

The fact that both structures are secretory sources is why the correct answer is 'Both'. In practice, endometrial cups are the primary, long-term secretory structures, while the chorionic girdle initiates the process.