What is the significance of ampullary–isthmic junction in the female reproductive tract?

During reproduction, the chromosome number (2n) reduces to half (n) in the gametes and again the original number (2n) is restored in the offspring, what are the processes through which these events take place?

What is the difference between a primary oöcyte and a secondary oöcyte?

How does zona pellucida of ovum help in preventing polyspermy?

Mention the importance of LH surge during menstrual cycle.