What is the structure into which the follicle is transformed after ovulation? What is the importance of that structure in the menstrual cycle?

The follicle that released the ovum suffers the action of LH and is transformed into the corpus luteum. The corpus luteum is very significant because it secretes estrogen and progesterone.

These hormones prepare the uterine mucosa, also called as endometrium, for nidation (implantation of the zygote in the uterine wall) and embryonic development as they stimulate the thickening of the mucous tissue, enhance its vascularity and make the appearing of uterine glycogen-producing glands.