LH, in the first half of the cycle, acts by increasing expression of precursors for estrogen. In later stages, it directs the final proliferation of follicular cells, release of ovum and conversion of the ruptured follicle to Corpus Lateum. The early follicle releases mainly Estrogen, while the CL releases mainly Progesterone. Progesterone has negative feedback action on Anterior pituitary and Hypothalamus. Estrogen initially (at lowish concentrations) has a similar negative action, but when it reaches high circulating concentrations that are sustained for 36 hours or more, the feedback switches to positive, and this results in the midcycle LH (and FSH) surge