It releases the hormone melatonin which acts on the ovaries to inhibit the estrus cycle.
An added benefit to spaying and neutering is cat owners will not have to withstand the pining of toms when neighborhood queens are in heat or the erratic nocturnal behavior of a queen in estrus.
When the second stage of the cycle approaches, aka estrus, the edges of the epithelial cells become rough or cornified, and there are fewer red blood cells.
Estrus - This is the second stage of the heat cycle in which the female becomes receptive to breeding.