The third major component of the female reproductive system is the ovary. During pregnancy or incubation (poultry), numerous oocytes are laid down in the cortex of the ovary. The number of available oocytes for the lifetime of a female is determined before birth or hatching (birds). During the pre- and postpubertal period, many oocytes begin to mature but become atretic due to a lack of a supportive hormonal environment. Only a very small percentage of oocytes with the surrounding follicular layers ever reach full maturity and ovulate. After ovulation in mammals, the granulosa and theca layers undergo morphological and biochemical differentiation and become a corpus luteum (CL). The CL secretes copious amounts of progesterone and is dependent on LH in most species. Eventually, prostaglandin F2a (PGF2o!), produced by the uterus, causes structural and functional regression of the CL.

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