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* As the corpus luteum regresses between days 15-18, the colour rapidly changes to yellow, creamy yellow or buff.
 
* As the corpus luteum regresses between days 15-18, the colour rapidly changes to yellow, creamy yellow or buff.
 
* The mechanism of luteolysis is not fully understood in the sow.
 
* The mechanism of luteolysis is not fully understood in the sow.
** Corpora lutea are unresponsive to PGF2
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** Corpora lutea are unresponsive to PGF2α until 12-23 days after obulation.
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** During luteolysis, corpora lutea are invaded by macrophages.
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** Macrophages produce Tumour Necrosis Factor (TNF).
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** TNF and PGF2α probably work together to cause luteolysis.
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** TNF also inhibits oestradiol production, thus eliminating the luteotrophic source.
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* There is rapid regression of corpora lutea at the next oestrus, but throughout the next dioestrus the corpora lutea remain as distinct entities.
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* After this they rapidly regress to grey pin-head foci on the ovary.
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* During the luteal phase of the cycle, oestrogens are luteotrophic.  This may prolong the lifespan of the corpora lutea for several weeks.  The result of this is pseudopregnancy.
    
=== Endocrine Changes during the Oestrous Cycle ===
 
=== Endocrine Changes during the Oestrous Cycle ===
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