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=== Cyclic Changes in the Ovaries ===
 
=== Cyclic Changes in the Ovaries ===
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==== Anoestrus ====
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* In anoestrus, the ovaries are oval and slightly flattened.
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* In a medium sized breed, they measure approximately:
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** 1.4cm pole-pole
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** 0.8 cm from the attached end to the free border
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* No follicles can be seen.
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* On section, tiny remnants of the corpora lutea of previous cycles are visible as yellow/brown spots.
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==== Pro-oestrus ====
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* Developing follicles are about 0.5cm in diameter.
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* They enlarge until ovulation, where their size is 0.6-1cm.
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=== Oestrus ===
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* By the time of ovulation, the ovary is enlarged.
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* The size of the ovary depends on the number of ripe follicles present.
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* The shape of the ovary becomes irregular due to the projection of follicles from its surface.
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* The follicle wall is thick, so it is hard to distinguish between follicles and corpora lutea.
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* Prior to ovulation, the surface of follicles shows a slightly raised papule.  This is about the size of a pin-head. Also, the epithelium covering this papule is brown, whereas the rest of the follicle is flesh-coloured.
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* A unique feature of the bitch follicle is the thickness of its wall.
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** Due to hypertrophy and folding of granulosa cells which is visible to the naked eye.
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** This is evidence of pre-ovulatory luteinization.
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* Spontaneous ovulation usually occurs 1-2 days after oestrus onset.
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* Most follicles rupture over a period of 48 hours.
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* The oocyte can be fertilized for up to 108 hours after ovulation.
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==== Dioestrus ====
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==== Appearance of the Ovaries on Ultrasound ====
 
==== Appearance of the Ovaries on Ultrasound ====
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