Oestrous Cycle - Dog
- Monoestrous, with no seasonal influence.
- A single very long cycle, follwed by a long period of anoestrus.
- Cycle about 3 times every 2 years.
- The luteal phase is about the same length as gestation (~60 days), so pregnancy does not affect the length of the oestrous cycle. Thus, no luteolytic mechanism is required.
- Characterised by vulval oedema, swelling and sanguinous discharge.
- May not always be visible because some bitches will continually lick the perineum.
- Bitch is attractive to males, but will not accept them.
- Duration is ~9 days
- Accepts the male
- Vulva becomes less oedematous
- Vulval discharge lessens and becomes more clear.
- Duration is ~9 days
- Ovulation usually occurs 1-2 days after the onset of oestrus.
- Some follicles continue to ovulate 14 days later.
- Starts when the bitch will no longer accept the dog.
- There is dispute over the duration of this phase, with two main theories:
- Ends when corpora lutea have regressed at 70-80 days.
- Ends when endometrial repair is complete at 130-140 days.
- Transitions from metoestrus to anoestrus with no external signs.
- This is also true after parturition.
- Lasts about 3 months before return to pro-oestrus.
Cyclic Changes in the Uterus
- The endometrium shows considerable change during the oestrous cycle.
- In pro-oestrus and oestrus, endometrial glands are loosely coiled with obvious lumina and a deep epithelial lining.
- During metoestrus, the glands become larger. Lumina become smaller and the basal layer of the endometrium becomes more tortuous.
- As the bitch reaches anoestrus, there is a reduction in the number of glands and the amount that they coil.
- In metoestrus (~98 days after the onset of oestrus), there is desquamation of the endometrial epithelium.
- By day 120-130, the endometrium is restored by proliferation of cells from crypts of the endometrial glands.
Cyclic Changes in the Ovaries
- In anoestrus, the ovaries are oval and slightly flattened.
- In a medium sized breed, they measure approximately:
- 1.4cm pole-pole
- 0.8 cm from the attached end to the free border
- No follicles can be seen.
- On section, tiny remnants of the corpora lutea of previous cycles are visible as yellow/brown spots.
- Developing follicles are about 0.5cm in diameter.
- They enlarge until ovulation, where their size is 0.6-1cm.
- By the time of ovulation, the ovary is enlarged.
- The size of the ovary depends on the number of ripe follicles present.
- The shape of the ovary becomes irregular due to the projection of follicles from its surface.
- The follicle wall is thick, so it is hard to distinguish between follicles and corpora lutea.
- 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.
- A unique feature of the bitch follicle is the thickness of its wall.
- Due to hypertrophy and folding of granulosa cells which is visible to the naked eye.
- This is evidence of pre-ovulatory luteinization.
- Spontaneous ovulation usually occurs 1-2 days after oestrus onset.
- Most follicles rupture over a period of 48 hours.
- The oocyte can be fertilized for up to 108 hours after ovulation.
- At first, the corpus luteum contains a central cavity.
- By day 10 after ovulation, the cavity becomes filled by compact luteinised cells.
- At this time, the corpus luteum reaches its full size, measuring 0.6-1.0 cm.
- Corpora lutea comprise the greater mass of the ovary.
- Generally, equal amounts of corpora lutea are found on each ovary.
- The number of foetuses in the uterine horns frequently differ from the number of corpora lutea on the ovaries of their respective sides.
- Embryonic migration to the uterine horn of the opposite side is common.
- On section:
- Corpus luteum is yellow-pink
- It remains unchanged in the non-pregnant bitch until about day 30 after ovulation.
- It then slowly atrophies
- During pregnancy, corpora lutea persist at their maximum size thoughout the pregnancy, but regress rapidly after parturition.
Appearance of the Ovaries on Ultrasound
- Developing antral follicles can be seen at the beginning of pro-oestrus.
- Appear circular and anechoic
- In oestrus, antral follicles are increased in size, reaching a maximum diameter of 4-13mm on day 13 (day 0 is the onset of pro-oestrus).
- From day 10 onwards, walls of the follicles become thickened due to pre-ovulatory luteinisation.
- No evidence of follicular collapse associated with ovulation.
- 25-30 days after the onset of pro-oestrus, the ovaries are difficult to identify.
Endocrine Changes during the Oestrous Cycle
- The luteal phase is prolonged compared to other species, this is shown by persisting levels of progesterone in the bloodstream.
- Progesterone levels begin to rise 60-70 hours before ovulation has occured. This is evidence of pre-ovulatory luteinisation of mature follicles.
- The pre-ovulatory rise in progesterone may signal the bitch to accept the male.
- Oestrogens rise rapidly before the onset of standing oestrus.
- The LH surge rapidly follows and lasts longer than in other species.
- Ovulation occurs 24-96 hours after the LH surge.
- FSH levels peak at the same time as the LH surge.
- As progesterone levels fall towards the end of metoestrus or pregnancy, prolactin increases.
- Prolactin is a major luteotrophic hormone in the bitch.
- Almost all bitches show some evidence of pseudopregnancy during metoestrus.
- Covert pseudopregnancy: where the bitch is in metoestrus, but shows little or no signs.
- Overt pseudopregnancy: where the bitch shows signs ranging from slight mammary development and lactogenesis to mock-parturition with nesting, loss of appetite, straining, emotional attachment to inanimate objects and heavy lactation.
- There is no difference in progesterone concentrations in bitches with or without pseudopregnancy.
- Prolactin is probably responsible for initiating the changes.