Oestrous Cycle

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.

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.

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.

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.

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.

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.

Oestrous Cycle - Dog