Oestrous Cycle - Anatomy & Physiology
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Introduction
Ovarian cycles are from oestrus to oestrus. Oestrus is the easily identifiable external marker.
Classification of Oestrous Cycles
Based on Vaginal Cytology
Pro-Oestrus
- The phase immediately preceding oestrus.
- Marked increase in reproductive activity.
- Follicular growth and regression of the corpus luteum of the previous cycle (in polyoestrous species).
- Uterus enlarges slightly
- Endometrium becomes congested and oedematous, glands show secretory activity.
- Vaginal mucous becomes hyperaemic.
- Increase in cell numbers in the vaginal epithelium.
- Superficial layers become cornified.
- The bitch shows external evidence:
- Vulval oedema
- Hyperaemia
- Sanguinous vulval discharge
Oestrus
- The period where the female will accept the male for copulation.
- The onset and end of oestrus are the only stages of the oetrous cycle that can be accurately measured. They are therefore used to determine cycle length.
- Female seeks out the male and 'stands' to be mounted.
- Uterine, cervical and vaginal glands secrete increased amounts of mucus.
- Vaginal epithelium and uterine endometrium becomes hyperaemic and congested.
- Cervix is relaxed
- Ovulation occurs during this phase in all domestic species except the cow.
- The cow ovulates ~12 hours after the end of oestrus.
- Ovulation is spontaneous in all domestic species except the cat, rabbit and camelids. In these species it is induced by the act of copulation.
- During pro-oestrous and oestrus there is follicular growth with no functional corpora lutea present on the ovary (aka follicular phase).
- The main ovarian hormone production is Oestrogen.
Metoestrus
- The phase after oestrus
- Granulosa cells of the ovulated follicle give rise to lutein cells, which are responsible for the formation of the corpus luteum.
- Reduction in secretion from uterine, cervical and vaginal glands.
Dioestrus
- The corpus luteum is present on the ovary and secreting large amounts of progesterone.
- Uterine glands undergo hyperplasia and hypertrophy.
- Cervix becomes constricted
- Secretions of the tract are scant and sticky.
- Vaginal mucosa becomes pale.
Anoestrus
- Genital system is quiescent
- Follicular development is minimal and corpora lutea have regressed and are non-functional.
- Secretions are scanty
- Cervix is constricted
- Vaginal Mucosa is pale
Based on Secretory Patterns of the Ovary
Follicular Phase
- Begins after luteolysis, which causes the decline in progesterone.
- Gonadotrophins LH and FSH are produced, which cause ovarian follicles to produce Oestrogen.
- Ends at ovulation.
- Oestrus is designated as day 0.
Luteal Phase
- Begins after ovulation
- Includes the development of corpora luta, which produce progesterone.
- Includes luteolysis, which is brought about by Prostaglandin F2α.