Oestrous Cycle - Anatomy & Physiology

()Map REPRODUCTIVE SYSTEM (Map) FEMALE REPRODUCTIVE TRACT

Introduction

Ovarian cycles are from oestrus to oestrus. Oestrus is the easily identifiable external marker. It should be noted that Oestrus is the noun and oestrous is the adjective.

Classification of Oestrous Cycles

Based on Vaginal Cytology

Pro-Oestrus

• The phase immediately preceding oestrus.
• Lasts 2-5 days, depending on the species.
• Marked increase in reproductive activity.
• Endocrine transition from progesterone to oestrogen dominance under the influence of gonadotrophins LH and FSH.
• 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
• During early metoestrus, both oestrogen and progesterone levels are relatively low.
• Granulosa cells of the ovulated follicle give rise to lutein cells, which are responsible for the formation of the corpus luteum (luteinization).
• Progesterone secretion is detectable soon after ovulation, with significant quantities produced in 2-5 days. This marks a transition from oestrogen to progesterone dominance.
• Reduction in secretion from uterine, cervical and vaginal glands.

Dioestrus

• Longest phase of the oestrous cycle, which usually laste 10-14 days in polyoestrous females.
• The corpus luteum is present on the ovary and secreting large amounts of progesterone. The duration of this phase is directly related to the time that the corpus luteum remains functional.
• High progesterone levels prompt the uterus to prepare a suitable environment for development of the embryo, and eventual attachment of the conceptus to the endometrium (implantation).
• Uterine glands undergo hyperplasia and hypertrophy.
• Cervix becomes constricted
• Secretions of the tract are scant and sticky.
• Vaginal mucosa becomes pale.
• Ends with the regression of the corpus luteum (luteolysis).

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.

Based on Vaginal Cytology

Proliferative Phase

• Oestrogen dominant
• Includes proliferation of the endometrium.

Secretory Phase

• Progesterone dominant
• Includes secretions from the uterus, such as 'uterine milk' to sustain the fertilised ovum until implantaion.

Comparative Oestrous Cycles

Species	Mare	Cow	Sow	Ewe	Bitch	Queen
Length of Oestrous Cycle (days)	21	21	21	17	93	Variable
Duration of Oestrus	4-8 days	12-36 hours	48-96 hours	24-48 hours	7-13 days	4-10 days
Duration of Follicular Phase (days)	7 (variable)	4	7	3	18	Variable
Duration of Luteal Phase (days)	14	17	14	14	75	40

Mare

Oestrous Cycle

• Seasonal breeder (long day)
• Oestrous cyclicity from spring-autumn. During the winter, the mare is normally anoestrus.
• Winter anoestrus is follwed by a transition to regular cyclic activity. During this time, the duration of oestrus may be irregular or very long.
• Before the first ovulation, behavioural oestrus may not be accompanied by the presence of palpable follicles on the ovary.
• Some long oestrus periods in spring are anovulatory.

• Average cycle length is 20-23 days. The cycles are longest in spring and shortest from June-September.
• Oestrus lasts 6 days.
• Dioestrus lasts 15 days.
• Ovulation occurs on the penultimate day or last day of oestrus.
• The diameter of a ripe follicle ready to ovulate is 3-7cm. The day before ovulation, the tension in the follicle is reduced. Palpation of a large, fluctuating follicle is a reliable sign of impending ovulation.

• The onset of oestrus after foaling is known as the 'foal heat'.
• Occurs on day 5-10 post-parturition.
• Sometimes shorter than normal, lasting 2-4 days.
• The first two post-parturient cycles are a few days longer than normal.

• During Oestrus, a single egg is usually released. Thus, the mare is monotocious.
• Ovulation seems to occur more frequently from the left ovary.
• All ovulations occur from the ovulation fossa.
• Due to the reversed structure of the ovary, corpora lutea may only be seen sometimes at the ovarian hilus. However, because the ovary is curved and mostly covered by fimbrae of the oviduct, corpora lutea cannot be identified by rectal palpation.
• Only fertilized eggs can pass into the uterus.
• Non-fertilized eggs remain in the Utero-Tubal junction for months, where they finally disintegrate.

Changes on the Ovary

• Just before the onset of oestrus, several follicles enlarge to 1-3cm.
• By the first day of oestrus, one dominant follicle is significantly larger than the others, with a diameter of 2.5-3.5 cm.
• During oestrus, this dominant follicle matures and ruptures once it has attained a diameter of 3-7 cm.
• Several hours before ovulation, the ripe follicle becomes much less tense and can be recognised as an indentation on the surface of the ovary.
• There is usually haemorrhage into the follicle and the coagulum hardens within the next 24 hours.
• After ovulation, the other follicles regress.
• During the first 4-9 days of Dioestrus, there are no follicles over 1cm present on the ovary.
• For 3 days after ovulation, the leutinising mass can be felt, but later it normally has the same texture as the ovary.
• The corpus luteum reaches full size at 4-5 days after ovulation but does not protrude from the ovarian surface.
• On section of the ovary, the corpus luteum will appear brown and later yellow. It is triangular or conical in shape, with the narrow end on the ovulaiton fossa. The centre of the corpus luteum normally contains dark brown fibrin.
• The corpus luteum of the cycle (non-pregnant) starts to regress on about the 12th day of the cycle. At the time of regression, there is a fall in blood progesterone concentration.

• During winter Anoestrus, both ovaries are usually small and bean-shaped. They normally measure:
  • 6cm pole-pole
  • 4cm from the hilus to the free border
  • 3 cm side-side
• During the cycle, the size of the ovary depends on the number and size of the follicles.
• During oestrus, the ovary may contain 2-3 follicles. These can each measure 4-7 cm. These, combined with other less developed follicles, give the ovary a huge size.
• During Dioestrus, there is an active corpus luteum and atretic follicles present. However, these only give the ovary a size a little larger than in Anoestrus.

Endocrine Changes During the Oestrous Cycle

• Biphasic secretion of FSH with surges every 10-12 days.
  • One surge after ovulation and a second in mid-late Dioestrus (~10 days before the next ovulation).
• This increase in FSH secretion is unique to the mare.
  • Primes the development of a new generation of follicles, one of which will ovulate during the next oestrus.
• The pattern of LH secretion is also unusual.
  • No dramatic LH surge prior to ovulation.
  • LH gradually increases and elevated levels then persist for 5-6 days either side of ovulation.
• Oestrogens reach peak values during oestrus.
• Progesterone concentrations follow the changes of the corpus luteum closely.

Links

Pharmacological Manipulation of Oestrous Cycles