Oestrous Cycle - Anatomy & Physiology

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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 can be done based on vaginal cytology or secretory patterns of the ovary.

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 follicles give rise to lutein cells, which are responsible for the formation of the corpus luteum (luteinisation).
  • 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

Oestrus Cycles in:

Mare
Bitch
Queen
Cow
Ewe
Nanny Goat
Sow

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