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.

Cyclic Changes in the Ovaries

• 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.

Cow

Oestrous Cycle

• Polyoestrous
• The first ovulation in heifers is usually without behavioural oestrus. This is termed 'silent heat'.
• Cyclical activity persists except during pregnancy, for 3-6 weeks post-calving and during high milk yield.
• In heifers, the average length of the oestrous cycle is 20 days (18-22).
• In cows, the average length of the oestrous cycle is 21 days (18-24).
• The average duration of oestrus is ~ 15 hours (2-30 hours).
• The body temperature of dairy cows dalls ~0.5◦C the day before oestrus. It then increases in oestrus before falling by ~0.3◦C at ovulation.
• Vaginal pH also fluctuates throughout the oestrous cycle, but is lowest (7.32) on the day of oestrus.
• Ovulation is spontaneous, occuring ~12 hours after the end of oestrus.
• Within 2 days of service, there is yellow-white vulval discharge of mucus. This contains neutrophil leucocytes from the uterus.
• At ~48 hours post-oestrus, irrespective of service, there is a bright red sanguineous discharge from the uterine caruncles.

Cyclic Changes in the Vagina

The main variations are in the epithelial cells of the anterior vagina and in the secretory function of the cervical glands.

• Oestrus: Anterior vaginal epithelium becomes greatly thickened due to cell division and the growth of the tall, columnar, mucus-secreting superficial cells.
• Dioestrus: Cells of the cranial vaginal epithelium vary from flat to low columnar. Invasion of the vaginal mucosa by leucocytes is maximal 2-5 days after oestrus.
• Copious secretion of mucus by the cervix and anterior vagina begins 1-2 days before oestrus.
• Secretions diminish to the 4th day after oestrus.
• Mucus is transparent and flows readily.
• Variations in cervical mucus:
  • During oestrus and for a few days after, crystals are disposed in a distinct aborisation pattern.
  • For the remainder of the cycle this pattern is absent.
  • This pattern, and the amount of cervical mucus depends on oestrogen concentration.
• Postoestrous vaginal mucus:
  • Floccules of leucocytes
  • Blood is frequently present
• Hyperaemia of vaginal and cervical mucosae is progressive during pro-oestrus and oestrus.
• At this time, the protrusion of the cervix into the cranial vagina is relaxed, so that 1-2 fingers can be inserted into the cervical os.
• During metoestrus, there is a rapid reduction in vascularity.
• From day 3-5 postoestrus:
  • Mucosa of the cervix is pale and quiescent.
  • The external os is constricted.
  • Mucus becomes scanty, sticky and pale yellow/brown.
• There are also cyclical changes in the vaginal pH:
  • pH falls from 7.0 to 6.72 one day before first signs of behavioural oestrus.
  • At the start of oestrus, pH falls again to 6.54.

Cyclic Changes in the Uterus

• Oestrus:
  • Uterus is congested
  • Endometrium contains oedematous fluid, which causes its surface to glisten.
  • Muscularis is contractile, so when palpated per rectum, the uterus feels turgid and the horns feel erect and coiled.
    • Tonicity is present the day before and the day after oestrus, but is at its maximum during heat.
  • Marked increase in vascularity
• 24-48 hours postoestrus:
  • Uterine caruncles show petechial haemorrhage. This gives rise to the discharge of blood from the vagina.
    • In heifers, there is also often perimetral subserous petechiae.
• Dioestrus:
  • Endometrium is covered with scanty secretion from the uterine glands.

Cyclic Changes in the Ovaries

• Usually one follicle ovulates, but twin ovulations occur in 4-5% of cows and triplet ovulations more rarely.
• In dairy cattle 60% of ovulations are from the right ovary, but in beef cattle, they are about the same.
• Size of the ovaries is dependent on the phase of the oestrous cycle.

Follicular Growth and Development

• There is follicular growth and atresia throughout the oestrous cycle.
• There are normally two waves of follicular growth, but 3 waves also commonly occur:
  • One begins on day 3-4
  • The second begins on day 12-14
• A normal follicle of 9-13mm is present between day 5-11 before becoming atretic.
• In the second wave, an ovulatory follicle develops, measuring 9-13mm between day 15-20.
• Ovulatory follicle is selected ~3 days prior to ovulation.
• Follicular growth is under the influence of FSH.
• One follicle normally obtains dominance and subsequently ovulates.
• Dominance does not involve inhibin. It is probably due to an intra-ovarian mechanism which does not suppress FSH secretion. Other hormones such as IGF-1 (Insulin-like growth factor-1) may also be involved in follicular growth patterns.
• During dioestrus, there are several large follicles on the ovary. These range in size 0.7-1.5cm in diameter. These follicles do not alter the contours of the ovaries, but cause variation in size.
• During Pro-oestrus and oestrus, the dominant follicle enlarged and ovulation occurs once it is at least 1.9cm.
• The ripening follicle is detectable by rectal palpation during heat.
  • It can be felt as a soft, bulging area on the surface of one ovary.
• Ovulation can occur from anywhere on the ovarian surface.
• The shape of the ovary when the corpus luteum develops will depend on the site of ovulation.
• The point of ovulation is usually an avascular area of the follicular wall. Therefore, haemorrage of the follicle is not seen with ovulation in the cow. However, after ovulation there is congestion around the point of follicular rupture and sometimes there is a small blood clot in the centre of the new corpus luteum.
• When the follicle ruptures, the ovum is expelled through a small breach in the surface.
• Most of the antral fluid escapes and the follicle collapses.
• The ovary feels flattened and soft on palpation.
• The surface from which ovulation has occurred is wrinkled and may be blood-stained.

Corpus Luteum of the Oestrous Cycle

• The corpus luteum develops by hypertrophy and luteinization of the follicular granulosa cells, this occurs rapidly.
• By 48 hours after ovulation, the corpus luteum is ~1.4cm in diameter. It feels soft and yields on palpation.
• It is dull cream in colour and the luteinized cells can be seen in loose pleats.
• It attains its maximum size by day 7-8 of dioestrus.
• Luteinized pleats are now compact.
• The body is a homogenous mass that is yellow/orange in colour.
• The shape varies, most are oval but they can be square or rectangular.
• Sometimes the centre is occupied by a cavity which is occupied by yellow fluid. This is known as a cyctic corpus luteum, but is rarely pathogenic and usually completely normal.
• If a cavity is present, it must be distinguished from a luteinisation of the walls of the follicle without ovulation (abnormal).
  • If it is a corpus luteum with a cavity, there will be a pin-head depression in the centre of the projection from the surface of the ovary. This depression indicates ovulation has occured.

Projection of the Corpus Luteum from the Surface of the Ovary

• As the corpus luteum enlarges, it pushes itself out of the ovary and so stretches the ovarian surface as it does so.
• By the time it reaches maximum development it often forms a disctinct projection from the surface of the ovary.
• Usually, this is a distinct bulge about 1cm in diameter with a clear-cut constriction where it joins the general contour of the ovary.
• Sometimes, it is nipple like.
• Another type of projection is indistinct and diffuse, occupying the greater part of the ovary.
• The type of protrusion depends on the extent of the ovarian surface that was occupied by the follicle just before ovulation.

Regressing Corpus Luteum

• The corpus luteum remains its maximum size until the onset of pro-oestrus (~24 hours before heat).
• When the cow comes into heat, the corpus luteum undergoes rapid reduction in size and changes colour and appearance.
• By the middle of oestrus, its diameter is reduced to 1.5cm and its protrusion from the surface of the ovary is less distinct. Also, its colour changes to bright yellow. (It is after this discovery in the cow that the corpus luteum was given its name. Corpus luteum is latin for 'yellow body').
• Its consistency is dense and it becomes invaded with scar tissue.
• By the second day of dioestrus, it is reduced to ~1cm and its outline is becoming irregular.
• The colour then changes from yellow to brown.
• By the middle of dioestrus, it has shrunk to 0.5cm and its protrusion from the ovarian surface is a bit larger than a pin-head.
• As it ages, its colour goes from red to scarlet.
• Small remnants can persist for several months.

Size of Ovaries

• Depends on the phase of the oestrous cycle and whether it contains an active corpus luteum.
• The presence of follicles does not alter the size of the ovary to anything like the extent of the corpus luteum.
• In the majority of heifers and young cows examined between day 6-18 of dioestrus, one ovary will be larger than the other.
• The larger ovary will be approximately:
  • 3.5cm pole-pole
  • 3cm from the attached side to the free border
  • 2.8cm from side to side
  • The corpus luteum will project from some point on the surface.
• The smaller ovary will be approximately:
  • 2.5cm pole-pole
  • 1.5cm from the attached side to the free border
  • 1.2 cm from side to side
• During the 4-5 days after oestrus, the developing corpus luteum has not attained sufficient bulk to influence the size of the ovary significantly and the regressing corpus luteum has lost significant bulk.
• During oestrus there will also be little difference in size.
  • If the ovary undergoing enlargement of a dominant follicle also contains a regressing corpus luteum, it will be a little larger than the other ovary but not strikingly so.

Ovaries in a Multiparous Cow

• Larger
• Partly due to progressive deposition of scar tissue from prolonged function.
• Can be due to large numbers of small, but visible follicles.
• Often, the ovary that does not contain a corpus luteum measures:
  • 4cm pole-pole
  • 3cm from the fixed edge to the free border
  • 2cm from side to side
• In mid-dioestrus it is still possible to detect the corpus luteum because, aside from its protrusion, the ovary appears plump. The ovary that does not contain a corpus luteum is flattened from side to side.
• Corpora lutea (active and regressing) and follicles approaching maturity are the same as those in the heifer.
• In addition, however, there are old scarred corpora lutea of previous pregnancies.
  • Generally white (corpus albicans - latin for white body), pin-head sized projections on the surface of the ovary.
  • Contain mainly scar tissue
  • Irregular outline
  • Maximum of 0.5 cm
• The corpus luteum of pregnancy does not atrophy as quickly after parturition as the corpus luteum of the cycle. It becomes invaded by scar tissue and remains throughout the cow's life.
• The fully developed corpus luteum is present by day 7 and persists unchanged until the onset of pro-oestrus at day 19-20.

Appearance of the Ovaries on Ultrasound

• Normal structures that can be identified:
  • Ovarian stroma: Mottled echotexture
  • Antral follicles: Anechoic (black) structures of variable size with a clear definition between the follicular wall and the antral cavity. They will not always be a regular, spherical shape.
  • Corpora lutea: Well defined border and a mottled echogenic appearance. They are less echogenic than the ovarian stroma. The fluid-filled lacuna can be identified as a dark, non-echogenic area in the centre.
  • Ovarian blood vessels: Black,non echogenic structures which are easily confused with antral follicles. Movevement of the transducer can be used to differentiate the two, as this shows their elongated structure.
• In addition, pathological structures like ovarian cysts can be seen.

Endocrine Changes during the Oestrous Cycle

• Hormones are secreted in a pulsatile manner and fluctuate considerably.
• Just before onset of behavioural oestrus, there is a sharp rise in plasma oestrogens, particularly oestradiol.
  • Peak values occur at the beginning of oestrus
  • Levels decline to basal concentrations at ovulation
• During the rest of the cycle, there are fluctuations but there is a discrete peak around day 6 of the cycle.
  • Related to the first wave of follicular growth
• Pre-oestrus rise in oestrogens stimulates the LH surge from the anterior pituitary.
  • This is necessary for follicular maturation, ovulation and maturation of the corpus luteum.
• A second, less discrete peak occurs 24 hours after the LH surge.
• Changes in progesterone concentrations mimic changes in the corpus luteum.
• Progesterone levels peak 7-8 days after ovulation and decline quickly from day 18.
• When progesterone levels fall to low basal levels, the negative feedback on the pituitary gland is removed. This allows a sudden release of gonadotrophins.

Links

Pharmacological Manipulation of Oestrous Cycles