Oestrous Cycle - Cattle

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 falls by ~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.
• For more general information on regression of the corpus luteum, click here

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.