Estrous Cycle

Polyestrus

Has many standing heats throughout the year

Seasonal Polyestrus

Period of anestrus induced by the length of day

Monoestrus

One cycle in a year

Anestrus is a result of

Pregnancy, poor nutrition, negative energy balance, lactation, and season

Cow length of estrous

21 days polyestrus

Ewe length of estrous

17 days seasonal polyestrus

Sow length of estrous

21 days polyestrous

Bitch length of estrous

6 months monoestrus

Queen length of estrous

17 days polyestrus

Developing follicles main hormone

Estrogen

Corpus luteum main hormone

Progesterone

Estrogen brings on

Standing heat

Progesterone maintains

Pregnancy

Reasons why a female may not cycle

Prepubertal, pregnant, seasonal, congenital

Progesterone lowers when

The CL dies

LH causes

Ovulation of dominant follicle

When are FSH and LH the highest?

During the follicular phase

Proestrus follicle

Small follicle grows into medium follicle

Uterus during proestrus

High mucus secretion, smooth muscle motility, and growth of uterine gland

How does proestrus begin?

Decline in progesterone due to luteolysis at the end of diestrus

Estrus follicle

Medium follicle turns into dominant follicle and ruptures during ovulation

Ovulation occurs

1-2 days after LH surge during estrus

Uterus during estrus

Uterine motility is increased towards uterus for optimal sperm transport

Estrus is dominated by estrogen which induces

Behavioral and physiological changes (Receptive to male)

Metestrus follicle

Ruptured follicle (CH) turns into CL

CH→CL process

Luteinization which occurs during metestrus

Uterus during metestrus

High uterine secretions and low muscle contractions

Hormones during metestrus

Estrogen and progesterone are low in the early part but progesterone increases later on days after ovulation

Diestrus follicle

Corpus luteum turns into corpus albicans when there is no pregnancy

Uterus during diestrus

Produces PGF2A, which kills the CL if there is no pregnancy

What stage is the CL fully functional?

Diestrus

Dominant hormone during diestrus

Progesterone

How does diestrus end?

When the CL is destroyed

Proestrus lasts about

2-4 days

Estrus in cows

12-18 hrs

Estrus in ewes

24-36 hrs

Estrus in sows

48-72 hours

Estrus in mares

4-8 days

Metestrus length

2-4 days

Diestrus length

10-14 days

Apparent anestrus

Failure to observe estrus or failure to recognize pregnancy that is erroneously interpreted as true standing heat

True anestrus

A condition where a female does not cycle due to insufficient hormonal stimuli

Gestational anestrus is

Physiological, lack of cyclicity - Major clue female is pregnant

Elevated P4 from the CL and placenta during gestational anestrus sends

Negative feedback to hypothalamus which reduces GnRH release from surge center and reduces gonadotropin release from anterior pituitary

Negative feedback sent by progesterone during gestational anestrus results in

No follicle maturation, standing heat, or surge of LH

Prior to parturition, what happens to hormones?

Progesterone drops rapidly and estrogen increases

After parturition:

HPG axis needs to be repaired before she cycles again so progesterone drops

What hormones are absent during gestation?

GnRH, FSH, LH

Lactational Anestrus is

Physiological, many mammalian females experience anestrus while nursing their young

Which mammals cycle soon after birth

Mares and alpacas

Cyclicity being delayed in cattle is influenced by

Degree of suckling

Suckling two times or less per day causes cyclicity to

Return in cattle

When the number of suckling sessions is between 3 to 20 per day:

Amplitude and pulse frequency of blood LH is below the threshold and the cow remains in anestrus

When the number of suckling sessions is limited to 2 or less per day:

Amplitude and pulse frequency of blood LH increase dramatically and cow will begin to cycle

Lactational Anestrus is not due to

Sensory stimulation of teat

Nutritional anestrus

Negative energy balance due to low intake of energy or protein

2DG is an inhibitor of glucose =

Dysregulates metabolic signals

What affects the hypothalamic control of GnRH secretion

Glucose

Lack of GnRH pulses causes

Inadequate secretion of gonadotropins which cause inactive ovaries

Nutritional anestrus is common in

Primiparous females because they are often young and still growing

Adequate nutrition =

Estrous cycles resume

Photoperiod

Period of time during the day where there is daylight

Long day breeder

16 hours of daylight, 8 hours of dark for at least 60 days

Example: Horse

Short day breeder

8 hours of daylight, 16 hours of dark for at least 60 days

Example: Sheep, goats, and deer

Effects of environment on the female

Photoperiod, temperature, and moisture

Moisture controls nutrients, therefore

Seasonal changes in feed availability, which effects the female

Silent ovulation occurs in between

First LH surge after anestrus and first estrus

Why does silent ovulation occur?

Hypothalamus must be primed with progesterone to express estrus in response to estrogen

Long days → Short night →

Low melatonin

Short days → Long night →

High melatonin

The menses occurs due to

Growth of the endometrium that shreds

During the menses, all hormones remain

Low

Menstrual vs Estrous follicular phase

Menstrual: 50% of cycle

Estrous: 20% of cycle

Menstrual vs Estrous ovulation

Menstrual: Middle of cycle

Estrous: Beginning and end of cycle

Luteolysis in menstrual cycle

Ovarian PGF2A

Luteolysis in estrous cycle

Uterine PGF2A

Menopause is well characterized in which cycle

Menstrual

Sexual receptivity is well defined in which cycle

Estrous