Estrous Cycle

What does polyestrous mean

cycle occur continuously throughout the year (cattle, pigs)

What is seasonally polyestrous

cycles occur only during certain seasons

what are short-day breeders

breed when days shorten (sheep, goats)

What are long-day breeders

breed when days lengthen (horses)

what is monoestrous

one cycle followed by long anestrus (dogs)

What is anestrus

Period when cycles stop (pregnancy, stress, season, lactation)

What are the two main phases of the estrous cycle

follicular phase and luteal phase

what happens in the follicular phase

• short (20%)

• Dominated by estradiol

• Ends with ovulation

What happens in the luteal phase

• long (80%)

• dominated by progesterone

• uterus prepares for pregnancy

Which hormones control the estrous cycle

GnRH → FSH & LH → ovaries (E2 & P4)

what does GnRH do

Stimulates release of FSH and from pituitary

What does FSH do

Stimulates follicle development

What does LH do

Triggers ovulation and CL formation

What triggers the LH surge

High estradiol from dominant follicle (positive feedback)

what happens during the LH surge

• Stops E2 production

• Causes follicle rupture

• Restarts meiosis

• Leads to ovulation (24h later)

what hormone dominates the luteal phase

Progesterone P4

What does progesterone do to GnRH/LH

Decreases pulse frequency (negative feedback)

Why does luteal phase feedback matter

Prevents additional ovulations during luteal phase

What causes corpus luteum regression

Prostaglandin F2α (PGF2α) from endometrium.

when does luteolysis occur

around day 12-14 of cycle

what happens when progesterone decreases

• less negative feedback

• increased GnRH and LH pulses

• follicle maturation resumes

what is inhibin

hormones from follicles that suppresses FSH

How is estradiol produced in follicles

• Theca cells → testosterone (LH)

• Granulosa cells → convert to estradiol (FSH)

what does estradiol do to uterus

• growth of endometrium

• thin cervical mucus (helps sperm)

What does progesterone do

• Thickens mucus (blocks sperm)

• Promotes gland secretion

• Reduces uterine contractions

What are the 4 stages of the estrous cycle

proestrus, estrus, metestrus, diestrus

What happens in proestrus

• progesterone decreases

• estradiol increases

• follicles grow

what define estrus

• sexual receptivity (heat)

• High E2

• Ends with ovulation

Behavioral signs of estrus

mounting, vocalization, restlessness

what happens in metestrus

• CL forms

• Progesterone increases

• Uterus prepares for embryo

What happens in diestrus

• High progesterone

• Max uterine gland activity

• longest stage (10-14 days)

what are follicular waves

cycles of follicle growth and regression during estrous cycle

what controls follicular recruitment

FSH pulses

what is required for dominant follicle selection

frequent LH pulses

when does ovulation occur in menstrual cycle

middle (day 14)

is there endometrial shedding in estrous cycle

no

is there endometrial shedding in menstrual cycle

yes (menses)

why use progesterone in reproduction management

to synchronize estrus for breeding/AI

how does progesterone synchronization work

• P4 suppresses GnRH, FSH, LH

• Removal → rapid return to estrus (2-3 days)

What is CIDR

Progesterone-releasing intravaginal device