07: Reproductive Cyclicity

Estrous cycle

All the events between subsequent estrus periods

Anestrous

Period of no cyclicity

Significance of the female having a reproductive cycle

Allows the female multiple chances to get pregnant

Two jobs of the female reproductive tract

Sperm acquisition and gestation

Primary, defining behavioral characteristic of estrus

Sexual receptivity and copulation

What happens to female cyclicity if she gets pregnant

She enters a period of anestrus that lasts for the entire length of gestation, uterine involution and part of lactation

Types of estrous cycle categorizations by frequency

• Polyestrous

• Seasonally polyestrous

• Monoestrous

Types of animals that are polyestrous

Cattle, swine, rodents

Types of animals that are seasonally polyestrous

• Short day: sheep, goats

• Long day: horses

Types of animals that are monoestrous

Canids and foxes

Major phases of the estrous cycle

Follicular and luteal phases

What defines the different phases of the estrous cycle

The predominant ovarian structure

Predominant ovarian structure during the follicular phase

Growing follicles

When is the female in the follicular phase

From CL regression to ovulation

Predominant ovarian structure during the luteal phase

CL

When is the female in the luteal phase

Ovulation to CL regression

Predominant hormone and source during the follicular phase

E₂ from the growing follicles

Predominant hormone and source during the luteal phase

P₄ from the CL

Subphases of the follicular phase

Proestrus and estrus

Subphases of the luteal phase

Metestrus and diestrus

Key events of proestrus

• Luteolysis (CL regression)

• Decreasing P₄

• Transition to E₂ dominance

• Follicles maturing to become ovulatory follicles

Hormones that drive proestrus

FSH and LH

Key events of estrus

• E₂ dominance

• Sexual receptivity

• Repro tract changes

Posture that indicates a female is in standing estrus

Lordosis

Key events of metestrus

• Reorganization of follicular tissue into the CL

• Low E₂ and P₄ initially

• Rising P₄ as the CL develops

Key events of diestrus

• Fully functional CL

• High and rising P₄

Primary function of P₄ in the luteal phase

Induces changes in the uterus to prep for housing an embryo

Variations in the subphases of the canine estrous cycle

• Proestrus

• Estrus (long)

• Diestrus

• Anestrus (LONG)

Since canines don’t have a metestrus phase, when does the CL develop

During the really long estrus phase (9 days!)

Variations in the subphases of the feline estrous cycle

• Proestrus

• Estrus

• No breeding/ov → postestrus

• Breeding/ov → diestrus and anestrus

Why is the estrous cycle of the queen so weird

Induced ovulator

What induces the pre-ovulatory LH surge in queens

Neuroendocrine reflex starting at the cervix

Hormonal changes that drive anestrus in all species

Low GnRH → inactive ovaries

Things that induce anestrus

• Pregnancy

• Lactation

• Presence of offspring

• Season

• Stress

• Disease

• Nutrition

How does lactation induce anestrus

Threshold number of sucklings a day prevents cycling

How does gestation induce anestrus

P₄ from the maintained CL (or placenta) → negative feedback on GnRH release → no FSH or LH

Factors that control cyclicity in seasonal polyestrous species

Photoperiod and temperature

Evolutionary factor that determines if a seasonal polyestrous species is a short or long day breeder

Length of gestation: to maximize survival, the goal is to have offspring when there are lots of resources (spring and summer)

How does photoperiod induce/prevent cyclicity in seasonal polyestrous species

Light → retinal neurons → hypothalamus → decreased norepinephrine → decreased melatonin → increased RFRP-3 release from neurons → increased/decreased kisspeptin-10 secretion (long/short day breeder dependent)

How does a female return to cyclicity after anestrus

The hypothalamus has to release enough GnRH (like puberty!)

Species that do not experience lactational anestrus

• Mares

• Alpacas

• Queens (not uniform)

How are the phases in the menstrual cycle different from the estrous cycle

In the menstrual cycle, the follicular and luteal phases are split 50-50

Events in the menstrual cycle follicular phase that are different from the estrous cycle

• Menses: sloughing of endometrium

• Proliferative phase: endometrial thickening in response to increasing E₂

Events in the menstrual cycle luteal phase that are different from the estrous cycle

• The CL secretes both E₂ and P₄

• Endometrium proliferates to maximum thickness to prepare for secretory activity

Menopause

Lack of cyclicity due to depletion of follicles

Amenorrhea

Lack of menstrual cyclicity in females of reproductive age