Ch.7 Female Reproductive cyclicity basics

cycles of reproductive cyclicity

estrous cycle and menstrual cycle

estrous cycles

consist of a series of predictable reproductive events beginning at estrus (heat) and ending at the subsequent estrus that continue thorughout the adult female’s life and is only interrupted by pregnancy, nursing, and season of the year in some species.

anestrus

a period when cyclicity stops

purpose of estrous cycles

provide females with repeated opportunities to copulate and become pregnant 

3 types of estrous cyclicity

• polyestrus

• seasonally polyestrus

• monoestrus

polyestrus

characterized as having a uniform distribution of estrous cycles throughout the entire year 

seasonally polyestrus

display “clusters” of estrous cycles that occur only during a certain season of the year:

• long-day breeders 

• short-day breeders 

monoestrus

having only one cycle per year

two major phases of the estrous cycle

follicular phase and luteal phase

follicular phase 

the period from the regression of the corpus luteum to ovulation (~20% of estrous cycle).

primary ovarian structure and hormone during follicular phase

large antral follicles and estradiol (secreted by follicles)

luteal phase

the period from ovulation until the corpous lutem regresses (~80% of estrous cycle)

primary ovarian structure and hormone during luteal phase

corpus luteum and progesterone (secreted by corpora lutea).

four stages of the estrous cycle

proestrus, estrus, metestrus, and diestrus

these are subdivisions of the follicular and luteal phases of the cycle

stages present in the follicular phase

proestrus and estrus

stages present in the luteal phase 

metestrus and diestrus 

proestrus (period, duration, hormones involved)

• begins when progesterone declines as a result of luteolysis (from the release of prostaglandin F2alpha) and ends at the onset of estrus

• ~2-5 days 

• characterized by the major transition from progesterone dominance to estradiol dominance 

• period when antral follicles mature for ovulation 

estrus (period, duration, hormones involved)

• characterized by visible behavioral symptoms such as sexual receptivity and mating

• peak estradiol secretion

metestrus (period, duration, hormones involved)

• defined as the period between ovulation and the formation of a functional corpora lutea

• the transition period from an estradiol dominance to progesterone dominance

• corpora lutea takes around 2-5 before being fully formed

diestrus (period, duration, hormones involved)

• defined as the period of maximum corpus luteal function and maximum progesterone secretion

• the longest stage of the estrous cycle (~10-14 days)

• ends with luteolysis

anestrus

• when the female doesn’t exhibit estrous cycles due to insufficient GnRH release

• ovaries are relatively inactive and neither ovulatory follicles nor functional corpora lutea are present 

superfecundation

occurs in canids when multiple ovulations produce multiple oocytes during a single estrus period that are fertilized by spermatozoa from different males 

metestrus in bitches

no defined metestrus

long-day breeders

seasonal breeders that begin to cycle as the day length increase, like in spring

short-day breeders

seasonal breeders that begin to cycle during the shorter days of fall

two primary factors that infleunce the onset of the breeding season

photoperiod and temperature

menstrual cycle

defined as the events that occur between the onset of two successive menstrual periods (~28 days)

menses (Menstruation)

the sloughing of the endometrium to the exterior

ways the menstrual cycle differs from the estrous cycle:

• no defined period of sexual receptivity

• a period of endometrial sloughing (menses/menstruation)

• the timeline for description of the cycle begins with menses, not ovulation or estrus