Reproductive Female Physiology: Estrous Cycle, Ovulation, and ARTs

Gonads and Hormonal Regulation

• Gonads produce sex cells and hormones; in males, the gonad produces sperm and testosterone.

• In females, the cycle is regulated by the hypothalamic-pituitary-gonadal (HPG) axis acting like a thermostat that turns on/off hormone release to maintain homeostasis.

  • Analogy: if you set a thermostat to 75 and temperature drifts, the system kicks on to return to 75; similarly, the HPG axis adjusts release of hormones to maintain reproductive homeostasis.

  • The HPG axis includes hypothalamic GnRH, pituitary LH and FSH, and ovarian steroids (estrogen/estradiol and progesterone).

• The HPG axis also interacts with the hypothalamic-pituitary-thyroid and hypothalamic-pituitary-adrenal axes (HP thyroid, HP adrenal), which can be covered in other courses.

Puberty and Onset of Reproductive Function

• Puberty marks the point when reproductive capability begins; in many species, onset can be influenced by environmental and social cues.

• Breed, body condition, and exposure to intact males can affect puberty onset (e.g., ram/boar effects accelerating puberty in some contexts).

• External stimuli such as pheromones, presence of a male, and intrinsic factors from within the animal influence puberty timing.

Female Reproductive Anatomy and Oocytes

• The uterus is the site of fetal development; it produces estrogen and progesterone along with the oocytes housed in ovarian follicles.

• Oocyte = the egg; in mammals, oocytes are produced in follicles within the ovaries.

• Uterine anatomy varies by species:

  • Humans: simple uterine body with fallopian tubes (oviducts) and no uterine horns.

  • Cattle/Sheep/Goats: uterus with horns (bicornuate), with oviducts attached near the horns.

  • Mares (horses): cervix has longitudinal folds; other species have cervical rings.

• Cervical anatomy and sperm transport:

  • In mares, the cervix has longitudinal folds and is relatively easier for a cervix to traverse compared to species with multiple vaginal/cervical rings.

  • Boar corkscrew penis is adapted to fit the corkscrew cervix of the sow; this is a species-specific mating adaptation.

  • In cows/sheep/goats, cervical rings make passage more selective until the appropriate phase of the estrous cycle.

• The cervix changes during the estrous cycle:

  • During heat (estrus), the cervix becomes more open to allow sperm passage.

  • During pregnancy, the cervix is tight to protect the uterus from pathogens.

• Oviducts (Fallopian tubes) transport the oocyte from the ovary to the uterus for fertilization.

Estrous Cycle Overview and Terminology

• Estrous cycle vs menstrual cycle:

  • Estrous cycle: typical in farm animals; roughly 21 days in many species; estrus (heat) is the period of sexual receptivity.

  • Menstrual cycle: typical in humans; ~28 days with menses.

• Estrus vs estrous:

  • Estrus: noun referring to the heat/period of sexual receptivity.

  • Estrous (with o): adjective describing the cycle as a whole (e.g., estrous cycle).

• Estrous cycle phases:

  • Follicular phase: short phase leading up to ovulation; dominated by estradiol (E2).

  • Luteal phase: longer phase after ovulation; dominated by progesterone (P4).

• Cycle length and ovulation timing:

  • Most farm species: estrous cycle ~21 days; ovulation occurs during the follicular phase, often toward or at the end of estrus (varies by species).

  • Sheep: ~17 days cycle.

• Seasonal patterns:

  • Seasonally polyestrous animals cycle multiple times per year but only during specific seasons (e.g., mares = long-day breeders; sheep/goats = short-day breeders).

  • Anestrus: absence of estrous cycles, due to season, pregnancy, illness, or other factors.

Ovarian Folliculogenesis, Ovulation, and Corpus Luteum

• Follicular recruitment and development:

  • A cohort of primordial follicles is recruited to grow each cycle.

  • Some follicles mature; one becomes the dominant preovulatory (Graafian) follicle in mono-ovulatory species; in multi-ovulatory species, several follicles may enlarge.

  • Follicular growth creates an antrum (fluid-filled cavity) within the follicle.

• Ovulation and luteinization:

  • Ovulation is triggered by a surge in luteinizing hormone (LH).

  • After ovulation, the ruptured follicle undergoes luteinization to form the corpus luteum (CL).

  • Corpus luteum (CL) is temporary and secretes progesterone (P4), essential for maintaining pregnancy.

  • If pregnancy does not occur, luteolysis (destruction of CL) occurs, driven by prostaglandin F2α (PGF2α):

  • $PGF_{2 ext{alpha}} <br>ightarrow ext{luteolysis}$

• Corpus luteum and pregnancy maintenance:

  • Progesterone from the CL supports pregnancy; without progesterone, pregnancy cannot be sustained.

  • If pregnancy is established, maternal recognition of pregnancy signals sustain CL function and prevent luteolysis (timing around day ~15–16 in many species).

• Hormone sources and feedback during the cycle:

  • Large follicle and its granulosa/thecal cells produce estrogen (estradiol, E2).

  • After ovulation, the CL produces progesterone (P4).

  • Reciprocal feedback between ovarian steroids and the hypothalamus/pituitary regulates GnRH, LH, and FSH release.

• Estradiol and progesterone dynamics during the cycle:

  • Follicular phase: E2 rises; estrus behavior; ovulation near end of the follicular phase.

  • Luteal phase: P4 rises; uterus prepared for potential pregnancy; if no pregnancy, P4 declines after luteolysis.

• Graph-like cycle dynamics (conceptual):

  • Early cycle: $E<em>2 ext{ high}, P</em>4 ext{ low}$

  • Post-ovulation: $E<em>2 ext{ low}, P</em>4 ext{ high}$

Hormonal Regulation Across the Cycle

• GnRH, LH, FSH axis:

  • Hypothalamus releases GnRH, which stimulates the pituitary to release LH and FSH.

  • FSH stimulates follicle growth and recruitment of follicles.

  • LH triggers final maturation of the dominant follicle and ovulation; also supports CL formation (luteinization).

• Key hormones and roles:

  • FSH (follicle-stimulating hormone): follicle recruitment and growth.

  • LH (luteinizing hormone): maturation of the dominant follicle and ovulation; luteinization and CL maintenance in luteal phase.

  • Estradiol (E2): produced by growing follicles; drives estrus behaviors and prepares the reproductive tract.

  • Progesterone (P4): produced by the CL after ovulation; maintains pregnancy.

• Ovulation = LH surge; luteinization follows ovulation to form CL.

• Feedback mechanisms regulate GnRH and gonadotropins to optimize timing of estrus, ovulation, and potential pregnancy.

Estrus Detection, Induced Ovulation, and Reproductive Strategies

• Estrous detection methods and signs:

  • Cattle: estrus detection patches monitor mounting; mounting behavior indicates estrus.

  • Pigs: standing heat and behavioral changes indicate estrus.

  • Horses: teasing with a stallion; signs include clitoral exposure, tail raising, winking; dangerous in non-estrous (diestrus) states if exposed to a stallion.

• Induced ovulators and related examples:

  • Species that require stimulation to ovulate: cats, rabbits, ferrets, mink.

  • Mechanism often involves tactile stimulation triggering a neuroendocrine response; some camelids are induced but via different mechanisms.

  • Cat penis has spines that induce ovulation in the female after mating.

• Controlled internal drug release (CDER):

  • A progesterone-releasing device implanted in the cow’s vagina to regulate estrus and breeding schedules.

  • Can be combined with GnRH or other hormones to synchronize estrus.

Pregnancy, Gestation, and Reproduction Across Species

• Maternal recognition of pregnancy:

  • Around day ~15–16 in many species, the embryo signals the uterus to maintain the CL and prevent PGF2α–driven luteolysis.

  • If recognition fails, luteolysis occurs, cycles restart, and pregnancy cannot be sustained.

• Gestation lengths and seasonality:

  • Mare gestation ~11 months; seasonal breeding aligned to environmental conditions (long-day breeder, winter shutdown).

  • Sheep/goats: short-day breeders; cycles continue during certain seasons and pause otherwise.

• Parturition (birth) and fetal HP axis:

  • Initiated by the fetus when crowded or ready; triggers fetal HP axis activation and parturition.

  • Neonatal needs: all farm calves/foals/piglets require colostrum in the first 24 hours for passive immunity; colostrum is rich in maternal antibodies.

  • Piglets may require iron supplementation due to rapid growth and limited iron stores.

Assisted Reproductive Technologies (ARTs) in Animal Science

• Embryo transfer:

  • Transfer of embryos from one animal to a recipient uterus; embryos may be created in vivo or in vitro and then transferred.

• In vitro embryo production (IVP/IVF):

  • In vitro maturation (IVM) of oocytes, in vitro fertilization, and in vitro embryo culture to generate embryos.

  • Embryos produced in vitro can be transferred to recipients.

• Intracytoplasmic sperm injection (ICSI):

  • A single sperm is injected directly into the oocyte; widely used in humans and in some animal species (horses, etc.) where conventional fertilization in vitro is less effective.

• Somatic cell nuclear transfer (SCNT):

  • Cloning; transferring a somatic nucleus into an enucleated oocyte to create a clone.

• Cryopreservation:

  • Cryopreservation of semen and embryos is routine; oocytes can be cryopreserved in some species, though not all are equally cryo-friendly.

• Sex-sorted semen:

  • Sorting sperm by sex chromosome to bias offspring sex ratio.

Key Terms and Vocab for Quick Reference

• Anestrus: absence of anestrous cycle.

• Polyestrous: animals that have multiple estrous cycles year-round (typical of most farm species).

• Seasonally polyestrous: multiple cycles but only during specific seasons (mares = long-day breeders; sheep/goats = short-day breeders).

• Estrus vs estrous: estrus is the heat/reciprocal period of sexual receptivity; estrous describes the cycle as an adjective (e.g., estrous cycle).

• Estrace (adjective) vs estrus (noun): estrace refers to the cycle phase in relation to estrous events; estrus refers to the period of receptivity.

• Graafian (preovulatory) follicle: the dominant follicle that becomes the ovulatory follicle.

• Corpus luteum (CL): temporary endocrine structure that secretes progesterone after ovulation; essential for pregnancy maintenance.

• Luteolysis: destruction of the CL, driven by PGF2α, signaling the end of a luteal phase if pregnancy does not occur.

• Prostaglandin F2α (PGF2α): hormone that induces luteolysis.

• Estradiol (E2): estrogen produced by growing follicles; drives estrus behaviors and uterine preparation.

• Progesterone (P4): hormone produced by the CL; maintains pregnancy.

• GnRH: gonadotropin-releasing hormone from the hypothalamus.

• FSH: follicle-stimulating hormone; promotes follicular recruitment and growth.

• LH: luteinizing hormone; triggers ovulation and luteinization.

• IGHT: terminology used in the lecture to describe the interaction of hormones and cycles (thermostat analogy).

• Equations and symbols to remember:

  • $PGF_{2\alpha} \uparrow \Rightarrow \text{luteolysis}$

  • $E<em>2 \uparrow, P</em>4 \downarrow \text{ during the follicular phase (before ovulation)}$

  • $E<em>2 \downarrow, P</em>4 \uparrow \text{ during the luteal phase (after ovulation)}$

  • $LH \uparrow \rightarrow \text{ovulation}$

  • $FSH \uparrow \rightarrow \text{follicle recruitment}$

  • $P_4 \text{ supports pregnancy (via CL)}$

• Important cycle timing:

  • Estrous cycle length: approximately 21 days (sheep ~17 days).

  • Ovulation typically occurs during the follicular phase, often toward the end of estrus (varies by species).

  • Maternal recognition of pregnancy: around day 15–16 in many species.

  • Parturition is the initiation of birth, often driven by fetal cues and the fetal HP axis.