Estrous Cycle Review

Estrous Cycle

Definitions of Key Terms

• Follicular Phase

  • The phase of the estrous cycle where follicles mature and estrogen levels increase.

• Luteal Phase

  • The phase following ovulation characterized by the formation of the corpus luteum and high levels of progesterone.

• Atresia

  • The process of degeneration or resorption of ovarian follicles that do not reach ovulation.

• Follicle Recruitment, Selection, Dominance

  • Recruitment: Initial phase where a cohort of follicles begins to grow.

  • Selection: The phase where certain follicles are chosen for further development.

  • Dominance: The process where selected follicles continue to grow while others regress (atresia).

• Metestrus

  • The stage after ovulation where the corpus luteum begins to form.

• Diestrus

  • The longest phase of the cycle, characterized by the functioning corpus luteum.

• Proestrus

  • The phase preceding estrus where the follicles grow and estrogen levels rise.

• Estrus

  • The period when the female is receptive to mating; also known as heat.

• Estrous vs Estrus

  • Estrous: Adjective describing the cycle (e.g., estrous cycle).

  • Estrus: Noun for the specific phase (heat) within the cycle.

• Anestrus/Anestrous

  • A state where there are no reproductive cycles occurring; ovaries are inactive.

• Polyestrus

  • The condition of having multiple estrous cycles throughout the year.

• Seasonally Polyestrous

  • Having clusters of estrous cycles during certain seasons.

• Monoestrous

  • Exhibiting only one estrous cycle per year.

Learning Objectives

1. 2-Cell, 2-Gonadotropin Model of Estradiol Synthesis

   • Understand the types of cells involved, including the theca and granulosa cells, along with their respective gonadotropin receptors (LH and FSH).

   • Main intermediates include cholesterol and progesterone, and the key enzymes include P450scc, aromatase, and 3βHSD.

   • Identify the function of mitochondria and the importance of the Steroidogenic Acute Regulatory Protein (StAR) in transporting cholesterol into mitochondria.

2. Biochemical Structures

   • Familiarize with the chemical structures of cholesterol, progesterone, and estradiol.

3. Follicular Wave

   • A wave of follicular development characterized by stages of recruitment, growth, and atresia.

4. Communication between Ovarian Structures and Hypothalamus/Pituitary

   • Detailed interaction during the processes of recruitment, selection, and dominance of follicles.

5. Hormonal Changes

   • Understand the interrelationship and timing between FSH rise, follicular wave growth, and estradiol production.

   • Examine how LH, FSH, estradiol, and inhibin concentrations fluctuate throughout the follicular wave.

6. LH Pulse Frequency

   • Investigate how LH pulse frequency varies during the estrous cycle.

7. Functions of Major Hormones

   • Roles of estradiol, progesterone, LH, FSH, and PGF2α, with specific targets for estradiol discussed.

8. Cycle Lengths & Timing

   • Review the duration of the estrous cycle, estrus period, and timing of ovulation across species: cow, mare, sow, ewe.

9. Reproductive Hormonal Dynamics

   • Tracker changes in major reproductive hormones throughout the estrous cycle.

10. Figures Interpretation

    • Ability to interpret and explain the figures and graphs presented during lectures.

Important Numbers to Know

• Estrous cycle length, duration of estrus, and ovulation timing for the cow, mare, sow, and ewe found in Table 7-1, Page 148 of the textbook.

Reproductive Cyclicity

• Estrous Cycle

• Menstrual Cycle

Estrous Cycle Explanation

• Estrous Cycle

  • A series of predictable reproductive events that commence at estrus and continue until the next estrus.

• Estrus (Heat)

  • The specific time frame when the female is receptive to mating.

• Estrous (Adjective)

  • Used to describe the length of the estrous cycle.

• Anestrus

  • A condition of reproductive inactivity; no ovulatory follicles or corpora lutea present.

Types of Estrous Cycles

• Polyestrus

  • Uniform distribution of cycles throughout the year.

• Seasonally Polyestrus

  • Clusters of cycles occurring at particular times of the year.

• Monoestrus

  • Only one cycle occurring per year.

Follicular Phase vs Luteal Phase

• Follicular Phase

  • Composed of proestrus, estrus, metestrus, and diestrus phases.

• Luteal Phase

  • Dominated by the actions of the corpus luteum.

Follicular Development Stages

• Stages of Follicles: Primordial, Primary, Secondary, Developing Antral (Tertiary), Ovulating Follicle, Corpus Luteum, Corpus Albicans.

  • Tunica Albuginea: Connective tissue capsule surrounding the ovaries.

  • Cortex: Outer layer containing follicles, blood vessels, and nerves; involved in follicle maturation.

  • Medulla: Inner layer containing connective tissue, while the hilus area allows entry of blood vessels.

Folliculogenesis

• Continuous growth of some primordial follicles throughout life.

• Maturity dependent on species and other factors:

  • Swine: 10-25 follicles per cycle.

  • Sheep: 1-3.

  • Cattle, Horses, and Humans: typically 1.

• Outcomes of follicle development include either ovulation or atresia.

Estradiol Production Pathway

• Cholesterol can be converted into estradiol through various enzymatic pathways involving:

  • P450scc, Pregnenolone, 3βHSD, Progesterone, 17α-Hydroxylase, Androstenedione, P450 Aromatase, leading to Estradiol production.

StAR Protein

• Steroidogenic Acute Regulatory Protein (StAR):

  • Essential for transporting cholesterol into the inner mitochondrial membrane.

  • Rate limiting step of steroid hormone synthesis.

Ovarian Structure Interactions

Theca Interna and Granulosa Cells

• Involvement of LH and FSH receptors in driving testosterone and estradiol production.

Hormonal Regulation of Ovarian Function

• FSH primarily triggers follicular development.

• LH stimulates the final maturation of the dominant follicle and ovulation.

• Inhibin plays a role in feedback inhibition during the selection phase.

Follicular Waves and Dominance

Recruitment and Atresia

• Each cycle may include multiple follicular waves with distinct recruitment and selection phases.

  • The first two waves often occur in the presence of high progesterone (metestrus and diestrus) leading many follicles to undergo atresia.

• The last wave occurs post-luteolysis, leading to ovulation of the dominant follicle.

Hormonal Concentration Dynamics Throughout the Cycle

• FSH levels increase during recruitment, while LH surges at dominance leading to ovulation.

• Estradiol (E2) and progesterone (P4) concentrations will exhibit cyclical changes that influence ovaries' responsiveness.

Figures

• Several graphs illustrating follicular dynamics, hormonal concentrations, and their respective stages in the estrous cycle are discussed throughout the material.