Follicle Phase and Ovulation
The Follicle Phase and Ovulation
Overview of Follicle Development
Follicle Development Types:
Monotocous Species: Ovulate one dominant follicle.
Polytocous Species: Ovulate multiple large follicles.
Heritability of Ovulation Rate:
The ovulation rate is heritable; noted in species such as pigs and sheep (and possibly humans).
Follicles on Ovaries During a Cycle
Presence of Follicles in Fertile Animals:
Numerous primary (1°), secondary (2°), and tertiary (3°) follicles are always growing in the cortex beneath the surface, with some visible on the surface (>1 mm).
Species Differences:
Certain species have large follicles only during the late follicle phase, which may include both spontaneous and induced ovulators.
Other species (like ruminants) have large/dominant follicles in both the luteal and follicle phases.
Ovulation Enhancement for Embryo Production
In various species, ovulation is enhanced to increase the potential for embryo production:
1 embryo
2 embryos
4 embryos
8 embryos
16 embryos
14 embryos
12 embryos
10 embryos
8 embryos
6 embryos
4 embryos
2 embryos
0 embryos
Note: The specific ovulation rates correspond to the number of embryos that can potentially be produced.
Litter-Bearing Species Ovulation Rate
Ovulation Rate Impact on Litter Size:
The ovulation rate is the first limiting factor to litter size in litter-bearing species.
Litter Size vs. Number of Eggs Ovulated:
This relationship forms a foundational principle in the reproductive biotechnology of these species.
The Follicle Phase Stages
Early Proestrus:
No species present dominant or ovulatory follicles.
Late Proestrus:
All species exhibit one or more dominant or ovulatory follicles.
Key Events Allowing Follicle Phase Start
Triggers for the Follicle Phase:
Puberty:
A signal initiates the advancement of follicles.
In Season:
Linked to photoperiod responsiveness.
Post-Partum Recovery:
Luteolysis:
Occurs in cycling animals that are not pregnant, often following a period of elevated progesterone.
Sequence of a Follicle Phase: Estrus and Ovulation
Key Events and Hormonal Patterns:
Concentration of Blood Hormones:
Major hormones involved during this sequence include:
Progesterone
FSH (Follicle-Stimulating Hormone)
Inhibin
Estradiol
GnRH (Gonadotropin-Releasing Hormone)
LH (Luteinizing Hormone)
Sequence: Proestrus and Estrus
Follicle Growth Dynamics:
Selected follicles grow to a large size while others regress (marked as X).
Estrogen Feedback:
Estrogen (E2) feedback leads to an increase in GnRH pulses and LH pulses.
Estrogen Threshold:
Reaches a level at which estrus begins (identified as ESTRUS).
LH Surge:
Leads to ovulation.
Days and Blood Concentrations:
Estrus occurs around day 9 of the cycle as described by hormone concentrations in the blood.
Follicle Lifespan
Recruitment Stages:
Primordial Follicles:
Leave resting state to enter the growing pool, transitioning from primordial to antral stage (approximately 1 month).
Antral Stage:
Follicles become sensitive to FSH.
Timeframe from antral stage to visibly surface follicles is approximately 1 month.
Growth from small to large follicles may take days to weeks.
Selection Process:
One or more medium follicles grow to the large (ovulatory) size.
Enhanced by active estrogen production, leading to increased inhibin production that reduces FSH and competition among follicles.
Follicle Sizes and Receptor Types
Follicle Size Classification (Surface Visibility):
Small: Follicles with FSH receptors.
Medium: Follicles with FSH and LH receptors.
Large: Follicles with LH receptors.
Structure of the Follicle
Follicular Anatomy Identified:
Germinal Epithelium and Tunica Albuginea
Theca Externa
Theca Interna
Granulosa
Antrum
Basal Lamina
FSH (Follicle-Stimulating Hormone)\n
Steroid Production in Follicles
2-Cell Production Mechanism:
The actions of FSH and LH on steroid production are observed in the following manner:
Granulosa Cells (C18) respond to FSH, converting androgens into estradiol (E2).
Theca Interna Cells (C27, C19) respond to LH to produce androgens.
Theca Externa: Contains capillaries; LH binds to receptors in theca cells to assist steroid production.
Paracrine Effects: The interaction between theca and granulosa cells leads to enhanced steroid production.
Comparative Follicle Size in Various Species
Sizes of Ovulatory Follicles (in mm):
Rat: 0.7 mm
Queen: 2 mm
Bitch: 7 mm
Ewe: 10 mm
Sow: 12 mm
Woman: 18 mm
Cow: 18 mm
Mare: 50 mm
Follicle Growth in Pigs
Surface Follicle Growth:
Over 4-5 days, pigs select ovulatory follicles with the following counts:
Small: 40
Medium: 25
Large: 18
Follicle Phase in Horses
Mare Follicle Phase:
Over 6 days, a mare develops and selects a single dominant large follicle which is retained until ovulation.
Estrus phase maintained for 5-7 days.
Follicle Development Waves in Cattle
Wave Patterns During the Estrous Cycle:
Cattle exhibit three waves of follicular development across weeks 1, 2, and 3.
Graphical Representation:
Shows fluctuation of dominant (DF), medium (MF), and small follicles (SF) across the estrus cycle duration (18-22 days).
Female Follicle Dynamics in Primates
Human Follicle Development Timeline:
Day 15: Recruitment of 3-30 follicles for growth post-ovulation.
Day 5: Selection of a cohort for advancement.
Day 9: Dominance and final selection process.
Day 14: Ovulation occurs.
Reproductive Features in Rats
Unique Rat Reproductive Dynamics:
Ovulated follicles have minimal luteal cells and produce limited progesterone.
Absence of cervical stimulation at estrus leads to no increase in PRL (prolactin) resulting in no development of CL (corpus luteum).
Cervical stimulation at estrus can induce pseudopregnancy lasting 12 days.
Canine Reproductive Cycle
Dog's Proestrus Phase:
Duration: 9 days, occurring 1-3 times per year.
Dogs will ovulate and form a corpus luteum (CL) lasting 60 days.
Citation: Payan-Carreira, R., Miranda, S., & Nizanski, W. (2011). Artificial Insemination in Dogs.
Feline Reproductive Behaviors
Cat's Reproductive Cycle:
During the season, cats may experience repeated proestrus periods lasting 10 days.
Peak estradiol occurs over 3 days.
Cats ovulate and form a CL only if mated; the CL lasts 60 days, regardless of pregnancy status.
Ovulation Types and Mechanisms
Spontaneous Ovulation:
Characterized by pre-ovulatory LH surge.
Estrogen induces dilation of ovarian arteries leading to increased blood flow and capillary permeability in the antrum.
After the LH surge, theca cells convert androgens into progesterone, stimulating collagenase production.
Resulting in the breakdown of tunica albuginea and rupture, leading to ovulation (egg and fluid expulsion).
Induced Ovulators:
During breeding seasons (spring/fall), large follicles develop and produce estrogen.
Vaginal or cervical stimulation by males causes neural action potentials through afferent nerve pathways to the CNS.
Resulting signals in the hypothalamus induce GnRH and LH surges, which leads to ovulation.
Ovulation can also be inhibited through hormonal means.
Comparative Estrous Cycles Overview
Key Cycle and Estrus Lengths for Various Species:
Pig:
Follicle Phase: 6 days
Estrus Duration: 2 days
Overall Cycle Length: 21 days
Cow:
Follicle Phase: 3-4 days
Estrus Duration: <1 day
Overall Cycle Length: 21 days
Horse:
Follicle Phase: 6 days
Estrus Duration: 5-7 days
Overall Cycle Length: 21 days
Sheep:
Follicle Phase: 3-4 days
Estrus Duration: <1 day
Overall Cycle Length: 17 days
Cat:
Follicle Phase: 9 days
Estrus Duration: 7 days
Overall Cycle Length: 9 days (repeats)
Dog:
Follicle Phase: 9 days
Estrus Duration: 9 days
Woman:
Follicle Phase: 14 days
Overall Cycle Length: 28 days
Rodent:
Follicle Phase: 3 days
Estrus Duration: <1 day
Overall Cycle Length: 5 days