Welcome to Folliculogenesis Part 2!
This is part 2 of a 3 part series all about ovarian follicles. For a fun visual learning experience I recommend watching my 3 minute video above.
After watching/reading you will learn:
What is the ovarian reserve?
In Folliculogenesis Part 1 we learned about the stages of folliculogenesis and how follicle growth all starts from the primordial follicle. These follicles are dormant and reside in the ovarian cortex - the outer 1mm of the ovary. Primordial follicles form in utero in the developing ovaries of the fetus. After they form, they become dormant. At birth, individuals with ovaries have all of the follicles they will ever have - around 500,000 - 1 million. These dormant primordial follicles make up an individuals ovarian reserve.
Ovarian reserve decline over time
The number of primordial follicles in the ovarian reserve undergoes a nature decline with age starting at birth and ending at menopause. After birth, the decline in follicle numbers is due to follicular atresia which is programmed cell death. One theory suggests that this happens to naturally select only the best quality oocytes.
Follicle recruitment and the dominant follicle
At puberty, cohorts of dormant primordial follicles activate and begin the process of folliculogenesis. Throughout each stage of growth, follicles undergo atresia until eventually only one follicle is selected as the dominant follicle that will ovulate. The dominant follicle is the largest and fastest growing. It produces the most hormones and is the most responsive to the hormones that cause follicles to grow. We end up with one dominant follicle because humans are a mono ovulatory species - only ovulating one egg. Other mammals, like dogs, are poly ovulatory meaning they ovulate many eggs. This is why we get litters of dogs and not humans. Of course, there are always exceptions to the rule like in the case of fraternal twins which come from two different ovulated eggs.
Next time…
Join me next time for Folliculogenesis Part 3 - the final installment of this series -where we will answer the questions: what hormones make follicles grow? what signals ovulation? and what happens to the follicle after ovulation?
If you want notifications on new posts please subscribe - it’s free! Free subscribers will also receive a visual infographic they can download about this week’s topic.
References
Schematics and illustrations created with BioRender.com and Canva
Slide 10 schematic adapted from here
Share this post