Welcome to Folliculogenesis Part 1!
This is part 1 of a 3 part series all about ovarian follicles. For a fun visual learning experience I recommend watching my 5 minute video above.
After watching/reading you will learn:
What is a follicle?
The follicle is the functional unit of the ovary. It consists of a single oocyte (or egg) surrounded by layers of hormone producing support cells. There are two types of hormone producing cells that make up the follicle. The cells immediately surrounding the oocyte are called granulosa cells and the cells on the outside of the follicle are called theca cells.
Folliculogenesis
One of the major functions of the ovary is to produce mature fertilizable eggs. This involves developing the follicle in a coordinated process called folliculogenesis. Most of us know about the final stage of follicle growth which is ovulation - when an egg is released from the ovary. However, there is a long process of development to get to a follicle and an egg that’s ready for ovulation. This process all starts with the primordial follicle. Primordial follicles are dormant meaning they are in a resting state waiting for activation signals to start the growth process. During their life cycle, these follicles activate from dormancy and begin to grow where they gain layers of granulosa cells and theca cells, the oocyte gets larger and develops, and eventually a fluid filled cavity called the antrum forms. The antrum is a source of nutrients that the follicle pools inside of it to aid in the development of the egg. This liquid is called follicular fluid. Finally, the follicle reaches its finally stage before ovulation.
From the size of a grain of salt to a beach ball
From the smallest stage of the primordial follicle to the final stage of the ovulatory follicle there is a huge volumetric expansion that occurs. The smallest primordial follicles are around 30µm or 0.03mm in diameter and ovulatory follicles can be as large as 30mm or 3cm. That means primordial follicles grow 1000x their size to reach their final stages! That’s like growing from the size of a grain of salt to a beach ball 😲. This entire process takes around 375 days.
Stages of follicle growth
Each stage of folliculogenesis is unique as many changes happen throughout the process. Below I have outlined the major stages of folliculogenesis with a brief description of each one. You will see a schematic version of the follicle stage on the left and then a histology image of an actual human ovarian follicle at that stage. Histology is the study of tissues and cells under a microscope and the staining is normally pink and purple! The nucleus of a cell will be stained in purple and the rest of the cell in pink.
Next time…
Join me next time for folliculogenesis Part 2 where we will learn about what follicle growth and decline looks like in practice within the ovary.
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References
Schematics and illustrations created with BioRender.com and Canva
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