Oogenesis and Follicular Development

The development of gametes (sex cells) was something that was quickly looked at during my undergraduate degree and I haven’t really revised anything on the formation and development of females gametes since. As an academic and research student I thought there must be better ways to improve my understanding than just reading from a textbook. I have tried to incorporate some histology, literature, drawings and flowcharts to show how I revised/learnt this topic in more depth.

Quick Overview of Folliculogenesis

Another important part of anatomy and its sub-disciplines in my opinion is to understand that etymology (origin of words) behind anatomical terms and processes. This is because most anatomical words are derived based on function or appearance and this can be really useful when jumping into a new area of study. I have provided a list of words and their origins down below before I start talking about Oogenesis.

Etymology

Oogenesis: “formation and development of the ovum” with the prefix: Oo – meaning “egg” originally derived from Greek or the cognate (having the same linguistic derivation) with the Latin term Ovum. The suffix: genesis meaning “birth”, “origin” or “creation” derived from Greek.

Oogenesis and Folliculogenesis

Before birth (prenatally) the primordial germ cells migrate from the wall of the yolk sac to the developing gonads around the 4th/5th week of development. The primitive germ cells undergo mitotic division and proliferate into oogonia. From the 5th to 7th month majority of oogonia degenerate and remaining become primary oocytes. They undergo their first meiotic division but stop when they enter the dictyotene stage (resting stage between prophase 1 and metaphase 1).

Primary oocytes do not finish their first meiotic division before puberty is reached
Jan Langman, M.D., Ph.D.

At birth there are ~ 2 million primary oocytes, but between birth and puberty a process known as atresia or degeneration occurs resulting in ~ 40 000 primary oocytes present at puberty. The primary follicle (follicular cells + primary oocyte) differentiates into secondary follicles and then into Antral follicles but wont differentiate any more until puberty. At puberty there is an increase is circulating Follicle Stimulating Hormone (FSH) which drives the further division and resulting in ovulation.

Here we can see a diagram showing the development of primordial follicles through to ovulation and then scar tissue (corpus luteum). As mentioed before before puberty the primary oocytes do not undergo further mitotic division. After puberty is reached there is an increase in follicle stimulating hormone and Lutenising Hormone (LH) leading to maturation of ~ 20 primordial follicles with usually one follicle being ovulated.

NOTE: Female (assigned female at birth) gametogenesis is different from male gametogenesis. Males undergo spermatogenesis which will result in two complete meiotic divisions, however the second meiotic division in oogenesis will not be completed unless fertilisation occurs.

Histology of the Ovary and Oocytes

Above we have from left to right: a histological section of the entire ovary (human), section showing primordial germ follicles (classic appearance where they are arranged in clusters), primary oocyte within a primary follicle (uni-layered (1 layer)) with its zona pellucida. 4th image is showing a multi-layered primary follicle (presence of two distinct layers of zona granulosa surrounding the primary oocyte and zona pellucida). The last image demonstrates an antral follicle, classic appearance with a large follicular antrum filled with fluid, layer of corona radiata cells (darker in appearance based on the stain) surrounding the primary oocyte (could possible be secondary oocyte depending on the size of the follicular antrum: small = primary, large = secondary) and the cumulus oophoros which is connecting the corona radiata to the stroma of the ovary.

I found that using histology really helped solidify my understanding of oogenesis and folliculogenesis. Histology inst often used in embryology classes but in my opinion it adds an extra layer of information that might be beneficial to visual learners.

The images above have been edited by me with annotations, it should be mentioned that these images were sourced from the Virtual Microscopy Database from J Chapman from the University of Tasmania and the original histological section can be viewed below

Histological Section of Ovary by Jamie Chapman from VMD

How I Taught Myself

Learning the histology and in-depth aspects to oogenesis was definitely interesting and fun. Although there are plenty of resources available I thought having my step by step process of how I learn informative and hopefully useful. I have attached a link to my notes that I have taken over the last two days with my drawings.

https://drive.google.com/file/d/14QqpPt2JY2Sf6H_X9pZ_hq_9LtJV62rY/view?usp=sharing

Resources

Jamie Chapman is a Senior Lecturer in Human Anatomy and creator of 3 Min Histology YouTube Videos. I will usually go straight to his videos to understand a histology topic rather than trying to find answers on google, online textbooks or in research papers. Both the videos below talk about the appearance of the ovaries and different types of follicles (in individuals assigned female at birth)

Other Resources