Dr. Dhafer M. Aziz, BVMS, MSc, PhD Professor, Department of Surgery and Theriogenology College of Veterinary Medicine, University of Mosul, Mosul, Iraq http://orcid.org/0000-0002-6283-5172 https://www.researchgate.net/profile/Dhafer_Aziz2 Female Fertility | 4th year 2019

Oogenesis and Folliculogenesis

Oogenesis

Early in embryogenesis, primordial germ cells migrate from the yolk sac endoderm to the genital ridge (developing ovary) where they take up residence and are called oogonia.

These diploid oogonia undergo several mitotic divisions prior to or shortly after parturition, thus providing the developing ovary with a large supply of future ova (eggs).

When oogonia begin the first meiotic division, they are called primary oocytes.

Primary oocytes are arrested in prophase of Meiosis until the female reaches sexual maturity. They grow in size during this arrested phase, but do not divide.

When the female reaches sexual maturity and under the influence of follicle stimulating hormone (FSH), a small number of primary oocytes are stimulated to continue through Meiosis.

During this process the number of chromosomes is reduced from the diploid number (2N) to the haploid number (1N).

The chromosomes are divided equally, most of the cytoplasm stays with the oocyte. The smaller polar body contains half the chromosomes but only a small amount of cytoplasm and will eventually degenerate.

After a primary oocyte completes the meiotic division, it is called a secondary oocyte (1N)

Ovarian follicles are composed of:

 The developing gamete  Associated follicular cells.

There are three stages in the development of follicles:

1. pre-ovulation

2. ovulation

3. post-ovulation.

Femal Fertility | Oogenesis and Folliculogenesis | Dr. Dhafer M. Aziz Page | 1 Pre-ovulation: Development of Follicles in the Ovary.

The primordial follicle consists of a primary oocyte and a single layer of flattened follicular cells.

The primary follicle consists of a primary oocyte with a single layer of cuboidal/columnar follicular cells.

The secondary follicle consists of several layers of cuboidal/columnar follicular cells, now collectively called the membrana granulosa which begin to secrete follicular fluid.

The Tertiary Follicle, the presence of pockets of follicular fluid within the membrana granulosa. As the follicle continues to develop, the separate pockets fuse to form one large pocket of fluid called the follicular antrum.

The mature follicle, is also called the pre-ovulatory follicle or Graafian follicle, has all of the components of the secondary-vesicular follicle but is much larger and contains one single large antrum of follicular fluid.

Femal Fertility | Oogenesis and Folliculogenesis | Dr. Dhafer M. Aziz Page | 2 As the oocyte and follicular cells are growing and developing in the ovary, the stromal cells differentiate and develop into the theca interna and theca externa cells.

As a follicle goes from a primary to a secondary follicle, the stromal cells immediately surrounding the follicle differentiate into the theca folliculi. The cells closest to the follicle become the theca interna cells, round, foamy cells that secrete androgens, including testosterone.

These two “male” hormones are converted by the granulosa cells to estrogens. The stomal cells farther away from the developing follicle become the theca externa cells, fibroblast-like cells arranged around the follicle outside the theca interna cells.

Femal Fertility | Oogenesis and Folliculogenesis | Dr. Dhafer M. Aziz Page | 3