Oogenesis

 

Introduction

Oogenesis is the female version of gametogenesis and is therefore the female equivalent of spermatogenesis. This process follows the immature, primordial ova right through to its maturation as a fertile ovum (egg).

Two processes which are important in cell division and therefore the creation of gametes (gametogenesis) are meiosis and mitosis, a quick recap on those process are shown below:

• Mitosis – Typical cell division where the diploid (2n) chromosome number is kept. Mitosis results in the production of identical cells.
• Meiosis – This type of cell division is usually associated with the production of gametes, it involves a reduction in the diploid (2n) chromosome count to a haploid (1n) chromosome count. Due to the occurrence of homologous recombination during crossover of genes, non-identical daughter cells are produced. Fertilisation sees two haploid cells (1n) fuse, which results in the restoration of the chromosome number to 2n.

Primordial Germ Cells

During embryonic life, primordial germ cells (the early stage sperm or ova) originate in the yolk sac endoderm where they migrate to the gonadal ridges (ventral to the lateral somites). The next step in their development depends on whether the embryo is destined to be male or female, whether the gonads are destined to be testes or ovaries:

• Testes – Primordial germ cells migrate to the medulla of the gonadal ridge and become surrounded by mesenchymal cells to form primitive sex cords (seminiferous tubule precursors)
• Ovaries – Primordial germ cells migrate to the cortex of the gonadal ridge. The germs cells undergo mitotic division to increase their number prior to puberty.

Stages of Oogenesis

The primordial germ cells differentiate into oogonia (s. oogonium), where by different processes they develop into oocytes. These oocytes enter interphase, undergoing meiosis until prophase 1, it is at this point all meiosis is halted, development will continue at fertilisation. All ova cells typically reach this stage before birth.

• Oogonia (2n) undergo mitosis (oocytogenesis) forming primary oocytes
• Primary oocytes (2n) undergo meiosis 1 (ootidogenesis) forming secondary oocytes, the primary oocytes are halted in prophase 1 of meiosis until ovulation
• Secondary oocytes (1n) undergo meiosis 2 (ootidogenesis), remaining in metaphase 2 until fertilised

Development of Follicles

Mesenchymal cells begin to surround the oocytes, which forms the primordial follicles. These surrounding cells eventually become cuboidal to form the granulosa cells. The follicle grows as the layers of granulosa cells increases; growth of the oocyte also contributes.

Gonadotrophins such as LH and FSH stimulate further follicular growth causing a cohort of follicles to emerge as a follicular wave (from which one will become dominant and be ovulated).  Fluid spaces develop in the granulosa cell mass (antrum formation) making the structure an antral follicle. The oocyte within the follicle remains attached to one edge of the internal follicle wall surrounded by cells of the cumulus (cumulus oophorus).

At regular intervals, from the cohort of follicles that initially emerge in the follicular waves some are selected to develop – they develop larger antral spaces. From the smaller group of selected larger follicles, typically only one becomes dominant – the others undergo atresia (the process of degeneration of follicles that are not ovulated in menstrual or oestrous cycle).

The oocyte within the follicle is still at the primary oocyte stage – it is still halted at prophase 1 of meiosis. However under the influence of a gonadotrophin surge associated with the process of ovulation, the process of meiosis resumes within the oocyte. The oocyte undergoes a meiotic division to form two differently sized cells, a large secondary oocyte and the first polar body. It is at this point ovulation takes place, meiosis again halted (until fertilisation).

Upon fertilisation by a spermatozoon, meiosis resumes, the secondary oocyte undergoes a second meiotic division to again produce two differently sized cells, a large zygote (a diploid [2n] cell resulting from the fusion of two haploid [1n] cells) and a smaller second polar body. The secondary oocyte, after being ovulated, has only a short period in which it can be fertilised – resulting in the development of the embryo.