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Ovarian Development
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Ovarian Development

Ovaries
As we work through this tutorial, be aware that there is a great deal of intertextual variation; for example, the timing of events is approximate, and terminology varies widely among sources.
< Week 5
Early in development, primordial germ cells migrate from the epiblast to the extraembryonic yolk sac. In week 5, when the primordial germ cells migrate from the yolk sac to the posterior body wall, medial to the mesonephros in the intermediate mesoderm.
  • Their arrival stimulates adjacent tissue growth and formation of the gonadal (aka, genital) ridges, medial to mesonephros.
    • If the primordial germ cells do not migrate to this location, the gonads do not form.
> Week 8
The arrival of primordial germ cells induces the formation of primitive sex cords, which, along with the proliferation of the adjacent mesenchyme, give rise to the gonadal ridges.
    • Cortex of ridges = primordial germ cells
    • Medulla = mostly primitive sex cords
In the second stage, primitive sex cords degenerate (mostly in medulla) and secondary sex cords form in the cortex.
  • Secondary sex cords develop from the coelomic epithelium of the cortex, where the primordial germ cells reside (so, also referred to as "cortical sex cords").
In the third stage, the oogonia cluster together to form cell nests (aka, cysts).
  • Research suggests that the nests are thought to provide an increased store of nutrients for future development, or possibly protection from mutagenesis.
  • The oogonia enter the first phase of meiosis I, to become oocytes. As we learn elsewhere, meiosis is then suspended until puberty, when pituitary hormones trigger meiotic re-activation.
  • Within cell nests, cytokinesis is incomplete, and cytoplasmic extensions hold cells close.
  • Secondary sex cords dissociate and give rise to follicular pre-granulosa cells that surround the cell nests (be aware that some authors consider the cell nest to include the somatic follicular cells, while others call the cell nest + follicular cells "ovigerous cords").
In the fourth stage, the cell nests break apart and give rise to the components of the ovarian follicle.
  • The follicles are the functional units of the ovary, as they produce hormones and mature oocytes.
  • Medulla = the neurovasculature
  • Cortex = the ovarian follicles
    • Individual oocytes + Follicular cells
  • Oocytes separated when the cell nests dissociated.
    • Several genes have been implicated in nest disintegration, during which apoptosis (programmed cell death) and, possibly, autophagy, reduce the number of sex oocytes by 1/3 or more.
    • Research indicates that oocyte survival is based on quality. Furthermore, it appears that communication between the oocytes and follicular cells guides nest break down.
  • Follicular cells re-organize and surround each individual oocyte; together, the somatic follicular cells and oocytes form a dynamically interacting primordial follicle.
    • Communication between the cells regulates the growth and maturation of the follicles.
Summary:
The primordial germ cells give rise to oogonia (the gametes, aka, sex cells). The gonadal ridge gives rise to the follicular granulosa cells and thecal cells, which support and interact with the oogonia throughout its development. Recall that the follicular cells arise from the secondary sex cords; though not addressed here, thecal cells contribute to ovarian follicles further along in development. The mesonephros is thought to give rise to the rete ovarii; though not included in our diagram, it has been suggested that this area of the ovary synthesizes retinoic acid (RA), which triggers entry into the first phase of meiosis.

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