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Ovarian Development
Here we will learn the embryology of the ovaries, which produce oocytes and hormones that are integral to the development of female sex characteristics.
Let’s start with the migration of primordial germ cells early in week 5.
We show the caudal portion of an embryo, with the aorta, dorsal mesentery, and hindgut; we the yolk stalk, which connects the primitive gut and the yolk sac.
We show the mesonephros.
Primordial germ cells migrate from the yolk sac wall, through the dorsal mesentery, and arrive at the gonadal ridges, which lie medial to the mesonephros.
In an XX individual, the indifferent gonad becomes an ovary under the direction of several genes, including WNT4, RSPO1/β-catenin signaling, and FOXL2
In approximately week 8, the developing ovary can be functionally divided into an inner medulla, which comprises the primitive sex cords (aka primary or medullary sex cords), and an outer cortex, which is where the primordial germ cells reside.
The primitive sex cords degenerate as the secondary sex cords (aka cortical sex cords) develop from the coelomic epithelium of the cortex (be aware that this is a simplification; supporting cells have complex origins and are the result of at least two developmental waves).
The primitive germ cells move through the stages of oogonia and early oocyte, and cluster together to form cell nests (aka, cysts) with follicular cells, derived from the secondary sex cords, surrounding them.
The oogonia enter meiosis I and become primary oocytes.
In the last stage, the cell nests break apart and give rise to the components of the ovarian follicle: individual primary oocytes, which are arrested in Prophase I, and their surrounding follicular cells.
Ovarian follicles are the functional units of the ovary; they produce hormones and mature oocytes.
Between the cell nest and individual follicle stages, apoptosis reduces the number of oocytes by 1/3 or more prior to birth.
The ovarian medulla comprises loose connective tissue and neurovasculature.
Primordial germ cells give rise to oogonia (which will develop into mature oocytes).
The gonadal ridge gives rise to the follicular granulosa cells, which support and interact with the oogonia throughout their development. Recall that the follicular cells specifically 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; we did not show this structure in our diagram, but know that it is thought to contribute to ovarian health and fertility.
Primordial Germ Cells
Primitive sex cords
Secondary sex cords
Cell Nests
Ovarian Follciles
Between the cell nest and individual follicle stages, apoptosis reduces the number of oocytes by 1/3 or more prior to birth.
The ovarian medulla comprises loose connective tissue and neurovasculature.
Summary table