Folliculogenesis - Phases of Development

Phases of Development

Folliculogenesis lasts for approximately 375 days. It coincides with thirteen menstrual cycles. The process begins continuously, meaning that at any time the ovary contains follicles in all stages of development, and ends when a mature oocyte departs from the preovulatory follicle in a process called ovulation.

The growing follicle passes through the following distinct stages that are defined by certain structural characteristics (unfamiliar terms will be defined in their respective sections):

In a larger perspective, the whole folliculogenesis, from primordial to preovulatory follicle, belongs to the stage of ootidogenesis of oogenesis.

Stage Description Size
Primordial Dormant, small, only one layer of flat granulosa cells Primordial follicles are about 0.03-0.05 mm in diameter.
Primary Mitotic cells, cuboidal granulosa cells Almost 0.1 mm in diameter
Secondary Presence of theca cells, multiple layers of granulosa cells The follicle is now 0.2 mm in diameter
Early tertiary The early tertiary follicle is arbitrarily divided into five classes. Class 1 follicles are 0.2 mm in diameter, class 2 about 0.4 mm, class 3 about 0.9 mm, class 4 about 2 mm, and class 5 about 5 mm.
Late tertiary Fully formed antrum, no further cytodifferentiation, no novel progress Class 6 follicles are about 10 mm in diameter, class 7 about 16 mm, and class 8 about 20 mm. It is common for non-dominant follicles to grow beyond class 5, but rarely is there more than one class 8 follicle.
Preovulatory Building growth in estrogen concentration, all other follicles atretic or dead

In addition, follicles that have formed an antrum are called antral follicles or Graafian follicles. Definitions differ in where this shift occurs in the staging given above, with some stating that it occurs when entering the secondary stage, and others stating that it occurs when entering the tertiary stage.

Until the preovulatory stage, the follicle contains a primary oocyte that is arrested in prophase of meiosis I. During the late preovulatory stage, the oocyte continues meiosis and becomes a secondary oocyte, arrested in metaphase II.

Read more about this topic:  Folliculogenesis

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