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Primary Oocyte (of Secondary Ovarian Follicle)

Primary Oocyte (of Secondary Ovarian Follicle)

Oocytus primarius folliculi ovarici secundariae

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Quick Facts

The primary oocyte is an oocyte that has begun but not completed the first maturation division; it is derived from an oogonium by differentiation near the time of birth (Dorland, 2011).

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Structure and/or Key Feature(s)

Although primary oocytes begin their first meiotic division in utero, the first meiotic prophase remains incomplete until just prior to ovulation, which means many remain arrested in their first meiotic division for 12 to perhaps 50 years.

The primary oocyte of the primordial ovarian follicle is about 30 μm in diameter and has a large round, eccentric, pale nucleus containing one or two large prominent nucleoli.

The cytoplasm of the oocyte is referred to as ooplasm and contains Golgi membranes and vesicles, endoplasmic reticulum, centrioles, and many mitochondria and lysosomes. These organelles are condensed into a region in the ooplasm referred to as a Balbiani body. The ooplasm also contains stacks of membranes called annulate lamellae.

Organelles increase their numbers and become scattered throughout the ooplasm as the oocyte matures. Lipid droplets and masses of lipochrome pigment may appear. Beneath the plasma membrane (oolemma) special secretory granules called cortical granules appear which contain proteases.

As the zona pellucida is being deposited, many irregular microvilli project from the oolemma into the perivitelline space between the oocyte and the granulosa cells to intermingle with similar microvilli projecting from adjacent granulosa cells.

When the first meiotic division is completed in a mature follicle equal share of chromatin is distributed to each daughter cell but only one receives most of the cytoplasm to become a secondary oocyte. The other daughter cell becomes the first polar body.

The second meiotic division commences immediately after the first meiotic division is completed (Ross and Pawlina, 2006; Mescher, 2013; McKinley, O'Loughlin and Pennefather-O'Brien, 2016).

List of Clinical Correlates

The long period of meiotic arrest may expose the primary oocyte to factors which could contribute to errors in meiotic division such as nondisjunction. Such an error results in anomalies such as trisomy of chromosome 21 (Down’s syndrome).


Dorland, W. (2011) Dorland's Illustrated Medical Dictionary. 32nd edn. Philadelphia, USA: Elsevier Saunders.

McKinley, M. P., O'Loughlin, V. D. and Pennefather-O'Brien, E. E. (2016) Human Anatomy. 5th edn.: McGraw-Hill Education.

Mescher, A. (2013) Junqueira's Basic Histology: Text and Atlas. 13th edn.: McGraw-Hill Education.

Ross, M. H. and Pawlina, W. (2006) Histology: A text and atlas. Lippincott Williams & Wilkins.

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