Fine structure of human oogonia in the foetal ovary

Foetal ovarian tissue is now being cultured or frozen, to generate oocytes for assisted reproduction, an emerging technology. This study examines the ultrastructure of oogonia at 13-15 weeks of gestation, which could be used as a control for culture and freezing of foetal ovaries. Oogonia are largel y located in the ovarian cortex, whilst primordial germ cells (PGC) and som atic follicle cells compose the surface epithelium. Oogonia and PGC have la rge vesicular nuclei with clear cytoplasm, compared to dense follicle cells , which have polymorphic nuclei. Follicle cells intermingle with oogonia an d establish close contacts - beginning of folliculogenesis. Nuclei of oogon ia contain one to three highly reticulated nucleoli, reflecting high levels of RNA synthesis at the onset of growth. Rough endoplasmic reticulum (RER) form stacks of cisternae associated with numerous ribosomes. Prominent org anelles in the ooplasm are elongated mitochondria with dense matrices and t ubular cristate presenting a multilocular appearance. Typical Golgi complex es, dense bodies and dear vacuoles are present and microfilaments are locat ed beneath the plasma membrane. The most remarkable feature of oogonia is t hat they have typical juxtanuclear centrioles (diplosomes) with dense peric entriolar material, which nucleate microtubules, characteristic of function al centrosomes organizing the cytoskeleton. The mature oocyte has no centri oles, since the maternal centrosome is inactivated or reduced, while the pa ternal is dominant. Centrioles are most likely involved in mitosis of oogon ia. (C) 2000 Published by Elsevier Science Ireland Ltd. All rights reserved .