Cytogenetic investigations on development and degeneration of oocytes from juvenile mice

Summary

Oocytes were isolated by mechanical disruption of the ovarian tissue of juvenile mice aged between 8 and 28 days p.p. Different types of oocytes could be distinguished by direct observation using an inverted light microscope. Their relative amount within the oocyte population isolated was determined in relation to age of the animals. The following types were isolated and examined:

• 1. “Normal” oocytes containing an intact germinal vesicle (that is, a nucleus in dictyate stage). In all age groups 55 to 83% of the oocytes were of this type. Their nuclear developmental stage was determined by studying their capacity to resume meiosis in culture as a function of age. 45% of the oocytes isolated from 15 day old mice were competent to resume meiosis. In mice aged 21 days more than 80% of the oocytes measuring about 80 μm in diameter resumed meiosis in vitro. No further increase in mean diameter or in the amount of competent oocytes could be found in older mice.

• 2. Degenerating oocytes. Five different types could be distinguished.

• a) “Atretic” oocytes present in all age groups investigated with pyknotic or disintegrating nuclei;

• b) “Precociously matured” oocytes first observed in mice aged 15 days containing normal diakinesis-and metaphase II-chromosomes;

• c) “Vacuolized” oocytes first isolated on day 18 having metaphase II-chromosomes scattered within the cytoplasm;

• d) “Fragmented” oocytes also first observed 18 days p.p. containing chromatin dissociated into micronuclei of different sizes within the cytoplasmic fragements;

• e) “Ghost” oocytes present in all age groups, characterized by the disintegration of the cytoplasm so that only the surrounding zona pellucida can be clearly seen.