Oocyte growth

The growth of oocytes usually takes place over a long period, and the increase in size is often dramatic: the frog oocyte is an extreme example of this. The young oocyte, with a diameter of less than 50 μm, grows over a period of three years to reach a final diameter of 1500 μm – this represents an increase in size by a factor of over 20 000. Mammalian oocytes are much smaller, with a time scale of weeks rather than years in their growth period. However the increase in size is also considerable. For example, the mouse oocyte grows from some 20 μm to a final diameter of 70 μm, an increase in size by a factor of 40. All oocytes are large: certainly larger than the average somatic cell, which is usually about 10 μm in diameter. The size of the full-grown oocyte depends principally on the amount of stored foodstuffs in the cytoplasm, although the nucleus also enlarges to some extent. The characteristic nucleus of the immature oocyte is called the germinal vesicle. Yolk is the major food storage product, although large quantities of lipid and glycogen granules are also found in some oocytes. The chemical composition of yolk varies from species to species according to its protein: fat ratio. In invertebrates and lower vertebrates, yolk is usually found in the form of small granules, evenly distributed throughout the cytoplasm and contributing around 20–30% of the total oocyte volume.