Oocyte differentiation in Urechis caupo (Echiura): a fine structural study

The fine structure of oocytes of Urechis caupo is described for seven arbitrary stages ranging from the smallest oocytes (7 mum in diameter) in the coelom to the mature oocytes (115 mum in diameter) in the storage organs. Although most types of cytoplasmic organelles accumulate more or less continuously, yolk granules do not appear until oocytes reach a diameter of 35 mum, and there is stage-specific synthesis of cortical granules in 60-80 mum oocytes. In the nucleus a single nucleolus first appears when an oocyte is 15 mum in diameter. Then a nucleolus satellite, which is about 3 mum in diameter, forms in 30 mum oocytes; this nucleolus satellite later (60-70 mum oocytes) becomes surrounded by 750 nm dense spherical bodies. Large (2-4 mum in diameter) juxtachromosomal spherules occur only in the nuclei of mature oocytes. Microvilli become progressively more numerous and longer until the oocyte reaches a diameter of 90 mum; their tips project 1 mum beyond the fibrous surface coat, which is 2 mum thick when well developed. Near the end of oocyte growth, the microvilli retract into the surface coat leaving their pinched-off tips adhering to the outside of the coat.