The nuclear envelope in the developing oocytes of the Tunicate, Boltenia villosa

Summary

• 1. In the developing oocyte of Boltenia villosa, five distinct morphological manifestations have been observed, all involving the nuclear envelope. None of them is continuous throughout the growth of an oocyte but each occupies a definite period of oocyte development. These different manifestations reflecting different activities at the nucleo-cytoplasmic boundary in B. villosa oocytes are listed as follows, starting from very young to almost mature cells: 1. the loss of the conventional two-membrane condition on the part of the nuclear envelope; 2. the formation of intranuclear vesicles; 3. probably concurrent with 2, the intensive formation of ribosomes involving the outer membrane of the nuclear envelope; 4. the formation of endoplasmic reticulum by the outer nuclear membrane; and 5. the formation of the annulate lamellae on both sides of the nuclear envelope.

• 2. These morphological changes, all occurring at the nucleo-cytoplasmic boundary but each occupying a definite stage of oocyte development, have been regarded as possibly due to differential gene activation.

• 3. The significance of the fact that the intranuclear vesicles and the intranuclear annulate lamellae, products of two of the activities occurring at the nucleo-cytoplasmic boundary, remain within the nucleus has been touched upon. These two types of structures, being intranuclear, furnish morphological evidence of at least one of the mechanisms whereby the germinal vesicle enlarges as the oocyte developes.

The question has also been raised whether these products of nucleo-cytoplasmic reactions remain within the nucleus to activate other genes or, as finished “information”, to find temporary shelter.