ORIGIN AND DIFFERENTIATION OF THE INNER FOLLICULAR CELLS DURING OOGENESIS IN MOLGULA-PACIFICA (UROCHORDATA), AN ASCIDIAN WITHOUT TEST CELLS

In Molgula pacifica small previtellogenic oocytes are found between ce lls of the ovarian epithelium. Each oocyte subsequently grows within a compartment of the epithelium known as a primary follicle. The wall o f the primary follicle is composed of outer follicular epithelial cell s. While growing from about 15-70 mum in diameter, each oocyte gradual ly recruits a set of about 950 non-epithelial inner follicular cells. These cells co-differentiate in sets with each oocyte, but test cells never appear. The first filamentous components of the vitelline coat a ppear on the surface of an oocyte in places where it is in contact wit h undifferentiated (stage 2) inner follicular cells. Each fully differ entiated inner follicular cell stores adhesive precursors in a large c ompartment of the endoplasmic reticulum and probably secrets component s of the vitelline coat. There is no evidence that the outer follicula r epithelial cells transform into inner follicular cells by dedifferen tiation as has often been assumed. Inner follicular cells, in stage 1, are nearly identical to hemoblasts. Hemoblasts may form the inner fol licular cells, but to do this they would have to cross the outer folli cular epithelium and this phenomenon has not yet been seen.