Ultrastructural Observations on the Ovary and Eggs, and the Development of Egg Adhesion in Pacific Herring (Clupea harengus pallasi)

Changes in Pacific herring ovarian tissues and eggs during the four stages, late vitellogenic, ovulating, ovulated, and fertilized, were investigated by transmission and scanning electron microscopy. During the late vitellogenic and transitional stages the egg membranes consisted of five layers: the externus, distal inter-layer, internus, proximal inter-layer, and the sub-internus. The proximal and distal inter-layers do not appear to have been described elsewhere. A class of small, non-vitellogenic oocytes which were observed in late vitellogenic and ovulating ovaries, but not in ovulated ones or spawned eggs, are tentatively described as 'pre-ovulatory atretic oocytes'. Transmission electron microscopy (TEM) results demonstrated a collapse of the pores in the internus which may serve as a mechanical means of achieving egg membrane semipermeability during the first, or ovulation hydration. A thin, electron-dense layer was deposited on the external surface of the externus during ovulation. This epi-layer expanded on contact with seawater and became strongly adhesive. The results of the analyses are discussed in relation to an industrial roe-product quality problem called 'fragile roe' — an expression which describes a condition of reduced adhesion of eggs.