Ultrastructure of female germ cells in Haliotis asinina Linnaeus

Abstract

Germ cells in the ovary of H. asinina are divided into six stages: oogonia and five stages of oocytes. The oogonium is a scallop-shaped cell 8-10 μm in diameter, closely adhered to a trabecula. Its nucleus exhibits small blocks of heterochromatin along the nuclear envelope and a small nucleolus. The cytoplasm contains abundant ribosomes. The stage I oocyte is a round cell 12-25 μm in diameter. Its nucleus contains numerous lampbrush chromosomes consisting of chromatin fibers with three sizes, i.e., 100-200, 40-60 and 7-12 nm in diameter. The cytoplasm has numerous mitochondria, few rough endoplasmic reticulum, and abundant ribosomes. The stage II oocyte is a round cell 25-35 μm in diameter. Its nucleus exhibits increasingly decondensed chromatin and a nucleolus, and the nuclear envelope exhibits numerous nuclear pores. The cytoplasm contains numerous and well-developed Golgi bodies, rough endoplasmic recticulum and abundant ribosomes. There are two types of secretory granules: both have a spherical shape, 350-450 nm in diameter, with an electron-dense and electron-lucent matrix, respectively. The stage III oocyte is a pear-shaped cell about 35×70 μm in size. Lampbrush chromosomes are almost completely unraveled. The two types of secretory granules are greater in number and cluster around the Golgi bodies. Larger and more electron-dense ovoid-shaped yolk granules start to appear. The stage IV oocyte is a flask-shaped cell about 50×80 μm in size. Its nucleus contains completely decondensed chromatin and a highly enlarged nucleolus. The cytoplasm is filled with lipid droplets (1.5-3 μm in diameter) and yolk granules (1.5-2.5 μm in diameter). The vitelline-cum-jelly coat starts to develop, and could be derived from the first type of secretory granules which are translocated to be exocytosed at the plasma membrane. The stage V oocyte is similar to the stage IV oocyte except its vitelline-jelly coat achieves maximum thickness and appears fibrous in comparison to the amorphous appearance at stage IV.