An ultrastructural study of oogenesis and cell dynamics during cocoon shell secretion in the subterranean freshwater planarian Dendrocoelum constrictum (Platyhelminthes, Tricladida)

The ultrastructure of the ovary and the female atrium during cocoon formation was investigated in the subterranean freshwater planarian Dendrocoelum constrictum. In the peripheral portion of the ovary, the oogonia are recognized as undifferentiated germ cells, which are morphologically similar to neoblasts that have a high nucleus/cytoplasm ratio. Oocyte maturation is characterized by a marked growth of the cytoplasm because of the accumulation of cytoplasmic organelles and inclusions. The Golgi complexes begin to increase within the ooplasm and produce vesicles with an electron-dense content that fuse to produce larger spherical globules with homogeneous and electron-dense material. In the mature oocyte, the spherical globules migrate toward the cortical ooplasm, forming a continuous monolayer. We confirm that these spherical globules, which represent cortical granules rather than eggshell globules, vary in size up to 2 μm and their electron-dense content shows concentric thin bands. After leaving the ovary through the oviduct, the mature and fertilized oocytes reach the female atrium where they are packaged with thousands of vitelline cells in the cocoon shell. Based on our ultrastructural analysis, we demonstrate that the wall of the cocoon shell is composed of two layers, each of which has a different origin. The shell granules extruded from the vitelline cells are involved in the secretion of the inner layer of the cocoon shell, whereas the outer layer of the cocoon shell is synthesized by the epithelial cells in the genital atrium.