An ultrastructural study of oocyte growth within the endoderm and entry into the mesoglea in Actinia fragacea (Cnidaria, anthozoa)

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Abstract
Sea anemone gametes arise in the endoderm but migrate into the mesoglea at an early stage. In order to observe this process, large individuals of Actinia fragacea were collected from the same intertidal location at regular intervals over a 2‐year period, and their gonads were examined by light and electron microscopy. The cellular origin of the oocytes is unclear, but the smallest recognizable oocytes are rounded cells, 6‐8 μm in diameter, with relatively large nuclei which may contain synaptinemalcomplexes. Their cytoplasm contains numerous ribosomes, a flagellar basal‐body‐rootlet complex, and distinctive dense structures also present in male germ cells but not found in anemone nonger‐ minal cells. During the endodermal phase of growth, the density of the oocyte nucleus increases, a single nucleolus becomes prominent, and mitochondria and glycogen accumulate in the cytoplasm. Most oocytes, but not all, only begin major vitellogenesis after entry into the mesoglea. Most oocytes enter the mesoglea before they attain a diameter of 25 μm. The oocytes migrate toward and enter the mesoglea by a process resembling amoeboid movement. During entry, the oocytes are constricted into a characteristic “hourglass” shape and become covered by a basal lamina continuous with that of the gonad epithelium. The last part of the oocyte to enter the mesoglea forms an intimate relationship with the surrounding endodermal cells, which is maintained after entry is complete, and is thought to be important in the establishment of the trophonema.

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