The fine structure of mitochondria and the mitochondrial cloud during oogenesis on the sea anemone Actinia.

The appearance and arrangement of the mitochondria during all stages of oocyte growth in the sea anemone Actinia fragacea (Cnidaria: Anthozoa) have been examined by electron microscopy. In small oocytes, the mitochondria are generally squat, with a dense matrix and numerous cristae, although a proportion may show an unusual arrangement of prismatic cristae. During early oogenesis, the mitochondria tend to be arranged in aggregates rather than randomly scattered, and may be associated with nuage material. With the onset of vitellogenesis, a large mitochondrial aggregate forms next to the nucleus. During early vitellogenesis this aggregate enlarges and comes to resemble the mitochondrial clouds found in some amphibian oocytes. Within the cloud, many mitochondria appear to be highly elongate and irregular in shape. The cloud begins to fragment and disperse midway through vitellogenesis at about the time when cortical granules appear. In fully grown oocytes, some mitochondria may have a much less dense matrix and fewer cristae than the remainder, which may be related to their state of activity.