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Summary
The marine acoel flatworm Childia groenlandica (phylum Platyhelminthes, order Acoela) has been examined by electron microscopy in order to elucidate the interactions that occur between developing oocytes and their associated somatic cells during oogenesis. In mature worms the female germ cells form an elongated mass on either side of the body. Previtellogenic oocytes at the anterior end of each germinal mass measure 10–20 μm in diameter and lack a complete sheath of enveloping somatic cells. More posteriorly, oocytes begin to produce yolk granules and become fully covered by somatic cells.
The surrounding somatic cells contain a well developed rough endoplasmic reticulum and form complex interdigitations with vitellogenic oocytes. As vitellogenesis proceeds, somatic cells secrete material in the direction of the vitellogenic oocytes, and the peripheral ooplasm of the oocytes exhibits coated pits as well as putative endocytotic vacuoles. During the later stages of vitellogenesis, small clumps of electron-dense material can be seen fusing with developing yolk granules. Collectively these ultrastructural observations suggest that yolk granules formed in the ooplasm are supplemented with vitellogenic precursors produced by the surrounding somatic cells. Such a heterosynthetic contribution to yolk formation would constitute the first example of a non-autosynthetic type of vitellogenesis in a flatworm that produces entolecithal eggs. The possible phylogenetic significance of these observations is discussed.
