Ovariole Sheath Structure and Its Relationship With Developing Embryos in a Parthenogenetic Viviparous Aphid

Couchman, J. R., King, P. E. 1980. Ovariole sheath structure and its relationship with developing embryos in a parthenogenetic viviparous aphid. (Zoology Department, University College, Swansea, U.K.) — Acta zool. (Stockh.) 61(3): 147–155. The ovariole sheath of the viviparous parthenogenetic aphid Brevicoryne brassicae has been the subject of an ultrastructural examination, particularly with regard to its relationship with embryos at various stages of ontogeny. The sheath is a unilaminar, squamous epithelial layer enclosing germarium and embryos, but does not have an intrinsic trophic role until the connection between the embryo and germarial trophocytes, in the form of the trophic cord, is lost. At this time there is a degree of thickening and an increase in the number of cell organelles with ultrastructural indications of metabolic activity, a situation paralleled in the embryonic serosa. There is also evidence that haemolymph components may have direct access to the embryos through intercellular spaces in the sheath. Intercellular junctions, microtubules and cell overlap in regions of the sheath adjacent to embryos at a more advanced state of development are all implicated in a mechanism which resists deformation, maintains sheath integrity and allows embryo passage. A tunica propria and microfilament bundles are absent in the ovariole sheath of this species.