Structure of yolk granules in oocytes and eggs of Blattella germanica and their interaction with vitellophages and endosymbiotic bacteria during granule degradation

Oocytes and embryos of the cockroach Blattella germanica were examined by optical and electron microscopy to study yolk granule degradation during embryo development. During vitellogenesis, progressively larger yolk granules are formed in the ooplasm and by chorionogenesis, the mature granules are packed so tightly that their shape is highly distorted. Throughout ovarian development, endosymbiotic bacteria lie at the follicle cell/oocyte interface. Just prior to chorionogenesis the endosymbionts transit the oocyte plasma membrane and cluster at the periphery. Bacteria become more numerous over the ventral region of the egg by day 1 postovulation and begin to invade the interior of the yolk mass from the ventral periphery. At that time, lysis of the nearby yolk granules occurs while those in the central ooplasm remain intact and free of bacteria up to day 4. Vitellophages become evident by day 2 postovulation. These cells are also distributed over the egg's periphery but are most numerous in the ventral region. Vitellophages, in association with the endosymbionts, protrude toward the yolk granules and extend filo- and lamellipodia over the granule surface. Portions of the yolk granules are then engulfed and sequestered as large vacuoles in the vitellophage's cytoplasm. The vacuoles then become vesiculated. As embryo development proceeds, the vesiculated portions partition into smaller multivesicular bodies. This study describes the dynamics of yolk granule-vitellophage interaction in embryos of B. germanica and suggests that yolk utilization entails the cooperative efforts of both vitellophages and endosymbiont bacteria.