Figure 5

Complete sequence of the development of the inner epithelium leading to spore formation in Buddenbrockia plumatellae. Confocal optical horizontal sections. F-actin (green), nuclei (red). Initially, the inner epithelium is formed of a layer of cylindrical cells (A), which then acquire large vacuoles on the basal side, restricting perinuclear cytoplasm to the apical side (B). The cells detach from each other and float freely in the inner cavity (C) and (D). They increase massively in size (E) and (F) and finally undergo cell division, leading to multicellular spores (G) and (H) in mature worms. ie, inner epithelium; mb, muscle block; sc, sporogonic cell; sp, spore.