Development of the tegument and alimentary tract in a trematode, Fellodistomum fellis

Abstract

Formation of the tegument and alimentary tract in developing Fellodistomum fellis cercariae has been examined by electron microscopy. Morphogenesis of the tegument is a two-stage event: (1) ectodermal cells at the germ ball periphery coalesce beneath a closely apposed and transitory primitive epithelium of sporocyst origin, forming a syncytial boundary of nucleated tegument. The primitive epithelium is then lost and the nuclei in the surface layer of the early tegument degenerate; (2) cells in the mesenchyme of the cercarial embryo differentiate as secretory cells and connect with the anucleate layer to form the true tegument. The foregut develops as an apical invagination of early nucleated tegument, with the primordial digestive cells differentiating behind the oesophagus. When fully formed, the surface anucleate layer of oesophagus tegument extends posteriorly over the apposed surfaces of the developing digestive cells, thus isolating them from each other and from the gut lumen. An apical cavity develops in each of the digestive cells and later communicates with the gut lumen through a pore in the overlying caecal tegument. Lamellated extensions of digestive cell cytoplasm project into the cavity, and development of GER and a Golgi apparatus in the cell mark the onset of digestive secretion.