Ultrastructural study of the tetrathyridium of Mesocestoides corti Hoeppli, 1925: tegument and parenchyma.

The study of dividing and non-dividing tetrathyridia using electron microscopy shows that the mode of multiplication by antero-posterior fission of these larvae is due to a particular tissue which is called the "apical massif". The apical massif is a part of the tegumental syncytium. It is located at the top of the scolex. It represents a polynucleated cell mass which has cytomorphogenetic power. During asexual multiplication, it differentiates into tegumental syncytium, sub-tegumental muscles, glycogen-storing parenchyma cells, and other cell types. Parts of it remain undifferentiated. The hypothetic origin of the apical massif is discussed. Longitudinal growth of the tetrathyridia occurs by invasion of migrating cells into the tegumental syncytium. These cells also originate from the apical massif. During asexual multiplication and longitudinal growth, filamentous microtriches are synthesized below the plasmalemma of the superficial cytoplasm of the tegumental syncytium. It is supposed that the blade-like microtriches derive from filamentous forms.