The comparative ultrastructure of the plerocercoid and adult primary scolex of Haplobothrium globuliforme Cooper, 1914 (Cestoda: Haplobothrioidea)

Abstract

Plerocercoids of Haplobothrium globuliforme recovered from the liver of Ictalurus nebulosus and adult cestodes from the intestine of Amia calva were examined using light and electron microscopy. Both plerocercoids and adults have four eversible proboscides on the scolex. Using light microscopy, a lumen is evident in the centre of each proboscis, through which the proboscis retracts during inversion into the muscular sacs contained within the scolex. Each proboscis contains numerous glandular sacs. Staining with Heidenhain's azan indicates that cells in the neck region of the cestode probably produce the secretory material for these glandular sacs. Ultrastructurally, the tegument of plerocercoid and adult scolices are remarkably similar. The distal tips of each proboscis are covered with short microtriches (0.25 μm long) which have poorly defined electron-dense tips. Nearer the base of the proboscides, the tegumental microtriches elongate and bear delicate, slender tips. Around the base of the proboscis, these microtriches are interspersed with robust spinelike microtriches (6.0 μm long) which have a clublike tip and which are anchored into the tegument by cup-shaped roots. Throughout the entire length of the proboscis, subtegumental glandular sacs are present. These have been seen to fuse with the tegumental cytoplasm. Covering the lateral surfaces of the scolex are the slender, delicate microtriches interspersed with angular, robust microtriches (2.0 μm long). The remainder of the scolex and neck regions are covered with small peglike microtriches which are somewhat longer in the adult (1.0 μm) than in the plerocercoid (0.8 μm). The tegumental cytoplasm in the neck region of the plerocercoid has a stratified appearance, with membrane-bound electron-lucent vesicles occupying the distal portion, and lamellar bodies and large mitochondria forming a layer near the inner plasma membrane. The adult tegumental cytoplasm lacks this stratification and contains electron-dense discs and occasional electron-lucent vesicles. The plerocercoid of H. globuliforme appears to be the developmental stage that preadapts the cestode for its adult existence in the definitive host's intestine.