Surface ultrastructure of Plagiorchis muris growth and developmental stages in rats, the final host.

In the excysted metacercaria, the whole fluke surface was covered with peg-shaped tegumental spines. Ciliated sensory papillae (type I) were concentrated around the oral sucker, and non-ciliated sensory papillae (type II) were aligned on the lip of oral and ventral suckers. Several type II papillae were aligned laterally with linear symmetry between the oral and ventral suckers. In juvenile flukes, cytoplasmic processes were band-shaped on the anterior half of the body surface and velvety on the posterior half. In adult flukes, cytoplasmic processes were differentiated into velvety processes, and densities of tegumental spines were reduced on the posterior half of the body. Flukes grew to be elongated and leaf-like as adults and retained a surface ultrastructure that was similar to that of juveniles in terms of the distribution of tegumental spines and sensory papillae. From the above results, the authors suppose that the marked differentiation of cystoplasmic processes and the reduced density of tegumental spines observed on the posterior half of the body surface are closely related to the development of reproductive organs therein.