Ultrastructural investigation of NI3S2-induced rhabdomyosarcoma in Wistar rat: comparative study with emphasis on myofibrillar differentiation and ciliar formation.

Nickel-sulfid-induced rhabdomyosarcomas were studied by both light and electron microscopy. The successive stages differentiating tumor cells were described, and two differentiation types of rhabdomyoblasts could be observed 1) with the characteristic pattern of fetal differentiation--i.e., myofilament apparation before Z-line formation--and 2) with synthesis of these elements in the reverse order. An organized T-system is not evident. The sarcoplasmic reticulum is irregular and its cisternae often contain a granular substance or microcrystals. The only well-developed element of tumoral myofibrils is the Z-line; the other zones of sarcomeres are seldom clearly defined. Several unusual granular structures were observed. No virus particles were found. The formation of cilia appears only in interphase rhabdomyoblasts and has to be considered as aberrant and temporary formations from centrioles. They generally possess a "9 + 0" microtubular pattern, but often could be observed as rudimentary forms with a "7 + 2" microtubular arrangement. This case is another example demonstrating the relationship of cilia formation with cell division, especially after suppression of mitotic control. The histology and the electron microscopy results are discussed in relation to the differentiation pattern and the ultrastructural features of embryonic, regenerating and pathological muscle differentiating in vitro.