Ultrastructure of Sporogenesis of Thelohanellus nikolskii Akhmerov, 1955 (Myxozoa: Myxosporea) from the Common Carp, Cyprinus carpio

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Abstract
Plasmodia of T. nikolskii occur in epidermal cysts in the fins of carp (C. carpio) from Hungary. The plasmodium, believed to be a grossly hypertrophied host cell, was limited by a membrane with numerous pinocytotic channels and associated vesicles. The peripheral ectoplasmic zone of the plasmodium contained large vesicular nuceli, mitochondria, Golgi apparatus, smooth endoplasmic reticulum, and lipid inclusions. Among these organelles and extending into the central region of the plasmodium were various stages of sporogenesis. These consisted of single and associated generative cells and sporoblasts ranging from the 2 cell stage of pansporoblasts containing mature spores. In young sporoblasts, the development of the 2nd capsulogenic cell was abortive resulting in the formation of a single polar capsule in each of the 2 spores. The capsular primordium and external tubule developed simultaneously and the rudimentary polar filament appeared in both structures. The external tubule was withdrawn into the capsule by a process of invagination and the mouth of the mature polar capsule was plugged by a corklike stopper. Posterior to the capsulogenic cell was a typical binucleate sporoplasm. The 2 halves of the spore wall were formed by the flattened valvogenic cells that were joined around their circumference by a long, continuous junctional apparatus which was located in a flangelike projection. The outer edge of the flange contained up to 50 microtubules in each valvogenic cell. The junctional apparatus was reinforced by dense fibrillar structures. The components of the tripartite wall of mature spores were formed in the valvogenic cell.