ULTRASTRUCTURAL FEATURES OF THE ACTINOSPOREAN PHASE OF MYXOSPOREA (PHYLUM MYXOZOA) - A COMPARATIVE-STUDY

Ultrastructural features of the actinosporean phase(A) of the myxozoan life cycle are presented and compared with corresponding structures o f the myxosporean phase (M). In both A and M phases, the same essentia l cell structures can be found : mitochondria, sometimes with plate- o r tubule-like cristae and various inclusions; Golgi in typical or modi fied form; rer; ribosomes, sometimes forming helices of polyribosomes; sporoplasmosomes; phagosomes; various vesicles; reserves such as lipi d bodies and beta-glycogen granules; typical microtubular bundles as r esidues of the mitotic spindle; cell junctions in form of more or less elaborate gap- or septate junctions or desmososmes and pseudopodia-li ke surface projections. Centrioles are absent. There is no essential d ifference in the main myxozoan character, polar capsule formation and structure; the identity includes the 11-12 nm fibers on the surface of the nascent polar filament. Very simple stem cells endowed with small amount of cytoplasm and fragments of rer (= inner cells of the pre- a nd extrasporogonic cycles of M and infectious cells in the A sporoplas m) have the potential to give rise to specialised cells such as sporob last or plasmodial cells with a wide variety of cell constituents. The greatest diversity is found in M sporogonic plasmodia, with a variety of vesicles, fibrillar structures and surface modifications. In both M and A phases, unique modifications of er membranes may be found. Com parison of A and M fine structure reveals an identical structural patt ern and supplies another confirmation of the unity of myxozoan life cy cle. Polar capsules of myxozoa and nematocysts of cnidarians are homol ogous in their structure and way of origin. Septate cell junctions are typical both of cnidaria and of myxozoa. These two features, in addit ion to a plethora of other characters support the postulated kinship o f both groups. The discrepancies in the mode of mitosis, however, warr ant further research of their phylogenetical relationships.