Fine structure of uterus and non-functioning paruterine organ in Orthoskrjabinia junlanae (Cestoda, Cyclophyllidea).

Some cyclophyllidean cestodes provide protection for their eggs in the external environment by providing them with additional protective layers around the egg membranes. In attempting to examine such adaptations, the microanatomy and fine structure of the uterus of pregravid and gravid proglottids of the cyclophyllidean cestode Orthoskrjabinia junlanae, a parasite of mammals that inhabit a terrestrial but moist environment, were studied. In the initial stages of uterine development, developing embryos locate freely in the lumen of a saccate uterus that later partitions into chambers. Each chamber that forms encloses several embryos. The chambers are surrounded by muscle cells that synthesize extracellular matrix actively. The paruterine organs consist of stacks of flattened long outgrowths of muscular cells, interspersed with small lipid droplets. In the gravid proglottids, the size of paruterine organ increases and consists of flattened basal and small rounded apical parts separated by constrictions. The fine structure of the organ wall remains the same: sparse nuclei and stacks of flattened cytoplasmic outgrowths but internal invaginations or lumen in the paruterine organ are absent. Completely developed eggs remain localized in the uterus. Based on the comparative morpho-functional analysis of uterine and paruterine organs and uterine capsules in cestodes, we conclude that these non-functioning paruterine organ in O. junlanae is an example of an atavism. We postulate that the life cycle of the parasite, which infects mammals living in wet habitats, where threats of desiccation of parasite ova is reduced, has favoured a reversion to a more ancestral form of uterine development.