Hymenolepis microstoma Cysticercoid Activation and Excystation In vitro (Cestoda)

Abstract

Excystation was studied in vitro and the events of excystation in this species were histologically determined and compared with previous reports for Hymenolepis diminuta. Occlusion of the anterior canal during early cysticercoid development in H. microstoma necessitates a method of excystation different from that of H. diminuta in which the anterior canal remains open and provides egress for the larval scolex. Supplementary studies of the qualitative and quantitative effects of various artificial digestive solutions on H. microstoma activation and excystation are included. A possible protective function of the cysticercoid wall during passage through the mouse stomach is discussed in conjunction with histological studies of the priming effect of artificial gastric juice. Natural infections of Hymenolepis microstoma (Dujardin, 1845) occur when mice eat arthropods harboring infective cysticercoids. Activation of the larval scolex and excystation occur in the duodenum subsequent to gastric dissolution of outer cysticercoid membranes (Dvorak, Jones, and Kuhlman, 1961). The adult tapeworm becomes established in the bile duct. Consequently, studies concerning the effects of physicochemical conditions of the vertebrate alimentary canal on cysticercoids contribute to an understanding of both cysticercoid wall function and host specificity. Rothman (1959) employed in vitro techniques to resolve responses of cyclophyllidean larvae to artificial digestive juices. Goodchild and Harrison (1961) studied histologically the excystation of Hymenolepis diminuta cysticercoids and reported that the scolex emerged via a patent anterior canal. We have examined excystation of Hymenolepis microstoma cysticercoids in vitro and have investigated the histology of excystation. In this species, the anterior canal of the cysticercoid is occluded during early development, and therefore it requires a method of excystation different from H. diminuta. MATERIALS AND METHODS Hymenolepis microstoma adults were maintained in inbred C3H/Crgl mice, and cysticercoids were established in adult Tribolium confusum by exposure for 72 hr to macerated, gravid proglottids. Before feeding, proglottids were stored overnight in 0.85% NaCI at 4 C (Collings and Hutchins, Received for publication 7 December 1971. 735 1965). Flour beetles were starved for 10 days prior to exposure and subsequently maintained on rolled oats at 22 to 25 C until used. Depending upon the particular experiment, cysticercoids ranging in age from 25 to 50 days were recovered by dissection in 0.85% saline. Artificial digestive media, prepared immediately prior to use, consisted of 0.5% (w/v) solutions of pepsin (Merck and Co., Inc.), trypsin (Fisher Scientific Company), sodium taurocholate (Nutritional Biochemicals Corporation), or trypsinsodium taurocholate combined in 0.85% NaCl. The pH of these solutions was adjusted with HC1 or NaHCO3 to the desired value. For quantitative studies of excystation, larvae were incubated in various digestive media at 37 C, and counts of excystation events were made using a dissecting microscope with a stage temperature of 37 C. Larvae were considered excysted when the entire scolex was free of the cyst wall. Detailed observations of excystation in living material were made with a compound microscope equipped with a stage incubator (Lab-Line; C. S. and E. Incu-Stage) adjusted to 37 C. Cysticercoids for histological study were incubated in the various digestive fluids at 23 C and were ultimately fixed in Zenker's fluid and embedded in paraffin. Specimens were serially sectioned at 5 Au and stained with Harris's hematoxylin and Triosin or with Mallory's triple stain (Galigher and Kozloff, 1964) for selective staining of fibrous elements (Voge, 1963).