Observations on the Histochemistry of Parastrigea mexicanus (Strigeidae: Digenea) with Emphasis on Egg Shell Formation

Abstract

Egg shell formation for Parastrigea mexicanus is described. Eleven histochemical tests were used to elucidate the process and to characterize the oogenotop. The egg is formed in the ootype (a region of high secretory activity); an oocyte and 12-17 vitelline cells are enclosed by a coalescence of vitelline shell globules. The vitelline cells also contribute copious amounts of glycogen and RNA. There was no direct histochemical evidence for Mehlis or uterine gland function; however, secretory functions were inferred from morphology and histochemical reactions. The basic mechanism of egg shell formation in P. mexicanus closely follows that of Fasciola, but it differs in some regards from the process described for the latter trematode. Helminthologists have studied egg shell formation in trematodes for 82 years now and their efforts have concentrated on widely separated taxa. In spite of this long period of time and the many investigators involved, only a few of the more than 100 families of digenetic trematodes have been studied in this regard. Our approach has been a comparative one and to date, we have completed studies on representatives from the following families: Fellodistomidae, Hasstilesiidae, Hemiuridae, Microphallidae, Notocotylidae, Pronocephalidae, and Syncoelidae. Fasciola hepatica has received the greatest attention; Haematoloechus has been studied in some detail also. Egg shell formation in the digenetic trematodes appears to follow a generalized mechanism which has been elucidated by a number of authors and reviewed by Smyth & Clegg (1959). Since that date, a number of papers have described variations on the central theme. Recently, Ebrahimzadeh (1966) studied 27 species (from ten families) on a comparative basis and he thus established a standard of excellence for students of egg shell formation to follow. During the course of other studies on parasites of the black-necked stilt, the author collected a large number of Parastrigea mexicanus Coil, 1955 (Strigeidae). Although the strigeoids have received much attention concerning their life cycles, it has been only recently that details of their morphology have been studied; many of these have emphasized the holdfast organ (Erasmus, 1968; and others). There are certain striking features of these worms which warrant further studythe large amount of tissue devoted to vitelline production, the extensive Mehlis gland tissue, the extremely large egg, and the nature of the uterus. P. mexicanus provides excellent material for study. The worm is short, compact, and easily handled; sectioned material can be prepared without difficulty. Microdissections are difficult, however, due to the muscular body wall and the heavy cuticle. This is the fifth report in our series of comparative studies on egg shell formation in digenetic trematodes. 1This investigation was supported by Public Health Service Grants AI 05145 and GM 12587. 2The author is indebted to Dr. Clarence Cottam and Mr. Caleb Glazener, both of the Welder Wildlife Foundation, for their help in providing facilities, equipment, and supplies for the early parts of this study. TRANS. AMER. MICROSC. Soc. 88(1): 127-135. 1969. This content downloaded from 207.46.13.21 on Tue, 27 Sep 2016 05:13:10 UTC All use subject to http://about.jstor.org/terms 128 TRANS. AMER. MICROSC. SOC., VOL. 88, NO. 1, JANUARY 1969