Vitellocytes and vitellogenesis in cestodes in relation to embryonic development, egg production and life cycle

Vitellocytes have two important functions in cestode embryogenesis: (1) for mation of hard egg-shell (e.g. Pseudophyllidea) or a delicate capsule (e.g, Cyclophyllidea), and (2) supplying nutritive reserves for the developing e mbryos. During evolution any of these two functions can be reduced or inten sified in different taxa depending on the type of their embryonic developme nt, degree of ovoviviparity and life cycles. Within the Cestoda, there are three monozoic taxa with only one set of genital organs: Amphilinidea, Gyro cotylidea and Caryophyllidea. In these monozoic taxa and some polyzoic grou ps with well developed vitellaria (e.g. Pseudophyllidea, Trypanorhyncha) a single oocyte [ = germocyte] and a large number of vitellocytes (up to 30) are enclosed within a thick, hardened egg-shell, forming a type of eggs typ ical for the basic pattern of Neodermata. Only one type of egg-shell enclos ures, the so-called 'heterogeneous shell-globule vesicle' is common for the above mentioned cestode taxa. Each membrane-bounded vesicle of mature vite llocytes contains numerous electron-dense shell globules embedded in a tran slucent matrix. In free-living Neoophora and Monogenea there are two types of vesicles with dense granules; the second is considered to be proteinaceo us reserve material. Within the Cestoda, the numbers of vitellocytes per ge rmocyte are reduced in those taxa forming eggs of the 'Cyclophyllidean-type ' (e.g. Cyclophyllidea, Tetraphyllidea, Pseudophyllidea). This is particula rly evident in Cyclophyllidea, for example, in vitellocytes of Hymenolepis diminuta (Hymenolepididae) there are numerous vitelline granules of homogen eously electron-dense material: in Catenotaenia pusilla (Catenotaeniidae) t here are three large, homogenous vitelline vesicles. while in Inermicapsife r madagascariensis (Anoplocephalidae) there is only one large vitelline ves icle, containing homogeneously electron-dense material, which occupies most of the vitelline cell volume. In this respect the Tetraphyllidea and Prote ocephalidea. in forming eggs that lack a hard egg-shell, hold an intermedia te position. A comparison of interrelationships which exist among types of vitellocytes, vitellogenesis, types of embryonic development, ovoviviparity and life cycles indicates parallelisms and analogies in adaptation to the parasitic way of life in different groups of cestodes. Knowledge on cestode vitellogenesis may also have an important applied aspect. Vitellocytes, du e to their high metabolic rate, represent a very sensitive target for analy sing effect of anthelminthic drugs upon the egg formation (ovicidal effects ). rapid degeneration of vitellocytes is usually accompanied by a cessation of egg production. (C) 2000 Australian Society for Parasitology Inc. Publi shed by Elsevier Science Ltd. All rights reserved.