Abstract
The origin, differentiation and functional ultrastructure of oncospheral or egg envelopes in Echinococcus multilocularis Leuckart, 1863 were studied by transmission electron microscopy (TEM) and cytochemistry. The purpose of our study is to describe the formation of the four primary embryonic envelopes, namely vitelline capsule, outer envelope, inner envelope and oncospheral membrane, and their transformation into the oncospheral or egg envelopes surrounding the mature hexacanth. This transformation takes place in the preoncospheral phase of embryonic development. The vitelline capsule and oncospheral membrane are thin membranes, while the outer and inner envelopes are thick cytoplasmic layers formed by two specific types of blastomeres: the outer envelope by cytoplasmic fusion of two macromeres and the inner envelope by cytoplasmic fusion of three mesomeres. Both outer and inner envelopes are therefore cellular in origin and syncytial in nature. During the advanced phase of embryonic development, the outer and inner envelopes undergo great modifications. The outer envelope remains as a metabolically active layer involved in the storage of glycogen and lipids for the final stages of egg development and survival. The inner envelope is the most important protective layer because of its thick layer of embryophoric blocks that assures oncospheral protection and survival. This embryophore is the principal layer of mature eggs, affording physical and physiological protection for the differentiated embryo or oncosphere, since the outer envelope is stripped from the egg before it is liberated. The embryophore is very thick and impermeable, consisting of polygonal blocks of an inert keratin-like protein held together by a cementing substance. The embryophore therefore assures extreme resistance of eggs, enabling them to withstand a wide range of environmental temperatures and physicochemical conditions.
Highlights
In the family Taeniidae, the genus Echinococcus includes species of great medical and veterinary importance, causing the important zoonotic infections cystic and alveolar echinococcosis
Echinococcus multilocularis is the zoonotic agent of human alveolar echinococcosis or alveolar hydatid disease
The purpose of the present study is to describe the origin, differentiation and functional ultrastructure of the oncospheral envelopes surrounding the eggs of Echinococcus multilocularis and to compare the results with published data on taeniids and other cyclophyllideans
Summary
In the family Taeniidae, the genus Echinococcus includes species of great medical and veterinary importance, causing the important zoonotic infections cystic and alveolar echinococcosis. Echinococcus multilocularis is the zoonotic agent of human alveolar echinococcosis or alveolar hydatid disease. Human infections with the metacestode of E. multilocularis result in alveolar hydatid disease which is still very common in different countries. Comparative study has been very difficult largely because of the lack of uniform terminology used by various authors. As early as 1966, Rybicka was trying to review and compare different terms used by various authors for the same oncospheral and egg envelopes (see Table 1 of Rybicka 1966). Very recently, the problem of standardised terminology of embryonic envelopes of tapeworm was reviewed and updated by Conn and Świderski (2008) and is used in the present study
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