The egg envelopes of Polymorphus minutus (Acanthocephala)

Abstract

The series of egg envelopes around the acanthor in the mature egg of the palaeacanthocephalan, Polymorphus minutus, has been investigated with light and electron microscopy. An attempt has been made to characterize the macromolecules from which the envelopes are constructed using histochemistry, an analysis of stabilizing bond types and comparisons of envelope fine structure with that of other structural protein systems. The outermost envelope (I) of the mature Polymorphus egg is a fertilization membrane. It is produced by membrane-bounded inclusions in the cytoplasm of the unfertilized oocyte which discharge their contents into the extracellular space around the egg. Envelope II is composed largely of keratin, and its subzones consist of different packing arrangements of 8 nm diameter electron-lucid filaments in an electron-dense matrix. The electron microscopical appearances which this filamentous keratin can present due to section orientation artefacts are discussed. The innermost envelope (III) is 20–30 nm thick and highly corrugated. The spaces between envelopes I and II and between envelope III and the inner surface of envelope II contain material with a granular or fibrous ultrastructure. These spaces show histochemical evidence for the presence of acid mucopolysaccharides.