Ultrastructural analysis of chorion formation in the silkmoth Bombyx mori

Abstract

The formation of the chorion in the silkmoth Bombyx mori (Lepidoptera) is analysed in detail by means of electron microscopy. The secretion of the proteinaceous chorion layers is being accomplished by the follicular epithelial cells which surround the oocyte. The mature chorion exhibits a tripartite ultrastructure; the trabecular layer closest to the oocyte, the inner lamellar layer, and the outer osmiophilic layer. The trabecular layer is deposited first upon the vitelline membrane and further modified possibly by rearrangement and (or) changes of its constituent proteins. A thin porous (sieve) layer is firmly attached to the tips of the follicle cell microvilli and appears to be implicated in the process of secretion of chorion proteins. In early choriogenesis, chorion is a thin (0.7–6 μm) lamellate and simply organized zone becoming thicker (12–20 μm) and more complex as choriogenesis proceeds. Chorion lamellae consist of helicoidally twisted fibrils embedded in an amorphous matrix. A denaturing agent (urea) was used to derive more information concerning assembly of chorion fibrous lamellae. The agent does not seem to eliminate the lamellar organization of chorion although it does modify it. There are suggestions that the agent disrupts many weak interactions, causing conformational changes in the structure of chorion protein components. The surface of the mature chorion is sculptured by follicle cell imprints. Small holes, the aeropyles, which extend radially into chorion down to the trabecular layer presumably serve for respiratory purposes. An elaborate depression in the anterior pole of the chorion, the micropyle, serves for sperm entry.