Morphogenesis of silkmoth chorion: Initial framework formation and its relation to synthesis of specific proteins

Morphogenesis of the silkmoth eggshell is described at the ultrastructural level. Four zones are each assembled in a distinct manner and during a distinct developmental period: the innermost vitelline membrane and the adjacent trabecular layer appear consecutively, followed by a thin sieve layer, and a thick, lamellate chorion. Once formed, the sieve layer remains attached to microvilli, and thus all components which assemble into lamellae must pass through the sieve layer. Initially, lamellogenesis (and sieve layer formation) occurs in patches overlying trabeculae. Lamellae quickly fuse and new ones are added, presumably by apposition. Distinct types of lamellae seen in the mature chorion are already distinguishable in early lamellogenesis. The final lamellar number is attained before the developing chorion is one-half its final thickness or one-fifth its final dry weight. The early lamellae constitute a framework which is subsequently modified through expansion and densification. Proteins which may represent components of various parts of the eggshell have been identified on the basis of their timing of synthesis, relative amino acid compositions, and spatial distributions within the chorion.