The eggshell ofDrosophlla melanogaster : VII. Formation of the micropylar canal and the role of the paracrystalline structure.

The micropylar apparatus (MA) inDrosophila melanogaster (Diptera: Drosophilidae) is a 25 μm long protrusion on the anterior pole of the eggshell, and it contains the 0.8 μm wide micropylar canal through which the spermatozoon penetrates the eggshell. The canal terminates in the paracrystalline structure thus forming the "pocket". The MA is secreted during oogenesis by the micropylar follicle cells (border cells and peripheral cells). Morphogenesis of the micropylar canal starts before stage 11A, when two of the border cells form two thin extensions containing microfilaments and penetrate the paracrystalline structure. Microtubules found at the base of the extensions participate in the formation of two projections. Adjacent follicle cells secrete the chorionic part of the MA, whilst the two projections elongate and twist during stages 12 and 13. Microtubules run parallel to the long axis of the projections and probably are related to cellular elongation during the formation of the projections. The paracrystalline structure is composed of lamellae having a periodicity of about 50 nm, which is maintained possibly due to the lateral fibrils connecting the lamellae. These lamellae consist of small fragments secreted during stages 10-11 by the border cells. The paracrystalline structure and the spongy vitelline membrane are condensed after egg maturation.