The ICL is formed by a thin patchwork of contiguous 3D crystals comprised of “chorins” which encapsulate an adjacent, stabilizing wax layer, the vitelline membrane and the unhatched larva in Drosophila eggs. Although the precise function of the ICL is not known, it is though to provide connectivity between the outer endochonrion and the vitelline membrane, thereby helping to anchor these features and allowing the larva to cleanly separate from the egg case during hatching.
The ICL when viewed from the side is comprised of multiple paired layers, with two paired layers forming a 2D crystalline sheet; multiple stacked 2D crystals are aligned to form 3D crystals that are found in patches within the extended structure. (top) Thin section of collapsed empty Drosophila egg case stabilized with tannic acid during embedding. (bottom) Close up view with 3D crystal side view (left) and optical diffraction pattern (right). [Akey & Edelstein, 1987]
(a-c) Stacked and aligned 2D crystalline sheets form patches in the ICL with distinctive 3D crystals that form a range of related crystal polymorphs based on a central chorin octamer with variable 222 symmetry and in-plane lattice dimensions. (d) The overall geometry of the two basic chorin octamer subunits undergoes reciprocal stretching and compaction motions along local 2-fold axes, coincident with the changed dimensions of the larger unit cell, that may be indicative of an underlying mechanism to adjust to mechanical strain or local curvature. [Akey & Edelstein, 1987]
3D structure of isolated 2D crystalline ICL sheets preserved in negative stain reveal the chorin octamer to ~25 Å resolution. The structure was obtained by tilting a single crystal and collecting multiple low dose images. (top left) Reciprocal space plot showing the missing cone, plot based on the tilting method of Unwin and Henderson (1975). (top right and bottom) Contour maps of horizontal sections at different levels along z are shown with density shading added, on stacked perspex sheets imaged on a light box. This approach reveals the overall shapes of the ɑ and β chorin “subunits”, which form an octamer within the 90º crystal form with 222 point group symmetry. [Akey, Crepeau and Edelstein, 1987]
Publications:
Akey CW, Edelstein SJ. The innermost chorionic layer of Drosophila. I. The role of chorin octamers in the formation of a family of interdigitating crystalline plates. (1987). J Mol Biol. 193:673-83. doi: 10.1016/0022-2836(87)90350-0. PMID: 3112410.
Akey CW, Crepeau RH, Edelstein SJ. The innermost chorionic layer of Drosophila. II. Three-dimensional structure determination of the 90 degrees crystal form by electron microscopy. (1987). J Mol Biol. 193:685-92. doi: 10.1016/0022-2836(87)90351-2. PMID: 3112411.
