Fig. 2: The morphogenesis of guanine crystals in a juvenile scallop eye by cryo-SEM (left column) and TEM (right column).

a, b Pre-crystallization: a Spherical vesicles containing small type i intraluminal vesicles and some disordered fibrils. Insert; high magnification image of a type i intraluminal vesicle. b Ellipsoidal iridosomes are characterized by two intraluminal fibrils stretching across the length of the vesicle. Ellipsoidal iridosomes prior to crystallization are characterized by two types of intraluminal vesicles. Type ii ILVs shown here, are bound to the intraluminal fibers. Type iii ILVs are typically unbound to the fibrils and are shown in Supplementary Fig. 5. c–e Nucleation and growth: c As morphogenesis proceeds, the iridosomes elongate and a small guanine crystal nucleates between the fibrils. The thin immature crystals are elongated along the H-bonded direction and exhibit an electron diffraction pattern characteristic of β-guanine (insert). d The intraluminal fibrils template the growth of the crystal along the H-bonded direction. The fibrils become tightly bound to the (100) face of the crystals. e Mature crystals stretch and re-shape the iridosome membrane, which condenses on the surface of the faceted crystal. Yellow arrows; intraluminal fibrils. The juvenile scallops were approximately 2 cm in diameter, with eyes averaging 200 µm in diameter. Scale bars: 100 nm.