title: Hierarchical assembly of the siliceous skeletal lattice of the hexactinellid sponge Euplectella aspergillum.
abstract: Despite its inherent mechanical fragility, silica is widely used as a skeletal material in a great diversity of organisms ranging from diatoms and radiolaria to sponges and higher plants. In addition to their micro- and nanoscale structural regularity, many of these hard tissues form complex hierarchically ordered composites. One such example is found in the siliceous skeletal system of the Western Pacific hexactinellid sponge, Euplectella aspergillum. In this species, the skeleton comprises an elaborate cylindrical lattice-like structure with at least six hierarchical levels spanning the length scale from nanometers to centimeters. The basic building blocks are laminated skeletal elements (spicules) that consist of a central proteinaceous axial filament surrounded by alternating concentric domains of consolidated silica nanoparticles and organic interlayers. Two intersecting grids of non-planar cruciform spicules define a locally quadrate, globally cylindrical skeletal lattice that provides the framework onto which other skeletal constituents are deposited. The grids are supported by bundles of spicules that form vertical, horizontal and diagonally ordered struts. The overall cylindrical lattice is capped at its upper end by a terminal sieve plate and rooted into the sea floor at its base by a flexible cluster of barbed fibrillar anchor spicules. External diagonally oriented spiral ridges that extend perpendicular to the surface further strengthen the lattice. A secondarily deposited laminated silica matrix that cements the structure together additionally reinforces the resulting skeletal mass. The mechanical consequences of each of these various levels of structural complexity are discussed.
authors.affiliation: Department of Molecular, Cellular and Developmental Biology, Institute for Collaborative Biotechnologies, and the Materials Research Laboratory, University of California, Santa Barbara, CA 93106, USA.
authors.forename: James C
authors.initials: JC
authors.lastname: Weaver
authors.forename: Joanna
authors.initials: J
authors.lastname: Aizenberg
authors.forename: Georg E
authors.initials: GE
authors.lastname: Fantner
authors.forename: David
authors.initials: D
authors.lastname: Kisailus
authors.forename: Alexander
authors.initials: A
authors.lastname: Woesz
authors.forename: Peter
authors.initials: P
authors.lastname: Allen
authors.forename: Kirk
authors.initials: K
authors.lastname: Fields
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Porter
authors.forename: Frank W
authors.initials: FW
authors.lastname: Zok
authors.forename: Paul K
authors.initials: PK
authors.lastname: Hansma
authors.forename: Peter
authors.initials: P
authors.lastname: Fratzl
authors.forename: Daniel E
authors.initials: DE
authors.lastname: Morse
mesh_terms: D000818:Animals; D008854:Microscopy, Electron; D011161:Porifera / Q000737:chemistry* / Q000648:ultrastructure*; D012822:Silicon Dioxide / Q000737:chemistry*; D012863:Skeleton
chemical_list: D012822:Silicon Dioxide