Figure 41.

Nakahara’s³²⁵ TEM study of the growing surface of columnar nacre from two gastropods. The samples were carefully fixed with a buffered 5% glutaraldehyde solution, and then embedded in araldite to preserve the delicate structure of the growing surface. In images 4–7, the mineral was removed with EDTA, which then appears as white voids in the electron micrographs. (a) Low power TEM (decalcified and stained with uranyl acetate), showing three nacreous stacks that had a cut through the top crystals (T) in Turbo. (b) High power TEM of the top of the crystal stacks. A delicate network of organics can be seen in the space between the pre-existing stacks, as well as multiple layers of packed sheets at the top membrane. The newly formed crystals at the top of the stack (T) appear to be pushing up the membrane as the sheets become separated into a new compartment. (c) Unstained section cut vertically to the growth surface of nacre of Tegula shows stacks of crystals (C), with interlamellar organic sheets (S), and a dense surface sheet (D). (d) Enlarged view of left image in b) shows the emergence of some form of mineral in the newest forming compartments. Nakahara indicates that “the growing crystals (C) and top crystals are always surrounded with envelope (E).” Rather than assuming this envelope is arising from specific protein adsorption, I suggest that this could simply be condensed organic matter as the mineral phase penetrates into the dense organic matrix, and as it crystallizes, it will further exclude the polymeric impurities such that they become concentrated at the surfaces and edges of the forming tablets. (Reprinted with permission from ref 325. Copyright 1979 Malacological Society of Japan.)