The lipoprotein crystals of cyclostome yolk platelets ( Myxine glutinosa L., Lampetra planeri [Bloch], L. fluviatilis [L.

Abstract

The crystal-like main portion of the yolk platelets in a myxinoid ( Myxine glutinosa L.) and two petromyzonoids ( Lampetra planeri [Bloch], L. fluviatilis [L.]) is organized according to a monoclinic lattice (averaged unit cell: a = 20.0 nm, b = 9.0 nm, c = 9.2 nm, β = 107°, V = 1580 nm 3 ) as results from an electron diffraction study of material fixed with glutaraldehyde-OsO 4 , embedded in glutaraldehyde-urea resin, and double-stained in thin sections with uranyl acetate and lead citrate. The packing unit (there are two such per unit cell above) appears to be a thorn apple-like microchamber (approximately 10 × 10 × 10 nm 3 ) with an unestablished internal organization. The crystal can be interpreted as being a close-packed structure of such chambers forming layers in various spatial orientations; this interpretation results from the analysis of electron micrographs of crystals (11 indexed projections reproduced) and freeze-fracture replicas. This architecture of the yolk platelet must have been conserved in cyclostomes for 250 million years and its physiological relevance is seen in rendering the yolk platelet particularly suitable for electrolyte storage. Electron-probe microanalysis of frozen-dried cyclostome yolk platelets has, in fact, revealed the presence of Na, Mg, K, Ca, and Cl (!)—mostly absent following washing the platelets in distilled water—in addition to covalently bound P and S.