Ultrastructure of spinach thylakoids as seen in low-temperature and conventional embeddings

Abstract

Low-temperature embeddings of glutaraldehyde-fixed spinach chloroplasts were obtained by dehydrating the organelles with ethanol and infiltrating them with a Lowicryl resin at −35°C or below. Polymerization was achieved by uv light at the infiltration temperature. In uranyl acetate-stained sections of the embedded material the thylakoids appeared, when studied with the TEM, as broad, light bands separated by thin, dark strata. The central, structureless space, the lumen, present in osmium tetroxide-fixed specimens embedded at room temperature was not observed. However, in thylakoids of chloroplasts infiltrated with Lowicryl at 0 °C, a lumen appeared, consisting of an almost homogeneous dark layer or of a row of dark spots interspersed by light ones. Our results suggest that the width of the lumen in thylakoids of living cells is subelectron microscopical at least under certain physiological conditions, since low-temperature embedding favors a life-like preservation of cell structures.