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The surface of thin sections of aldehyde-fixed biological material shows a specimen-related relief of 2-6 nm with Lowicryl. Epon sections are about three times smoother. The relief is the consequence of thin-sectioning being in reality a cleavage. Epitopes are supposed to be laid open (or set free) because cleavage follows the interfaces between protein and Lowicryl. We have developed a simple theory on this basis and have theoretically estimated the efficiency of on-section labeling and compared it with experimental data. For randomly dispersed proteins in cytoplasm, Lowicryl sections will yield significant label only when the concentration of the antigen is about 10 microM or more. The complex situation of more compact proteins, as represented by fibers, sheets, and biological membranes is discussed and the difficulty of significant calculations is explained. Pre-embedding labeling and melted cryosections should give 10-30 times more label. The possible reasons for the observed much smaller gain of not more than two to three times are discussed.

The shape and size of the bacteriophage T4 head are dependent on genes that determine the scaffolding core and the shell of the prohead. Mutants of the shell proteins affect mainly the head length. Two recently identified genes (genes 67 and 68) and one already known gene (gene 22), whose products are scaffold constituents, have been investigated. Different types of mutants were shown to strongly influence the proportion of aberrantly shaped particles. By model building, these shape variants could be represented as polyhedral bodies derived from icosahedra, through outgrowths along different polyhedral axes. The normal, prolate particle is obtained by elongation along a fivefold axis. The mutations of the three core genes (genes 67, 68, and 22) affect the width mainly by lateral outgrowths of the prolate particle, although small and large isometric particles are also found. Many of the aberrant particles are multitailed, suggesting a correlation between tail attachment sites and shape.