Electron microscope studies of frozen‐substituted marine eggs. II. Morphology of ice crystal‐free unfertilized eggs

Abstract

Studies of ice crystal-free eggs of the clam Spisulu solidissirnu using freeze-substitution techniques are described. Formation of ice in cells may be avoided by removing sufficient intracellular water prior to freezing. After such treatment, most subcellular inclusions can be recognized and have dimensions similar to those seen after chemical fixation. Golgi regions, ribosomes, endoplasmic reticulum, nuclear envelope, annulate lamellae, yolk and the vitelline membrane resemble their counterparts in chemically fixed eggs. Mitochondria, however, lack the space normally found between cristae walls, and the resulting structure resembles two unit membranes "back to back." Several inclusions and structures not seen in chemically fixed eggs are described: light bodies are found in groups of two to eight near the Golgi regions; cores occur in the microvilli of the vitelline membrane; microtubules are found in the cytoplasm of the unfertilized egg and 220 to 250 A particles, which are not ribosomes, occur regularly arranged along microtubules.