American Journal of Anatomy

doi: 10.1002/aja.1001300101pmid: N/A

Rebhun, L. I.; Sander, G.

doi: 10.1002/aja.1001300102pmid: 4322098

Eggs of the marine clam Spisula solidissima were examined by the technique of freeze‐substitution. Morphology of the cytoplasm was investigated as a function of cellular water content using osmotic means to control water concentration. In eggs frozen in less than double strength sea water, ice crystallization usually occurred throughout the cell. In eggs frozen in double to triple strength sea water, 10–75% of the eggs were devoid of ice crystals in the cytoplasm and in triple strength sea water, also in the nucleus. From these results and from a detailed analysis of cells in which both ice crystals and ice crystal‐free regions occurred it is concluded that cells or parts of cells may be frozen free of ice crystals if their free water content is reduced. Reduction in free water content may occur in several ways: deliberate dehydration by osmotic means; accidental dehydration by evaporation of water from cells prior to freezing; osmotic dehydration of one region by an adjacent one in which ice crystallization is in progress. In ice crystal‐free cells, all common cell inclusions could be recognized.

Rebhun, L. I.; Sander, G.

doi: 10.1002/aja.1001300103pmid: 4322097

Studies of ice crystal‐free eggs of the clam Spisula solidissima 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 Å particles, which are not ribosomes, occur regularly arranged along microtubules.

Rebhun, L. I.; Sander, G.

doi: 10.1002/aja.1001300104pmid: 4322099

Eggs of the clam Spisula solidissima were studied by freeze‐substitution techniques at the time of the first meiotic division. The mitotic apparatus is organized into regions containing microtubules, endoplasmic reticulum, ribosomes and mitochondria which surround densely stained amorphous regions. The amorphous regions, which appear to be channels running through the mitotic apparatus, are radially arranged in the asters and are parallel to the spindle axis. Microtubule‐associated particles, each about 250 Å in diameter, are arranged in crystalline arrays around the microtubules of the mitotic apparatus. After a period of time in hypertonic sea water the mitotic apparatus disappears and the microtubule‐associated particles remain aggregated in hexagonally packed crystals. Histochemical tests suggest that the particles are composed of polysaccharide material. Other particles and inclusions show orientation relative to the mitotic apparatus axis or show non‐uniform localization in the egg. The possible meaning of such localization and orientation in the physiology of the egg is discussed.

Youson, John H.; McMillan, Donald B.

doi: 10.1002/aja.1001300105pmid: 4322100

The presence of microvilli and well‐developed organelles in the cells of the non‐ciliated intermediate segment of the ammocoete nephron suggests that this is not simply a conducting tubule but may be involved in the formation of urine. This region more closely resembles the thin limb of the loop of Henle of mammals than the intermediate segment of lower vertebrates.

Ross, Muriel D.; Burkel, William

doi: 10.1002/aja.1001300106pmid: 5540217

The rat acoustic nerve is separated into central and peripheral portions by an astrocytic glial dome which is convex peripheralward. The long central portion is of typical central nervous system structure with narrow extracellular space (100–200 Å in width), oligodendrocytes and astrocytes. The glial dome is penetrated by acoustic nerve fibers at a node of Ranvier; the basal lamina of the astrocytes is reflected back over the peripheral Schwann cells at this site. Centrally, the myelin is thinner than peripherally.

Martinez, I. Ricardo; Peters, Alan

doi: 10.1002/aja.1001300107pmid: 5540218

Membrane‐coating granules are found in close association with the Golgi apparatus, and their apparent size, shape and internal substructure varies with the plane of section.