ELECTRON MICROSCOPIC OBSERVATIONS ON THE SUBMICROSCOPIC MORPHOLOGY OF THE MEIOTIC NUCLEUS AND CHROMOSOMES

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Abstract
Thin sections of the testicular follicles of the grasshopper Laplatacris dispar were studied under the electron microscope. In the primary spermatocytes, during meiotic prophase, 3 main regions can be recognized within the nucleus the nucleolus and associated nucleolar material, the interchromosomal regions with the dense particles, and the chromosomes. The nucleolus is generally compact and is surrounded by nucleolar bodies that comprise aggregations of dense round particles 100 to 250 A in diameter. A continuous transition can be observed between these particles and those found isolated or in short chains in the interchromosomal spaces. Particles of similar size (mean diameter 160 A) can be found associated with the nuclear membrane and in the cytoplasm. The chromosomes show different degrees of condensation in different stages of meiotic prophase. The bulk of the chromosomes appears to be made of very fine and irregularly coiled filaments of macromolecular dimensions. Their length cannot be determined because of the thinness of the section but some of them can be followed without interruption for about 1000 to 2000 A, The thickness of the chromosome filaments seems to vary with different stages of prophase and in metaphase. In early prophase, filaments vary between 28 [plus or minus]7 A and 84 [plus or minus] 7 A, with a mean of 47 A; in late prophase the mean is about 70 A. In metaphase the filaments vary between 60 and 170 A thick, with a mean of about 100 A. Neither the prophase nor the metaphase chromosomes has a membrane or other inhomogeneities. The finding of a macromolecular filamentous component of chromosomes is discussed in relation to the physico-chemical literature on nucleoproteins and nucleic acids and as a result it is suggested that the thinnest chromosome filaments (28 [plus or minus]7 A) probably represent single deoxyribonucleoprotein molecules.