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PubMed1977

Synaptonemal complex karyotyping in spermatocytes of the Chinese hamster (Cricetulus griseus). I. Morphology of the autosomal complement in spread preparations.

Using the Counce-Meyer spreading technique, in over 70 spermatocytes it was possible consistently to obtain whole, flattened nuclei containing complete sets of pachytene SCs. The SCs are visible in both the phase and electron microscopes. Each SC is morphologically intact, preferentially stained, and attached to the nuclear envelope by a dense, terminal plaque. It is thus possible to trace each SC for its entire length. Also, a structure representing the kinetochore is clearly visible in each autosomal SC. Karyotypes comparable to the somatic karyotype can be constructed by arranging SCs according to length and kinetochore position. The observed regularity of SC morphology implies structural stability sufficient to withstand the stresses imposed by the procedure.--A coarse network of closely packed nuclear annuli connecting SC attachment plaques often provides end-to-end associations and may tend to immobilize SCs during processing.--Three kinds of perturbation of SC structure are encountered. Twists in the SC frequently occur, but no regular pattern or correspondence with chiasma distribution is observed. SCs occasionally hook around each other without disruption, but in two instances the unpaired axis of the X apparently was interlocked within an autosomal SC. Streching of the SC is infrequent; it is conspicous when it occurs and is usaully associated with other obvious distortions of the nucleus.--Distinctive morphologies of the X and Y chromosomes facilitate their identification inall preparations.--During zygotene, autosomal synapsis, i.e., the formation of SCs from the pairing of single axial elements, initiates at distal ends and terminates at the kinetochore region; neither initiation nor termination is synchronous among all autosomes.

Animals↗
PubMed1977

Synaptonemal complex karyotyping in spermatocytes of the Chinese hamster (Cricetulus griseus). II. Morphology of the XY pair in spread preparations.

The X and Y chromosome axes have a distinctive morphology at pachytene and are clearly distinguishable from autosomal SCs. The X and Y are totally unpaired at late zygotene when most of the autosomes are synapsed; however, their attachment points at the presumptive SC end are closely apposed. The X and Y axes pair to form a length of SC that is somewhat shorter than the unpaired portion of the Y. Unpaired axes may appear thin and sometimes double, or may thicken to form fusiform bulges that are sometimes hollow: two on the X and one on the Y. Discrete differentiations, tentatively identified as kinetochores, are often visible at the proximal end of the SC on the Y axis, and also between the fusiform differentiations on the X axis. Additional differentiations, in the form of loops and densely staining granular excrescences form on the X axis and sometimes on the Y. A further differentiation appears as a cloud-like sheath around the distal end of the X axis, often in association with a bi-lobed dense body. At late pachytene, the XY-SC remains intact, but the unpaired X and Y axes develop side branches. Just before diplotene, while the autosomal SCs are still intact, the XY-SC is lost, although the axes persist and remain together at the attachment point of the SC. The X and Y differentiations form a logical temporal sequence when grouped according to progressive structural complexity. The morphological types may serve as markers for the sub-stages of pachytene.

Animals↗
PubMed1977

Synaptonemal complex karyotyping in spermatocytes of the Chinese hamster (Cricetulus griseus). III. Quantitative evaluation.

Relative length is a constant and distinctive characteristic for each autosomal SC, despite variations in absolute length from cell to cell. Arm ratio is distinctive for each SC except for two of the three sub-acrocentrics, and serves, together with relative length, for identification. The constancy of relative length and arm ratios indicates biological stability and lack of physical distortion in these spread preparations. There is a 1:1 relationship between relative lengths of autosomal SCs and mitotic autosomes: their arm ratios are similar. These close parallels provide strikingly similar SC and somatic karyotypes. Variability was observed in sub-acrocentric arm ratios and in lengths of unpaired X and Y axes, correlated with the presence of constitutive heterochromatin. - Utilizing progressive differentiations of the X and Y chromosomes for staging, it is demonstrated that autosomal SCs decrease in length from late zygotene to mid-pachytene, and then increase at late pachytene. Within a nucleus, synchrony of length changes is maintained. It is concluded that the factors governing autosomal SC length are regular for any given bivalent from cell to cell and may be related to those that control somatic autosome length relationships. - The X and Y axes differ quantitatively as well as qualitatively from autosomal SCs. The SC portion of the X and Y is constant in length through most of pachytene; the unpaired axes shorten and lengthen, but not in proportion to autosomal SCs. X and Y relative lengths ard arm ratios vary throughout pachytene and do not maintain proportionality with somatic values. The evidence suggests, but does not prove, that the long arm of the X is paired with the short arm of the Y.-Twists occur in autosomal SCs at increasing frequencies throughout pachytene but cannot account for length changes. The number of twists per SC is directly proportional to SC length. Intertwining of SCs is random and proportional to SC length. End-to-end associations of autosomal SCs appear to be random; however, the ends of the X and Y are less often involved in such connections. - The length of axial material in all chromosomes at pachytene, expressed as an equivalent length of DNA double helix, represents 0.013% of the diploid DNA complement.

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