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.