Arc of separation (degrees of separation) , or central angle? which ever one should call it, i don’t know, between chromosome pairs in their respective chromosome territories. I see it everywhere in the FISH and other fluorescent imaging of chromosome territories. I see it in bone marrow spreads as well, clearly shown by areas of chromatin condensation of certain chromosome – or portions thereof.

So here is a bone marrow cell which has obvious chromosomal densities, and I am presuming that these are condensed by epigenetic or other influences but there is definite symmetry (in 3 axes) and differing angular dimensions to the presumptive pairs of chromosomes. The whole nucleus is in a V shape, the point of the V being “down” here but in terms of 3D space, it may not be. It certainly represents a portion of the nucleus around which the symmetry occurs. Simetimes instead of being seen as a “v” (or heart as I have depicted it here) a U is present. and this might represent a less well condensed form of the same type of compaction, on the way to becoming a multilobular white cell.

The arc drawn between what is felt to be homologous areas of condensed chromatin, billateral symmetrical dark spots (highlighted in black on images to the right of the unretouched (but color adjusted to red) giemsa stained white cell).  The arc is influenced of course by random position of the cell as it is smooshed onto the slide during preparation as well as the spot identified as the center of the nucleus… whatever that might turn out to be, but i bet it is somewhere near the apex (bottom of the heart).  The actual physical center of the nucleus is not well defined in the literature.  So i think the concept of radial chromosome territory positions might need to be altered to account for the z axis of symmetry and the position which ultimately will be described as the nucleolar “center”… which cant be the nucleolus only, since often there are multiple nucleoli…  maybe the term center of the nucleus should be called “centers of the nucleus” and the arc of separation determined from various sites.