Abstract
The relative extent of cell mixing in tissues of mouse chimaeras or mosaics can be studied by comparing the distributions of the two cell populations in the tissues. However, the mean patch size is misleading because it is affected by both the extent of cell mixing and the relative contributions of the two cell populations. Previous work suggested that effects attributable to differences in tissue composition among chimaeras can be factored out either by correcting the mean patch size or by using the median patch size for the minority cell population and restricting the analysis to grossly unbalanced chimaeras. In the present study, computer simulations of two-dimensional mosaic arrays of black and white squares (representing cells) were used to simulate chimaeric tissues. Random arrays simulated tissues with extensive cell mixing, arrays of cell clumps (representing coherent clones) simulated less mixed tissues, and striped arrays simulated tissues with elongated but fragmented descendent clones. The computer simulations predicted that (i) the median patch length (minority cell population) and the corrected mean patch length would both distinguish between random and clumped patterns and (ii) differences in the variation of the composition of two perpendicular series of one-dimensional transects would distinguished between stripes and randomly orientated patches. Both predictions were confirmed by analysis of histological sections of the retinal pigment epithelium from fetal and adult mouse chimaeras. This study demonstrates that two types of non-random two-dimensional variegated patterns (clumps and stripes) can be identified in chimaeras without two-dimensional reconstruction of serial sections.
Highlights
Spatial analysis of mosaic or chimaeric tissues, composed of two genetically distinct cell populations, can provide information about cell mixing during organogenesis
Computer simulations of black and white mosaic tissues comprising 100 · 100 arrays of cells were produced as described in the Materials and methods section
The corrected mean patch length and the median were numerically similar when p was low, but the median increased when p was high. This similarity has already been demonstrated for eyes of unbalanced E12.5 fetal chimaeras, where the corrected mean patch length is only equivalent to a single cell (West et al 1997)
Summary
Spatial analysis of mosaic or chimaeric tissues, composed of two genetically distinct cell populations, can provide information about cell mixing during organogenesis. A patch is defined as a group of cells of like genotype, which are contiguous at the time of consideration and is a unit that can be measured directly in chimaeric tissue. It may comprise one or several neighbouring ‘coherent clones’ of the same genotype. A coherent clone is defined as a group of clonally related cells, which have remained contiguous throughout the history of the embryo (Nesbitt, 1971). A descendent clone is any group of clonally related cells irrespective of whether they have remained contiguous throughout development
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