Quantitative and spatial information on the composition of chimaeric fetal mouse eyes from single histological sections

Lessons from mouse chimaera experiments with a reiterated transgene marker: revised marker criteria and a review of chimaera markers

Recent reports of a new generation of ubiquitous transgenic chimaera markers prompted us to consider the criteria used to evaluate new chimaera markers and develop more objective assessment methods. To investigate this experimentally we used several series of fetal and adult chimaeras, carrying an older, multi-copy transgenic marker. We used two additional independent markers and objective, quantitative criteria for cell selection and cell mixing to investigate quantitative and spatial aspects of developmental neutrality. We also suggest how the quantitative analysis we used could be simplified for future use with other markers. As a result, we recommend a five-step procedure for investigators to evaluate new chimaera markers based partly on criteria proposed previously but with a greater emphasis on examining the developmental neutrality of prospective new markers. These five steps comprise (1) review of published information, (2) evaluation of marker detection, (3) genetic crosses to check for effects on viability and growth, (4) comparisons of chimaeras with and without the marker and (5) analysis of chimaeras with both cell populations labelled. Finally, we review a number of different chimaera markers and evaluate them using the extended set of criteria. These comparisons indicate that, although the new generation of ubiquitous fluorescent markers are the best of those currently available and fulfil most of the criteria required of a chimaera marker, further work is required to determine whether they are developmentally neutral.

Increased corneal epithelial turnover contributes to abnormal homeostasis in the Pax6(+/-) mouse model of aniridia

We aimed to test previous predictions that limbal epithelial stem cells (LESCs) are quantitatively deficient or qualitatively defective in Pax6^+/− mice and decline with age in wild-type (WT) mice. Consistent with previous studies, corneal epithelial stripe patterns coarsened with age in WT mosaics. Mosaic patterns were also coarser in Pax6^+/− mosaics than WT at 15 weeks but not at 3 weeks, which excludes a developmental explanation and strengthens the prediction that Pax6^+/− mice have a LESC-deficiency. To investigate how Pax6 genotype and age affected corneal homeostasis, we compared corneal epithelial cell turnover and label-retaining cells (LRCs; putative LESCs) in Pax6^+/− and WT mice at 15 and 30 weeks. Limbal BrdU-LRC numbers were not reduced in the older WT mice, so this analysis failed to support the predicted age-related decline in slow-cycling LESC numbers in WT corneas. Similarly, limbal BrdU-LRC numbers were not reduced in Pax6^+/− heterozygotes but BrdU-LRCs were also present in Pax6^+/− corneas. It seems likely that Pax6^+/− LRCs are not exclusively stem cells and some may be terminally differentiated CD31-positive blood vessel cells, which invade the Pax6^+/− cornea. It was not, therefore, possible to use this approach to test the prediction that Pax6^+/− corneas had fewer LESCs than WT. However, short-term BrdU labelling showed that basal to suprabasal movement (leading to cell loss) occurred more rapidly in Pax6^+/− than WT mice. This implies that epithelial cell loss is higher in Pax6^+/− mice. If increased corneal epithelial cell loss exceeds the cell production capacity it could cause corneal homeostasis to become unstable, resulting in progressive corneal deterioration. Although it remains unclear whether Pax6^+/− mice have LESC-deficiency, we suggest that features of corneal deterioration, that are often taken as evidence of LESC-deficiency, might occur in the absence of stem cell deficiency if corneal homeostasis is destabilised by excessive cell loss.

Evaluation of methods for one-dimensional spatial analysis of two-dimensional patterns in mouse chimaeras

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.

Quantitative analysis of patch patterns in mosaic tissues with ClonalTools software

Quantitative analysis of mosaic tissues is a powerful method for following developmental lineages; however, analytical techniques are often subjective and repetitious. Here a flexible, semi-automated image analysis method for mosaic patterns is described. ClonalTools is a free customizable tool-set designed for the open-source image analysis package ImageJ. Circular, polygonal or linear one-dimensional mosaic arrays can be interrogated to provide measurements of the total number and width of positive and negative patches in a region of interest. These results are adjusted for the effects of random clumping using a previously described method to correct for differences in the contribution of the positive and negative cell type. The applicability of ClonalTools to different systems is discussed with reference to the analysis of mosaic patterns in the mouse corneal epithelium and adrenal cortex and in the outgrowth of neurites from explant cultures of mouse retina as example systems. To validate ClonalTools quantitatively, a recently published manual clonal analysis of the corneal epithelium of X-inactivation beta-Gal-mosaic mice was re-analysed. The semi-automated results did not differ significantly from the published data. Rapid quantification of such patterns to produce biologically relevant results represents a welcome improvement in terms of ease and speed of use over previous methods.

Mosaic analysis of stem cell function and wound healing in the mouse corneal epithelium

Background

The mouse corneal epithelium is a continuously renewing 5–6 cell thick protective layer covering the corneal surface, which regenerates rapidly when injured. It is maintained by peripherally located limbal stem cells (LSCs) that produce transient amplifying cells (TACs) which proliferate, migrate centripetally, differentiate and are eventually shed from the epithelial surface. LSC activity is required both for normal tissue maintenance and wound healing. Mosaic analysis can provide insights into LSC function, cell movement and cell mixing during tissue maintenance and repair. The present study investigates cell streaming during corneal maintenance and repair and changes in LSC function with age.

Results

The initial pattern of corneal epithelial patches in XLacZ^+/- X-inactivation mosaics was replaced after birth by radial stripes, indicating activation of LSCs. Stripe patterns (clockwise, anticlockwise or midline) were independent between paired eyes. Wound healing in organ culture was analysed by mosaic analysis of XLacZ^+/- eyes or time-lapse imaging of GFP mosaics. Both central and peripheral wounds healed clonally, with cells moving in from all around the wound circumference without significant cell mixing, to reconstitute striping patterns. Mosaic analysis revealed that wounds can heal asymmetrically. Healing of peripheral wounds produced stripe patterns that mimicked some aberrant striping patterns observed in unwounded corneas. Quantitative analysis provided no evidence for an uneven distribution of LSC clones but showed that corrected corneal epithelial stripe numbers declined with age (implying declining LSC function) but stabilised after 39 weeks.

Conclusion

Striping patterns, produced by centripetal movement, are defined independently and stochastically in individual eyes. Little cell mixing occurs during the initial phase of wound healing and the direction of cell movement is determined by the position of the wound and not by population pressure from the limbus. LSC function declines with age and this may reflect reduced LSCs numbers, more quiescent LSCs or a reduced ability of older stem cells to maintain tissue homeostasis. The later plateau of LSC function might indicate the minimum LSC function that is sufficient for corneal epithelial maintenance. Quantitative and temporal mosaic analyses provide new possibilities for studying stem cell function, tissue maintenance and repair.

Validity of the 21-OH/LacZ transgenic mouse as a model for studying adrenocortical cell lineage

Mosaic beta-galactosidase reporter staining patterns in the adult adrenal cortex of 21-OH/LacZ transgenic mice were compared to those observed in mouse chimeras and X-inactivation mosaics, which are known to have a lineage basis. This revealed similar patterns of blue and white radial stripes in all three experimental groups. Each blue stripe may contain one or more blue coherent clones of cells but this was taken into account by correcting the observed stripe numbers for the effects of different proportions of LacZ-positive (blue) and LacZ-negative (unstained) cells between adrenals. The corrected stripe numbers were similar in all three experimental groups, which supports the hypothesis that the stripes in the adrenals of 21-OH/LacZ transgenic mice are formed in a similar way to those in chimeras and X-inactivation mosaics (i.e., they have a lineage basis). This suggests that the 21-OH/LacZ transgenic mouse is likely to be a valid model for studying steroidogenic cell lineage in the adrenal cortex, thereby providing additional support for the centripetal migration hypothesis of adrenocortical cytogenesis.

Clonal analysis of patterns of growth, stem cell activity, and cell movement during the development and maintenance of the murine corneal epithelium

Patterns of growth and cell movement in the developing and adult corneal epithelium were investigated by analysing clonal patches of LacZ-expressing cells in chimeric and X-inactivation mosaic mice. It was found that cell proliferation throughout the basal corneal epithelium during embryogenesis and early postnatal life creates a disordered mosaic pattern of LacZ(+) clones that contrasts with patterns of proliferation and striping produced during the later embryonic stages of retinal pigmented epithelium development. The early mosaic pattern in the corneal epithelium is replaced in the first 12 postnatal weeks by an ordered pattern of radial stripes or sectors that reflects migration without mixing of the progeny of clones of limbal stem cells. In contrast to previous assumptions, it was found that maturation of the activity of limbal stem cells and the pattern of migration of their progeny are delayed for several weeks postnatally. No evidence was found for immigration of the progeny of stem cells until the 5th postnatal week. There are approximately 100 clones of limbal stem cells initially, and clones are lost during postnatal life. Our studies provide a new assay for limbal and corneal defects in mutant mice.

Developmental potential and survival of glycolysis-deficient cells in fetal mouse chimeras

Mouse embryos homozygous for a null allele of Gpi1 fail to complete gastrulation and die around E7.5. We produced E12.5 chimeric mouse conceptuses, composed of wild-type and homozygous Gpi1m/m null mutant cells to test whether the presence of wild-type cells allowed mutant cells to survive and, if so, whether they survived better in some tissue locations than others. Fourteen homozygous Gpi1m/m<-->Gpi1c/c chimeras were identified and these contained low levels of homozygous mutant cells in most tissues tested. Homozygous Gpi1m/m cells contributed better to the yolk sac endoderm and placenta than to the epiblast derivatives tested (retinal pigment epithelium, brain, tail, amnion, and yolk sac mesoderm). The depletion of mutant cells confirms that the gene acts cell autonomously, but the GPI deficiency is not always cell-lethal. When mixed with wild-type cells in chimeras, homozygous mutant cells can differentiate into many different cell types and survive until at least E12.5.