Quantitative evaluation of coat-color patterns in artificially produced chimeras of the mouse by means of a microcomputer-based video-image analysis system

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.

Neural crest progenitors of the melanocyte lineage: coat colour patterns revisited

Neural crest-derived melanoblasts are the progenitors of melanocytes, the pigment cells of the skin, hair and choroid. Previous studies of adult chimaeric mice carrying different coat colour markers have suggested that the total melanocyte population is derived from a small number of melanoblast progenitors, each of which generates a discrete unilateral transverse band of colour. This work also suggested minimal mixing of cells between clones. We have used two complementary approaches to assess the behaviour of migrating clones of melanoblasts directly in the developing embryo. First, we made aggregation chimaeras between transgenic Dct-lacZ and non-transgenic embryos, in which lacZ is a marker for melanoblasts. Second, we generated transgenic mice carrying a modified lacZ reporter construct containing a 289 base pair duplication (laacZ) under the control of the Dct promoter. The laacZ transgene is normally inactive, but reverts to wild-type lacZ at low frequency, labelling a cell and all of its progeny at random. Mosaic embryos containing labelled melanoblast clones were generated. In contrast to previous data, chimaeric and mosaic embryonic melanoblast patterns suggest that: (1) there is a large number of melanoblast progenitors; (2) there is a pool of melanoblasts in the cervical region; (3) different cell dispersion mechanisms may operate in the head and trunk regions; and (4) there is extensive axial mixing between clones.

Developmental fate of single embryonic stem cells microinjected into 8-cell-stage mouse embryos

Embryonic stem (ES) cells are pluripotent and capable of differentiating into somatic as well as germ cell lineages when conjoined with blastomeres of early mouse embryos. However, the developmental potential of single ES cells has not been fully investigated. We injected single murine ES cells (A3-1 cell line) of 129 origin into 8-cell mouse embryos (B6xBDF1) and examined the patterns of distribution of ES-cell-derived cells in the blastocysts as well as in the fully grown chimeric mice. The ES cells underwent 1-2 cycles of mitosis between the 8-cell and the blastocyst stage when they were introduced as single cells, whereas those introduced as groups of 2-5 cells did not proliferate during the same period of development. The ES cells and their daughter cells were predominantly incorporated into the ICM. From the 63 8-cell embryos which received single ES cells microinjected into the perivitelline space, 24 newborns were obtained, and 4 (2 fertile males, 1 sterile female and 1 hermaphrodite) of them (16.6%) were chimeric. The test breeding studies revealed that all the progeny of the two chimeric males were derived from spermatozoa of 129 genotype. The relative contribution of the host-derived and the ES-cell-derived cells in different tissues of the chimeric mice was assessed by PCR analyses of the microsatellite polymorphism of genomic DNA extracted from the tissues. In two male germ line chimeras, the testes, the kidneys and the dorsal skeletal muscles exhibited exceptionally high 129 contents. Our results demonstrated that single ES cells which maintain totipotency or pluripotency of high degree are present in a colony of ES cells, and that single ES cells conjoined with the blastomeres of 8-cell-stage embryos may colonize, if the circumstances allow, almost exclusively the germ cells and concomitantly the urogenital cell lineages. Possible correlation between the allocation of the germ line and the urogenital lineages is discussed.

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

The spatial distribution of cells in chimaeric tissues, composed of two genotypes, provides insights into the extent of cell mixing during development and growth. However, direct measurement of patch sizes is not usually meaningful because, when the proportion of one genotype is high, a single patch may encompass several adjacent coherent clones of like genotype (clone aggregation). Two previously used methods of comparing patch lengths were evaluated to overcome this problem. The corrected mean patch length (corrected for the predicted effects of random clone aggregation) is a more useful summary statistic than the median patch length of the minor genotype, because its use is not restricted to grossly unbalanced chimaeras, but its validity has been questioned. The two methods gave almost identical numerical summaries of patch sizes in the retinal pigment epithelium of fetal chimaeras, thereby validating the use of the corrected mean patch length for this tissue. The present study also showed that the corrected patch length was unaffected by the presence of cells hemizygous for the TgN(Hbb-b1)83Clo transgene and that the proportion of pigmented cells in a single histological section was representative of the overall composition of the chimaeric fetus.

Quantitative analysis of striped coat-color patterns in Large White-->Duroc chimeric pigs with special reference to the genetic control mechanisms of the dominant black-eyed white phenotype

Coat colors of four chimeric pigs produced by the microinjection of dissociated blastomeres of (Landrace x Large White) blastocysts to the blastocyst cavity of Duroc x Duroc) blastocysts (Kashiwazaki et al., 1992) exhibited characteristic horizontal stripe-patterns. We carried out quantitative analysis of those patterns in order to derive information concerning the genetic regulatory mechanisms of the dominant black-eyed white phenotypes in the pig. In the four chimeras, the theoretical mean widths of the single-clone stripe calculated from the estimated widths of minimal recognizable stripe (MRS) (Tachi, 1988) were 2.1 +/- 0.1, 2.23 +/- 0.15, 1.89 +/- 0.06, and 1.93 +/- 0.28 cm respectively. The estimated number of single-clone stripes in the thoracico-lumbar region of those animals were 42.3, 40.7, 46.3, 44.2, and about twice the mean number of vertebrae in the same region (Duroc, 20 or 21; Large White 21 or 22). Furthermore, the mean length of thoracico-lumbar vertebrae in two of the chimeric pigs, as measured on X-ray radiographs, was approximately twice the mean single-clone stripe width. It was concluded that the stripe-patterns of the chimeric pigs probably represented the dermatome patterns of epidermis; and in the pig, a single somite was likely to be derived from the clones of two primordial cells, as originally proposed by Gearhart & Mintz (1972) in the mouse. It was suggested, furthermore, that in the Large White-->Duroc chimeric pigs, melanocytes that migrated into the region of skin formed by a Large White dermatome could not survive, thus creating a clearly demarcated white stripe. Possible involvement of KL or c-kit in the dominant black-eyed white phenotype of the pig is discussed.

Expression of whey acidic protein (WAP) genes in tissues other than the mammary gland in normal and transgenic mice expressing mWAP/hGH fusion gene

Whey acidic protein (WAP) is a major whey protein secreted in rodents' milk. Murine WAP (mWAP) genes have been assumed to be expressed solely in the mammary gland. However, several heterologous genes fused with the mWAP promoter and artificially introduced into animal genomes as transgene were expressed not only in the mammary gland but also in other tissues as well. In the present study, we investigated, by means of the reverse transcription polymerase chain reaction (RT-PCR), the patterns of expression of endogenous WAP genes in tissues of normal mice and in transgenic mice carrying hGH gene coupled to the mWAP promoter sequence. The results revealed that the genes driven by the mWAP promoter, regardless of whether they are endogenous genes or transgenes, were transcribed in a variety of tissues other than the mammary gland of lactating normal female mice, although the expression levels are generally low. The expression of WAP genes in the cerebrum and the liver is regulated, as in the mammary gland, according to the reproductive stages. However, the tissue distribution of endogenous WAP gene expression in mature virgin transgenic female mice was the same as that in lactating normal female mice.

Analysis of coat-color patterns in aggregation chimeras between BALB/cA and C3H/HeN mice with special reference to migratory patterns and clone number of epidermal melanoblasts

Chimeras provide unique opportunities to study interactions between the phenotypically similar but genotypically allogeneic cell populations during embryogenesis in vivo. From the quantitative analysis of coat-color patterns in C3H/HeN----BALB/cA chimeras, a model was proposed stating that the aggregability of the C3H/HeN-derived melanoblasts in the chimeras was inversely related to the ratio between the mean free path of the epidermal melanoblasts in the normal C3H/HeN mouse and that in the chimeras. As a corollary, the possibility was suggested that during the migration of melanoblasts, mechanisms identical with or similar to contact inhibition of movement might operate after collision between the isogeneic, but not between the allogeneic melanoblasts. With regard to the number of melanoblast clones in the trunk region of the mouse, the present series of analyses yielded the value of 24-28 arranged unilaterally; the value closely approximated the number of the somites in that region and provided further support for the proposition made earlier by Tachi [Dev Genet 9: 121-154, 1988; "Development of Preimplantation Embryos and Their Environment." New York: Alan R. Liss, Inc., 1989, pp 263-274].