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Marker chromosome analysis of two mouse chimaeras.

Chimaeric mice were obtained by injecting embryonic (CBA/H-T6 X PDE)F1 cells into PDE blastocysts. Three of 15 young were overt chimaeras. One female and one male chimaera survived to adulthood and after completion of test breeding, which demonstrated chimaerism in the germ cells of both, they were killed for study when aged 32 and 33 weeks respectively. Chromosome spreads were scored for the presence or absence of the T6 marker chromosome in direct preparations from bone marrow, spleen, thymus, lymph nodes, Peyer's patches, corneas, gut epithelium and testes. Preparations from monolayer cultures of skin, kidney, ovary and gut from mitogen-stimulated blood cultures were scored in the same way. Both components of the chimaeras were identified in every one of 53 specimens studied, some of which, such as single lymph nodes, corneas, and segments of gut, may not have contained more than 10(5) proliferating cells. This result complements published evidence for fine-grained mixture obtained in morula-aggregation chimaeras by other methods and implies extensive cell movement during embryogenesis. Results obtained from the lymphomyeloid tissues show a clear partition into two groups in respect of the proportions of host-type to donor-type cells indentified. The one group consists of bone marrow, thymus and Peyer's patches, the other of spleen and lymph nodes. This result would be most simply explained in terms of two distinct stem cell pools and appears to conflict with the currently favoured hypothesis of a single stem cell pool for the whole lymphomyeloid complex located in bone marrow. Four groups of factors may, however, modify the relative representation of the two components in different lymphomyeloid sites: (1) The magnitude of embryonic founder populations. (2) Limited recruitment from the stem cell pool in post-natal life. (3) Variable size of clones produced by individual stem cells. (4) Differential cellular behaviour determined by genotypic differences.