Cell mixing in the spinal cords of mouse chimeras

With the aim of determining whether there is significant cell mixing during development of the spinal cord, experimental chimeric mice containing two genetically distinct cell populations were produced by aggregating BALB/c or BALB/c x LPT hybrid embryos with C3H/HeN embryos. The BALB/c and LPT hybrid spinal cord cells were distinguished histochemically from the C3H/HeN spinal cord cells by using beta-glucuronidase as an independent cell genotype marker. BALB/c and LPT hybrid cells have high levels of beta-glucuronidase activity, while the C3H/HeN cells have low levels. The spinal cords of the chimeras were dissected out regionally (i.e., cervical, thoracic, and lumbar areas) and were sectioned serially. Each region was then analyzed by scoring large- and medium-sized neuronal cell bodies (greater than or equal to 10 microns) whose genotypes were distinguished by their beta-glucuronidase levels. Observations of seven chimeric mice, with coat colors that varied from one extreme (5% albino) to the other (90% albino), suggest that each chimeric spinal cord is a relatively homogeneous population throughout its length. On average only 4 to 5 percentage point differences were observed when comparing left-right, cranial-caudal, and dorsal-ventral regions within a given chimera. The cell mixing, however, is not total, and regional variations were noted. Maximum left-right differences between different spinal cord levels never exceeded 18 percentage points, while in the entire cord the maximum left-right difference was 11 percentage points. When considering dorsal-ventral differences, 18 and 15 percentage points were observed within the spinal cord levels and the entire cord, respectively. However, when comparisons were made between smaller subregions (e.g., right-dorsal-cervical vs left-ventral-lumbar), larger differences of up to 30 percentage points were observed. In addition, the genotype proportions in the spinal cord were closely correlated with the visually estimated proportions of coat color genotypes.