Separation in distinct subpopulations by elutriation of liver cells following exposure of rats to N-nitrosomorpholine

Abstract

N-Nitrosomorpholine, administered with drinking water to SD rats at the daily dose of 2.4 mg/kg for 7 weeks, induces persisting changes in the hepatocytes as shown by electron microscopy and cytochemistry. In situ, the hepatocytes exhibit a heterogeneous reaction for glucose-6-phosphatase activity. Cells of large diameter, frequently deficient in this enzyme, contain a well-developed rough and/or smooth endoplasmic reticulum. Adult rat hepatocytes from control and N-nitrosomorpholine-treated rats were isolated by enzymatic perfusion. Isolated cell populations in both experimental models were composed of a few contaminating sinusoidal cells; small, intermediate, and large hepatocytes; and doublets or triplets of undissociated cells. Five distinct hepatic subpopulations were separated by elutraition or counterflow centrifugation and analyzed by morphological, morphometric, and cytophotometric methods. Fraction I is composed of small (16 to 18 micrometers) diploid hepatocytes; Fractions II and III consist of homogeneous populations of tetraploid cells (mean diameters, 20.5 and 22.4 micrometers); Fraction IV is enriched with large octoploid cells whose mean diameters reach 25.2 micrometers; and Fraction V contains large cells and cell aggregates. The counterflow centrifugation shows the higher proportion of hypertrophied and polyploid hepatocytes, obtained after carcinogen treatment, in the elutriated Fractions IV and V. The structural integrity of hepatocytes is not affected by the process of elutriation. Large hepatocytes, up to 30 micrometers in diameter, exhibit an abundant smooth endoplasmic reticulum, frequently disposed at the periphery of the cell where it forms a network of anastomosing tubules. Moreover, some of these cells present well-developed rough endoplasmic cisternae, closely associated in large fields. Under the scanning electron microscope, elutriated hepatocytes from control rats show numerous regularly distributed microvilli covering the entire cell surface, whereas hypertrophic hepatocytes from N-nitrosomorpholine-treated rats offer heterogeneous cell surfaces, characterized by the presence of patches of short, closely packed microvilli.