Supraoptic nuclei of rodents adapted for mesic and xeric environments: Numbers of cells, multiple nucleoli, and their distributions

Quantitative comparisons were made between the supraoptic nuclei (SON) of rodents from mesic and xeric habitats. Laboratory rats, Rattus norvegicus, mesically adapted, were compared with xerically adapted gerbils, Meriones unguiculatus, and kangaroo rats, Dipodomys merriami and Dipodomys spectabilis. As a close genetic control, California deer mice, Peromyscus californicus (mesic), were compared with cactus mice, Peromyscus eremicus (xeric), of the same subgenus. Measures were obtained separately for anterior (Soa) and tuberal (Sot) subdivisions of SON in each species; these measurements included numbers of cells, cell size and, following the discovery of multiple nucleoli in many cells, the proportion of cells showing 1, 2, 3, and 4 or more nucleoli. In mesic species, Sot had fewer cells than Soa; in all but one (D. spectabilis) of the xeric species, the two subdivisions had equal numbers of cells. Cell size was related to body size rather than adaptation. All xeric species showed higher proportions (20 to 60%) of cells with multiple nucleoli than did corresponding mesic species (0 to 20%). Adaptation to environments requiring conservation of body water is correlated with larger numbers of SON cells with multiple nucleoli; relatively larger numbers of cells in Sot but not in Soa; and there are no notable differences in size of cells between mesic‐ and xeric‐adapted species. The suggestion that number of nucleoli is realated to a SON cell's production of antidiuretic hormone is supported by other cytological studies.