Ultrastructural observations on the gallbladder of Richardson's ground squirrel (Spermophilus richardsonii)

The gallbladder of Richardson's ground squirrel shares many structural similarities with numerous other species, including the mouse, rabbit and man. Dark cells exhibit a condensed cytoplasm but contain all the typical organelles of light cells, although their total complement appears reduced. Edematous cells extruded from the epithelial lining were replaced through mitosis or had their space occupied by adjacent cells. Specializations of the basal plasma membrane and underlying basal lamina form peg-and-socket interdigitations, and desmosomes were observed on the lateral cell boundaries down to the level of the basal lamina. The latter remain intact during active water resorption while the former disappear. Adrenergic nerve fibers were observed intramuscularly and beneath the basal lamina. Random interepithelial migration of leucocytes was also observed. Some of these observations help to elucidate controversial issues whereas others have been previously unreported and may be species-dependent.