Quantitative morphological analysis of interatrial muscle cells in the ferret heart

Cells located in the interatrial septum of the ferret heart were examined and mean cell volume, surface area, length, width, as well as cell length/width and surface area/volume ratios were obtained. The muscle cells were from two different regions. One region was the area of the middle internodal tract while the other was from the area where the anterior and middle internodal tracts intermingled. Based on the data obtained, at least two different subpopulations of interatrial muscle cells could be defined. The larger cells had a mean cell length of 109.7 micrometer, a mean cell width of 13.1 micrometer, a length/width ratio of 8.61, a mean cell surface area of 5,057.6 micrometer2, a mean cell volume of 5960.8 micrometer3, and a surface area/volume ratio of 0.87. The smaller cells had a mean cell length of 58.0 micrometer, a mean cell width of 12.2 micrometer, a length/width ratio of 4.85, a mean cell surface area of 2494.1 micrometer2, a mean cell volume of 2553.6 micrometer3, and a surface area/volume ratio of 1.00. The large cell population had cells that were myofibril rich and also others that were myofibril poor. These quantitative data indicate that the regions of internodal pathways are not composed of a single specialized cell type, but rather are composed of at least two, if not more, cell types that intermingle with each other.