1. The packing density decreases with increasing cyst volume.
The packing density decreases with increasing cyst volume. (A) The packing density for 463 tissue cysts harvested at all time points plotted relative to the volume of the widest optical section reveals that larger tissue cysts tend to be more loosely packed. These results indicate that the rate of tissue cyst expansion exceeds the replication rate of bradyzoites and occurs independently of bradyzoite replication (scenario C in Fig. 4). The formula used to determine the packing density is shown in red. In this equation, N is the number of nuclei (bradyzoites) in the optical section, D is the diameter of the tissue cyst, z equals the number of optical sections (8 in this case), and πr2 is the area of the circle (r is the radius). Of note, smaller tissue cysts tended to be more densely packed while also exhibiting the greatest range of packing densities (green arrowheads). The differences in packing densities for same/similar sized cysts was observed for the intermediate (cyan arrowheads) and large (magenta arrowheads) cysts as well. Finally, tissue cysts of vastly different volumes can have very similar packing densities (red asterisks). (B) The mean packing densities (plus standard errors of the means [SEM] [error bars]) for tissue cysts harvested at weeks 3 (n = 175), 4 (n = 30), 5 (n = 124), 6 (n = 37), and 8 (n = 97) postinfection reveal evidence of a dynamic and potentially oscillatory pattern where changes in the packing density over the course of the infection reflect the differential rates of cyst expansion and bradyzoite replication. In general, increases in the packing density reflect increased replication rates, while decreases in the packing density are suggestive of tissue cyst expansion in the absence of bradyzoite replication.
Elizabeth Watts et al. mBio 2015; doi: /mBio