Histological findings
Then, we corresponded the anatomical/surgical classification to histological findings (Table 2). In terms of embryonic development, thymic and parathyroid tissues must strongly imply its generation from the 3rd pouch (Fig. 2 for Cases 1, 2, and 8). Immunohistochemical staining by calcitonin for detection of C cell is essential in defining the origin from the 4th pouch; however, staining was not conducted in our PSF cases except two negative cases. While negative C cells were supposed to develop from the 3rd pouch, we did not regard these two cases as 3rd pouch origin since immunohistochemical staining of calcitonin was technically difficult and the positivity seemed to depend on the slice. Therefore, based on the thymic tissues around the PSF, we classified PSF into two origins: 3rd pouch with thymic tissues around the fistulas (five cases with shadow in Table 2) and 4th pouch without thymic tissues (seven cases).
Case 10 was previously reported as a double-tract PSF. One fistula that was regarded as having 3rd pouch origin had epithelium, while another (considered as having 4th pouch origin) was filled with granulation tissue and did not have epithelium.