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.