The anal canal is a unique anatomic region that is characterized by blood supply and lymphatic drainage from two different origins. While the lower rectum and the upper part of the anal canal above the pectinate line share the internal iliac as the primary lymph node drainage site, which of the following structures shares the same primary lymph node drainage site as that of the lower part of the anal canal below the pectinate line?
- Sigmoid colon
Lymphatic drainage varies among different structures based on anatomic distance and more importantly based on embryological origins. Since the upper and the lower parts of the anal canal are derived from different embryological origins, their blood supplies and their lymphatic drainage are different despite their anatomic proximity. The pelvic region in general, with the exception of the genitals (testes and ovaries), drains primarily in the superficial inguinal lymph node. These tissues includes the scrotum, the superficial thigh, and the anal canal below the pectinate line.
Conversely, the testes and ovaries have the same unique drainage sites: the superficial and the deep para-aortic plexuses. These sites are usually the first lymph nodes involved in ovarian and testicular cancers.