What is the anatomy of the rectum?

Anatomy of the Rectum

The rectum is defined anatomically as the segment of bowel extending from the sigmoid take-off (where the mesocolon elongates and the sigmoid colon transitions to the mesorectum) down to the anorectal ring, which is formed by the palpable upper border of the puborectalis and anal sphincter muscles at approximately 3-5 cm from the anal verge. 1, 2

Anatomical Boundaries and Definition

Superior Boundary

• The sigmoid take-off represents the most widely accepted superior boundary, identified on cross-sectional imaging (MRI or CT) as the point where the mesocolon elongates and the sigmoid colon assumes a ventral and horizontal course on axial and sagittal views respectively 2
• Alternative definitions exist: NCCN guidelines define the rectum as the area below the line connecting the sacral promontory to the upper edge of the pubic symphysis on mid-sagittal MRI 1
• This lack of uniform definition has historically led to significant inconsistencies in treatment decisions and trial recruitment 1, 2

Inferior Boundary

• The anorectal ring marks the inferior boundary, formed by the palpable upper border of the anal sphincter and puborectalis muscles 1, 3
• This ring is palpable on digital rectal examination at approximately 3-5 cm from the anal verge 1, 3
• The functional anal canal begins at this point and extends to the anal verge 1

Peritoneal Coverage

Anterior Peritoneal Reflection

• The anterior aspect of the rectum has peritoneal covering down to the level of the anterior peritoneal reflection 1
• The relationship of rectal tumors to this reflection is clinically critical, as it predicts risk of both local and peritoneal recurrence 1

Posterior Surface

• Posteriorly, the nonperitonealized margin is represented by a triangular-shaped bare area that extends superiorly in continuity with the mesentery of the sigmoid colon 1

Mesorectal Envelope

Mesorectal Fascia (MRF)

• The rectum is surrounded by the mesorectal fascia, a multi-layered membrane that envelops the mesorectum 4
• This fascia contains the rectum, its blood vessels, lymphatics, and perirectal fat 1
• The optimal surgical plane for rectal cancer resection follows this fascial layer, producing an intact bulky mesorectum with a smooth surface 1
• Autonomic nerves pass between the mesorectal fascia and the parietal fascia, making sharp dissection along this anatomic plane essential to preserve function 4

Clinical Significance for Tumor Assessment

• Tumor involvement of the MRF is defined as tumor, metastatic lymph nodes, or extramural vascular invasion (EMVI) within ≤1 mm of the fascia 1
• This distance determines surgical planning and need for neoadjuvant therapy 1

Muscular Anatomy

Rectal Wall Layers

• The rectal wall consists of inner circular smooth muscle and outer longitudinal smooth muscle layers 4
• The internal anal sphincter arises as a continuation of the circular smooth muscle layer 3

Pelvic Floor Muscles

• Levator ani muscle complex: Forms the pelvic floor and includes the puborectalis muscle 1, 3
• Puborectalis muscle: Creates the anorectal angle and forms the superior boundary of the anal canal; palpable as a distinct muscular ring during digital examination 1, 3
• Anal sphincter complex: Includes internal (smooth muscle) and external (striated muscle) sphincters separated by the intersphincteric space 1, 3

Intersphincteric Space

• This is the anatomical plane between the internal and external anal sphincters 1, 3
• Critical landmark for identifying fistula tracts and determining surgical planes in abdominoperineal resections 1, 3

Adjacent Pelvic Structures

Organs at Risk for T4b Invasion

When rectal cancer invades beyond the visceral peritoneum or mesorectal fascia, it may involve 1:

• Urogenital organs: Ureters, bladder, urethra, prostate, seminal vesicles, cervix, vagina, ovaries
• Gastrointestinal structures: Small intestine, colon
• Musculoskeletal structures: Pelvic bones, pelvic floor muscles (coccygeus, piriformis, levator ani, anal sphincters), sacrococcygeal ligaments
• Neurovascular structures: Pelvic floor nerves, extramural rectal vessels

Lymphatic Drainage

Regional Lymph Nodes

• Mesorectal lymph nodes: Within the mesorectal envelope 1
• Distal sigmoid mesentery lymph nodes 1
• Para-rectal vessel lymph nodes 1
• Internal iliac lymph nodes 1

Lateral Lymph Nodes

• Include obturator and internal iliac nodes 1
• Lower rectal cancers (especially those extending to the dentate line/puborectalis) and cT3-4 tumors have higher risk of lateral lymph node metastasis 1
• If rectal cancer extends to the dentate line, inguinal lymph nodes are considered regional rather than distant metastases 1

Non-Regional (Distant) Lymph Nodes

• External iliac, common iliac, and obturator lymph nodes (unless tumor extends to dentate line) 1

Clinical Pitfalls to Avoid

• Location documentation: Always annotate the distance between rectal tumors and the puborectal muscle, and specify the involved quadrant, particularly the anterior 1/4 quadrant (10 o'clock to 2 o'clock position), as this correlates with circumferential resection margin involvement 1
• Imaging modality: MRI is the preferred modality for defining rectal anatomy and tumor relationships, used by 72% of experts 2
• Peritoneal reflection: Do not assume all rectal tumors are extraperitoneal; anterior tumors above the peritoneal reflection behave differently regarding peritoneal recurrence risk 1
• Sphincter assessment: During digital examination, distinguish between the anal sphincter complex (lower) and the puborectalis muscle (higher, forming the anorectal ring) 1, 3