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Pectinate line

The pectinate line, also known as the dentate line, is a key anatomical landmark in the human anal canal, consisting of an irregular, serrated circle formed by the anal valves at the inferior ends of the anal columns.^[1] It divides the anal canal—approximately 3–4 cm in length—into an upper two-thirds (colorectal zone) and a lower one-third (intermediate and cutaneous zones), located about 2 cm above the anal verge.^[2] This structure marks the transitional boundary between the hindgut-derived columnar epithelium above and the ectoderm-derived non-keratinized stratified squamous epithelium (anoderm) below.^[3] Embryologically, the pectinate line represents the junction between the postallantoic gut superiorly and the proctodeum inferiorly, which profoundly influences the region's neurovascular anatomy.^[3] Above the line, the arterial supply arises from the superior rectal artery (from the inferior mesenteric artery), venous drainage follows the portal system, innervation is autonomic via the inferior hypogastric plexus, and lymphatic drainage targets the internal iliac nodes.^[2] Below the line, blood supply shifts to the middle and inferior rectal arteries (from the internal pudendal artery), venous drainage is systemic, somatic innervation comes from the inferior rectal branches of the pudendal nerve, and lymphatics drain to the superficial inguinal nodes.^[1] These differences contribute to the line's role as a critical reference in understanding regional physiology and pathology. Clinically, the pectinate line is significant in conditions affecting the anal canal, such as hemorrhoids, where internal hemorrhoids (above the line) are typically painless due to visceral innervation, while external ones (below) cause pain from somatic nerves.^[3] It also serves as a landmark for anal gland infections that may lead to abscesses or fistulas, and it guides surgical interventions like hemorrhoidectomy or ileal pouch-anal anastomosis by delineating zones of varying sensitivity and vascularity.^[2]

Anatomy

Location and gross structure

The pectinate line, also known as the dentate line, is an irregular, scalloped demarcation that divides the anal canal into its upper two-thirds, derived from the hindgut, and its lower one-third, derived from the proctodeum.^[3]^[2] This transitional zone is located approximately 2-3 cm proximal to the anal verge within the 3-4 cm length of the anal canal.^[3]^[4] The line is formed by the anal valves, which are transverse mucosal folds connecting the distal ends of the vertical anal columns (also called columns of Morgagni), creating a zigzag or pectinate configuration that gives the structure its serrated, wavy appearance.^[1]^[2] These 6-10 anal columns project into the lumen of the upper anal canal, with the valves situated at their bases, collectively outlining the irregular circle of the pectinate line.^[4] Grossly, the pectinate line appears as a whitish or pale, scalloped ridge due to the abrupt transition in epithelial lining from columnar mucosa above to stratified squamous epithelium below, often described as the smooth, pale anal pecten.^[1] It is not readily visible on external inspection but can be observed during proctoscopy or under anesthesia with anal retraction, highlighting its role as a key anatomical landmark in the colorectal zone.^[3]^[2]

Surrounding structures and relations

The pectinate line serves as a key transitional boundary within the anal canal, which measures approximately 3 to 4 cm in length from the anorectal junction to the anal verge.^[1] Superiorly, it directly borders the rectal ampulla and the upper portion of the anal canal, where the canal is enveloped by longitudinal smooth muscle layers derived from the rectal muscularis and reinforced by the internal anal sphincter, a thickening of the circular smooth muscle that maintains basal tone.^[5] This superior relationship positions the pectinate line at the interface between the more distensible rectal reservoir and the narrower, sphincteric anal canal.^[2] Inferiorly, the pectinate line transitions abruptly to the lower anal canal, culminating at the anal verge, where it adjoins the external anal sphincter—a skeletal muscle layer under voluntary control that encircles the distal canal and integrates with the perianal skin.^[1] Below this line lies the anoderm, a specialized non-keratinized stratified squamous epithelium that provides a sensitive, hairless transition to the external perianal region without skin appendages.^[5] Circumferentially and laterally, the pectinate line is defined by its intimate association with the vertical folds of the anal columns (also known as columns of Morgagni) above, which project inward from the rectal mucosa and house vascular cushions, while below it aligns with the anal sinuses and valves—shallow recesses where glandular ducts open, forming a mucocutaneous junction that separates endodermal and ectodermal derivatives.^[2] This arrangement creates a scalloped, serrated appearance to the line, facilitating the anchorage of mucosal folds.^[1] In the broader perianal context, the pectinate line lies in close proximity to the puborectalis muscle, a U-shaped component of the levator ani that forms a sling around the anorectal junction to maintain the anorectal angle, and the overall levator ani complex, which supports the pelvic floor and bounds the ischioanal fossae laterally, allowing for canal expansion during defecation.^[6] These muscular relations underscore the pectinate line's position as a functional watershed approximately midway along the anal canal's length.^[1]

Embryology and development

Embryonic origins of the anal canal

The anal canal originates from the cloaca, an early embryonic structure representing the common terminal portion of the gastrointestinal and urogenital systems.^[7] During the fourth week of gestation, the cloaca is bounded caudally by the cloacal membrane and communicates with the allantois, which later contributes to the formation of the umbilical cord and bladder.^[8] The postallantoic portion of the hindgut extends caudal to the allantois attachment, providing the foundational endodermal lining for the future anorectal region.^[3] Around the seventh week of gestation, the urorectal septum, derived from mesoderm, begins to descend caudally, dividing the cloaca into an anterior urogenital sinus and a posterior anorectal canal.^[7] This septation process involves the growth of mesenchymal tissue and the incorporation of cloacal wall folds, such as the Tourneux and Rathke folds, which facilitate the separation.^[8] Concurrently, the rectoanal region undergoes caudal descent relative to the developing body axis due to differential growth and regression of the dorsal cloaca, including the resorption of the tailgut by Carnegie stage 15 (approximately 36 days post-fertilization).^[8] This positional shift positions the anorectal canal appropriately within the pelvic region. The upper portion of the anal canal derives from the endoderm of the hindgut, resulting in a lining of simple columnar epithelium continuous with the rectal mucosa.^[7] In contrast, the lower portion originates from the ectoderm of the proctodeum, an invagination external to the cloaca, and is initially separated from the anorectal canal by the anal membrane—a bilayered structure lacking mesoderm.^[3] This membrane ruptures around weeks 8 to 9 of gestation, establishing patency of the anal canal and marking the junction between endodermal and ectodermal contributions, which later corresponds to the pectinate line.^[7]

Formation of the pectinate line

The pectinate line forms during embryonic development as a critical transitional landmark in the anal canal, arising at the site of the anal membrane rupture around the eighth week of gestation. This rupture perforates the anal membrane, the dorsal remnant of the cloacal membrane that separates the anorectal canal from the exterior following septation, thereby establishing the external opening of the anus and delineating the junction between the superior portion of the anal canal—derived from the endodermal hindgut—and the inferior portion, originating from the ectodermal proctodeum. The site corresponds to the position after completion of cloacal septation by the urorectal septum's descent, which divides the cloaca into the urogenital sinus anteriorly and the anorectal canal posteriorly by weeks 7 to 8.^[7]^[9] Following membrane rupture, the irregular contour of the pectinate line emerges from the development of anal valves and columns between weeks 8 and 12. Anal columns, also known as columns of Morgagni, appear as longitudinal mucosal folds in the upper anal canal, while anal valves form as transverse folds connecting the bases of these columns, creating the characteristic dentate or serrated margin. These structures arise from invaginations and proliferations of the mucosa in the transitional zone, with the valves collectively outlining the pectinate line and contributing to its wavy appearance.^[9]^[5] The positioning of the pectinate line is further shaped by the regression of the cloacal folds and proliferation of surrounding mesenchyme during early weeks of development. As the cloacal folds regress in conjunction with tail fold involution, mesenchymal tissue proliferates around the cloaca from weeks 4 to 7, remodeling the region and facilitating the precise alignment of the urorectal septum with the anal membrane. This mesenchymal expansion supports the structural integrity of the emerging anal canal and anchors the pectinate line at its definitive location approximately 2 to 3 cm from the anal verge.^[10]^[11] Evolutionarily, the pectinate line represents a conserved vestige of cloacal division across vertebrates, reflecting the ancient partitioning of a shared cloacal chamber into distinct digestive and urogenital outlets that originated at the base of the craniate lineage. In mammals, this septation is a specialized adaptation, with the line marking the persistent embryological boundary that echoes the cloacal morphology retained in non-mammalian vertebrates such as birds and reptiles.^[12]^[13]

Histology

Epithelial and mucosal transitions

The pectinate line marks a critical histological transition in the anal canal, where the epithelium shifts abruptly from the endoderm-derived columnar epithelium above to the ectoderm-derived stratified squamous epithelium below. Above the line, the mucosa is lined by simple or stratified columnar epithelium containing numerous goblet cells that secrete mucus to lubricate the passage of fecal material. This mucin production supports the protective and lubricating functions of the upper anal canal mucosa. Immediately proximal to the pectinate line lies the anal transition zone (ATZ), approximately 0.5–1 cm long, featuring transitional epithelium with 4–9 layers and minimal mucin production.^[14]^[15] At the pectinate line, the epithelium transitions sharply to non-keratinized stratified squamous epithelium, forming the anal pecten, which provides a smooth, pale surface adapted for mechanical protection against friction. Further distally, toward the anal verge, this squamous epithelium gradually becomes keratinized, marking the shift to true skin. The anal pecten derives its characteristic whitish appearance from the unkeratinized squamous layers and lack of pigmentation or skin appendages in this zone.^[1]^[2] At the pectinate line, the mucosa features anal sinuses, which are small recesses formed between the anal columns and valves, housing tubular mucous glands that open into them to contribute additional lubrication. Unlike the upper region, where the mucosa adheres directly to the muscularis with minimal intervening connective tissue, the lower anal canal possesses a distinct fibroelastic submucosa that supports vascular structures and allows for mucosal elasticity.^[15]^[5]

Cellular and tissue characteristics

Below the pectinate line, the submucosa consists of dense fibroelastic connective tissue that directly underlies the stratified squamous epithelium of the anoderm, contrasting with the prominent muscularis mucosae layer present in the lamina propria above the line.^[14] This fibroelastic submucosa provides structural support and elasticity to the lower anal canal, facilitating dilation during defecation.^[3] Anal glands, resembling sebaceous glands histologically, are embedded within the submucosa and open into the anal sinuses at the level of the pectinate line.^[14]^[2] These glands secrete a mucoid lubricant to aid passage of feces and are implicated in the pathogenesis of infections, including perianal abscesses and fistulas due to their potential for bacterial harboring.^[3] Cellular components in the region include smooth muscle fibers originating from the internal anal sphincter, which blend into the submucosa around the pectinate line to contribute to continence.^[14] The tissue above the pectinate line lacks hair follicles and sweat glands, reflecting its endodermal origin, while below the line, the anoderm contains limited apocrine glands but no hair follicles or eccrine sweat glands until the transition to perianal skin at the anal verge.^[14]

Clinical significance

Implications for innervation and sensation

The pectinate line serves as a critical demarcation in the innervation of the anal canal, dividing it into regions with distinct neural supplies that profoundly influence sensation. Above the pectinate line, the mucosa receives visceral innervation primarily from the autonomic nervous system via the inferior hypogastric plexus, which integrates sympathetic fibers from the superior and inferior hypogastric nerves (originating from lumbar spinal levels) and parasympathetic fibers from the pelvic splanchnic nerves (S2–S4).^[5] This visceral supply renders the upper anal canal largely insensitive to localized pain and fine touch but responsive to stretch and distension, facilitating functions like defecation without discomfort from minor stimuli.^[5] In contrast, below the pectinate line, the anal canal is innervated by somatic nerves, specifically the inferior rectal branches of the pudendal nerve (S2–S4), which provide both motor supply to the external anal sphincter and sensory fibers to the anoderm and perianal skin.^[16] These somatic afferents convey sharp, well-localized sensations of pain, temperature, pressure, and touch, making the lower anal region highly sensitive to irritation or injury.^[16] This innervation supports voluntary continence and protective reflexes but also accounts for the acute discomfort in lower anal pathologies. At the pectinate line itself, a transitional zone exists with overlapping visceral and somatic contributions, resulting in variable sensory thresholds that can blur the distinction between painless distension and localized pain.^[4] This shift in innervation closely correlates with the epithelial transition from columnar mucosa above to stratified squamous epithelium below, as detailed in the histology section.^[5] Clinically, these innervation differences explain why lesions above the pectinate line, such as internal hemorrhoids, are typically painless due to the absence of somatic pain fibers, whereas those below, like external hemorrhoids or anal fissures, elicit severe pain from somatic stimulation.^[6]

Lymphatic and vascular drainage differences

The pectinate line serves as a critical anatomical watershed in the anal canal, demarcating distinct patterns of lymphatic and vascular drainage between the upper (above) and lower (below) regions, which arise from their differing embryological origins. Above the line, the mucosa is derived from endoderm and shares drainage characteristics with the rectum, while below the line, the ectodermal origin aligns it with perianal skin. These differences have significant clinical implications for conditions such as cancer metastasis and hemorrhoidal disease.^[5]^[3] Lymphatic drainage above the pectinate line follows the rectal pathway, primarily to the inferior mesenteric and internal iliac lymph nodes, facilitating upward spread in proximal malignancies. In contrast, below the line, lymphatics drain to the superficial inguinal nodes, directing potential metastases to groin nodes rather than pelvic ones. This divergence influences staging and treatment in anal cancers, as tumors above the line may involve abdominal nodes, while those below target superficial inguinal chains.^[5]^[3] Arterially, the region above the pectinate line is supplied by the superior rectal artery, a branch of the inferior mesenteric artery, ensuring continuity with hindgut perfusion. Below the line, supply shifts to the inferior rectal arteries, branches of the internal pudendal artery from the internal iliac system, reflecting somatic vascular origins. Venous drainage parallels this: above the line, veins drain via the superior rectal vein into the inferior mesenteric vein and thus the portal system, whereas below, the inferior rectal veins empty into the internal pudendal vein and systemic caval circulation.^[5]^[3] At the pectinate line itself, portosystemic venous anastomoses connect the portal and systemic systems, forming a vulnerable watershed zone. These connections contribute to the risk of hemorrhoid formation, as increased pressure in the portal system (e.g., from liver disease) can lead to variceal dilation and thrombosis in this anastomotic region, distinguishing internal hemorrhoids (above the line) from external ones (below). The line's role as a drainage divide also underscores its importance in understanding metastatic patterns, where lymphatic mismatch can alter disease progression and surgical planning.^[5]^[3]

Role in pathology and procedures

The pectinate line serves as a critical anatomical landmark in the pathogenesis of several anorectal conditions, distinguishing between regions with differing epithelial types, innervation, and vascular supplies that influence symptom presentation and disease behavior. Anal fissures typically originate below the pectinate line in the anoderm, where somatic innervation leads to severe pain during defecation due to the tear's location in a somatically sensitive area.^[17] In contrast, internal hemorrhoids develop above the pectinate line within the insensate visceral mucosa, often presenting as painless rectal bleeding rather than discomfort.^[5] Squamous cell carcinomas of the anal canal, strongly associated with human papillomavirus (HPV) infection—particularly high-risk types like HPV-16—predominantly arise from the squamous epithelium below the pectinate line, where chronic HPV exposure in the transformation zone promotes neoplastic progression.^[18] In diagnostic procedures, the pectinate line guides visualization and intervention during anoscopy and proctoscopy, enabling clinicians to localize lesions accurately for targeted biopsy, especially in evaluating suspicious masses or precancerous changes in the anal canal.^[18] For instance, it helps differentiate internal from external pathology, with rubber band ligation for internal hemorrhoids specifically applied 2-3 cm proximal to the line to avoid the painful somatic zone below, thereby minimizing post-procedure discomfort.^[19] Surgically, the pectinate line informs approaches in fistulotomy and lateral internal sphincterotomy, where incisions below the line must preserve somatic innervation to the external sphincter and avoid incontinence, as the region's dual nerve supply affects functional outcomes.^[20] In colorectal cancer staging, particularly for tumors involving the distal rectum or anal canal, the line's demarcation of lymphatic drainage—portal system above versus inguinal below—influences nodal metastasis patterns and guides treatment decisions, such as whether to include inguinal node irradiation.^[21] Historically, the pectinate line and its associated anal columns were first detailed by anatomist Giovanni Battista Morgagni in the early 18th century.^[22] Today, magnetic resonance imaging (MRI) enhances its visualization for tracking anal fistulas, delineating tracts relative to the line to plan sphincter-sparing interventions.^[23]

References

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The Anal Canal - Structure - Arterial Supply - TeachMeAnatomy
### Summary of the Pectinate Line
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All anal valves together form the pectinate line, a serrated line where the intestinal mucosa merges with the squamous epithelium of the anal canal.
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Abnormal dilatation of these subepithelial vascular cushions above the dentate line produce internal hemorrhoids and below the dentate line external hemorrhoids ...