Anococcygeal raphe revisited: a histological study using mid-term human fetuses and elderly cadavers

Abstract

Purpose: We recently demonstrated the morphology of the anococcygeal ligament. As the anococcygeal ligament and raphe are often confused, the concept of the anococcygeal raphe needs to be re-examined from the perspective of fetal development, as well as in terms of adult morphology.

Materials and methods: We examined the horizontal sections of 15 fetuses as well as adult histology. From cadavers, we obtained an almost cubic tissue mass containing the dorsal wall of the anorectum, the coccyx and the covering skin. Most sections were stained with hematoxylin and eosin or Masson-trichrome solution.

Results: The adult ligament contained both smooth and striated muscle fibers. A similar band-like structure was seen in fetuses, containing: 1) smooth muscle fibers originating from the longitudinal muscle coat of the anal canal and 2) striated muscle fibers from the external anal sphincter (EAS). However, in fetuses, the levator ani muscle did not attach to either the band or the coccyx. Along and around the anococcygeal ligament, we did not find any aponeurotic tissue with transversely oriented fibers connecting bilateral levator ani slings. Instead, in adults, a fibrous tissue mass was located at a gap between bilateral levator ani slings; this site corresponded to the dorsal side of the ligament and the EAS in the immediately deep side of the natal skin cleft.

Conclusion: We hypothesize that a classically described raphe corresponds to the specific subcutaneous tissue on the superficial or dorsal side of the anococcygeal ligament.

Fig. 1

Sagittal sections of the miadsagittal area including the coccyx (CO) and anococcygeal ligament. Masson trichrome staining. (A) 96 years old male, displays a thick external anal sphincter (EAS) in the 30 mm inferior side of the CO. (B) 84 years old female, exhibits a thin EAS 40 mm below the CO. The anococcygeal ligament (ACL) merges with the EAS as well as the longitudinal anal muscle (arrows) between the EAS and internal anal sphincter (IAS). Inserts in panels A and B are higher magnification views of a square in panel A or B, respectively. Insert in panel A shows smooth muscles in the ACL, while an inset in panel B striated muscles distributing in the inferior end of the ACL. Note a fibrous tissue (stars) in the dorsal side of the ligament. Asterisk in panel A (or in the insert in panel B) indicates the dorsosuperior reflection of the well developed EAS (or an artificial damage during the histological procedure).

Fig. 2

Transverse sections of the anal canal dorsal wall. (A) 86 years old female, 10 mm inferior side of the coccyx (CO), displays a small fibrous tissue mass (arrows) at a gap between the bilateral levator slings (LA). This gap was the smallest in specimens examined (3 mm). (B) 92 years old male, inferior end of the CO and the most superior part of the anococcygeal ligament (ACL). (C) Same specimen as panel B. Panel B corresponds to the, whereas panel C contains the inferior end of the ligament at a level 20 mm inferior side of the CO. In panel C, note a fibrous tissue mass (arrows) in the dorsal side of the external anal sphincter (EAS) and the ligament. (D) A higher magnification view of a square in panel B. (E) A higher magnification view of a square in panel C. Arrows in panels D and E show smooth muscles (panel D) or striated muscles (panel E) in the anococcygeal ligament, respectively. GM, gluteus maximus muscle.

Fig. 3

Transverse sections of the anal canal and natal skin cleft (85 years old male). (A) A level of the inferior end of the coccyx (CO) contains the most superior part of the anococcygeal ligament (ACL). (B) A level 40 mm inferior side of the CO. A fibrous tissue extends between the natal cleft (skin) and the external anal sphincter (EAS). (C) A higher magnification view of a square in panel A. Arrows show smooth muscles in the ligament. (D) A higher magnification view of a square in panel B. Arrows show striated muscles in the subcutaneous fibrous tissue. LA, levator ani muscle; GM, gluteus maximus muscle.

Fig. 4

Horizontal sections of the anal canal in a 12-week fetus. (A) Immunohistochemistry for smooth muscle actin (SMA). 0.2 mm superior side of panel C. A dorsal midsagittal structure (arrows) is positive for SMA and extending from the longitudinal smooth muscle coat of the anal canal toward the coccyx (CO). (B) Immunohistochemistry for SMA. The dorsal extension (arrows) of the external anal sphincter (EAS). (C) Immunohistochemistry for desmin (a striated muscle marker) of a section near panel B. All panels are prepared at the same magnification (scale bar in panel A). COM, coccygeus muscle; IC, ishchiocavernosus muscle; OI, obturator internus muscle. PF, pelvic fascia.

Fig. 5

Tilted horizontal sections of the anal canal dorsal wall in a 20-week fetus. HE staining. (A) The most superior side of the figure includes the inferior end of the coccyx (CO). Lower magnification than other panels while panels B, C and D are at the same magnification (B) 0.2 mm inferior to the panel A. (C) 0.2 mm inferior to the panel B. (D) The most inferior side of the figure and 0.2 mm inferior to the panel C. The levator ani muscle is located out side of the figure (much more superior and lateral). A connective tissue mass (asterisk) is present in the dorsal side of the CO. In the inferior side of the CO (panels C and D), another connective tissue mass (star) is seen connecting to the CO. Panels C and D include the back skin in the upper side of each panel. Fatty tissues (fat) start development in the ventral side of the gluteus maximus muscle (GM). In this specimen, there is no specific structure connecting between the external anal sphincter (EAS) and the inferior elongation of the coccyx (star).

Fig. 6

Anococcygeal raphe: a classical concept and our identification according to the present histology. (A) A classical view of the anococcygeal raphe (raphe) in dorsal views of the female perineum. (B) Our identification according to the present study. Muscles are not drawn in a dorsal part of the ischorectal fossa (asterisk). In the diagram, the ventral part of the female perineum is drawn according to our recent studies., ACL, anococcygeal ligament; ATLA, arcus tendineus for the levator ani; BS, bulbospongiosus muscle; CO, coccyx; COM, coccygeus muscle; EAS, external anal sphincter; IC, ishchiocavernosus muscle; LA, levator ani; OI, obturator internus muscle; PB, perneal body; PM, perineal membrane; SSL, sacrospinous ligament; STL, sacrotuberous ligament.