Embryology

The primitive gut is formed during the third week of gestation. The anorectal region in humans derives from four separate embryological structures: the hindgut, the cloaca, the proctodeum, and the anal tubercles [1]. The hindgut forms the distal third of the transverse colon, the descending colon, the sig-moid, the rectum, and the upper part of the anal canal to the level of the anal valves [2]. The end of the hindgut enters into the cloaca, an endoderm-lined cavity that is in direct contact with the surface ectoderm. The cloaca is initially a single tube that is subsequently separated by caudad migration of the urorectal septum into anterior urogenital and posterior intestinal passages. During the 10th week of development, the external anal sphincter is formed from the posterior cloaca as the descent of the urogenital septum becomes complete. By the 12th week, the internal anal sphincter is formed from a thickened extension of rectal circular muscle [3]. The proctodeal portion of the cloacal membrane disintegrates to form the anal tubercles that join posteriorly and migrate ventrally to encircle a depression, known as the anal dimple or proctodeum. The anal tubercles join the urorectal septum and genital tubercles to form the perineal body, completing the sepa ration between the rectum and the urogenital tract. The upper portion of the anal canal is derived from endoderm and is supplied by the inferior mesenteric artery, which supplies the hindgut. The lower third of the anal canal has ectodermal origins and is supplied by the rectal arteries, which are branches of the internal pudendal artery [2].

Anatomy Pelvic Floor

The pelvic floor is a dome-shaped muscular sheet [4] that predominantly contains striated muscle and has midline defects enclosing the bladder, the uterus, and the rectum. These defects are closed by connective tissue anterior to the urethra, anterior to the rectum (i.e., the perineal body), and posterior to the rectum (i.e., the postanal plate). Together with the viscera (i.e., the bladder and anorectum), the pelvic floor is responsible for storing and evacuating urine and stool.

The levator ani and the coccygeus muscle comprise the two muscular components of the pelvic floor or pelvic diaphragm. The muscles that constitute the levator ani complex are the puborectalis, the pubococcygeus, and the ileococcygeus. These muscles originate at different levels of the pubic bone, the arcus tendineus fascia pelvis (condensation of the obturator internus muscle fascia), and the ischial spine. These muscles are inserted at the level of the rectum, the anococcygeal raphe (levator plate), and the coccyx (Fig. 1).

It is unclear whether the puborectalis should be regarded as a component of the levator ani complex or the external anal sphincter. Based on developmental evidence, innervation, and histological studies, the puborectalis appears distinct from the majority of the levator ani [1]. On the other hand, the pub-orectalis and external sphincter complex are innervated by separate nerves originating from S2 4 (see below), suggesting phylogenetic differences between these two muscles [5].

Fig. 1. Pelvic view of the levator ani demonstrating its four main components: puborectalis, pubococcygeus, iliococ-cygeus, and coccygeus. Reprinted with permission from [6]

Rectum and Anal Canal

The rectum is 15- to 20-cm long and extends from the recto sigmoid junction at the level of third sacral vertebra to the anal orifice (Fig. 2). The upper and lower rectums are separated by a horizontal fold. The upper rectum is derived from the embryological hind gut, generally contains feces, and can distend toward the peritoneal cavity [7]. The lower part, derived from the cloaca, is surrounded by condensed extra peritoneal connective tissue and is generally empty

Longitudinal Circular muscle muscle space layer

Longitudinal Circular muscle muscle space layer llio-

Levator J cocygeus ani

Pubo-I cocygeus ischiorectal fossa

Gluteus

Anal columns

Anal sinuses

Parts of sphincter ani extemus

Sphincter ani internus llio-

Levator J cocygeus ani

Pubo-I cocygeus ischiorectal fossa

Anal columns

Gluteus

Anal sinuses

Parts of sphincter ani extemus

Sphincter ani internus

Fig. 2. Diagram of a coronal section of the rectum, anal canal, and adjacent structures. The pelvic barrier includes the anal sphincters and the pelvic floor muscles. Reprinted with permission from [8]

in normal subjects, except during defecation. In humans, there are fewer enteric ganglia in the rectum compared with the colon and very few ganglia in the anal sphincter [9,10].

The anal canal is an anteroposterior slit, with its lateral walls in close contact. The literature describes a longer (approximately 4.0-4.5 cm) "surgical" or "clinical" anal canal and a shorter (approximately 2.0 cm) "anatomical" or "embryological" anal canal. The anal valves and the distal end of the ampullary part of the rectum mark the proximal margin of the "short" and "long" anal canal, respectively. The proximal 10 mm of the anal canal is lined by columnar, rectal-type mucosa. The next 15 mm (which includes the valves) is lined by stratified, or a modified columnar, epithelium. Distal to that is about 10 mm of thick, non hairy, stratified epithelium (i.e., the pecten). The most distal 5-10 mm is lined by hairy skin.

The anal canal is surrounded by the internal and external anal sphincters. The internal sphincter is a thickened extension of the circular smooth muscle layer surrounding the colon that contains discrete muscle bundles separated by large septa [11]. In the rectum, the interstitial cells of Cajal (ICC) are organized in dense networks along the submucosal and myenteric borders. In the internal anal sphincter, the ICCs are located along the periphery of the muscle bundles within the circular layer.

The external sphincter is composed of superficial, subcutaneous, and deep portions; the deep portion blends with the puborectalis [7]. In men, this trilaminar pattern is preserved around the sphincter circumference. In contrast, the anterior portion of the external sphincter in women is a single muscle bundle. External sphincter fibers are circumferentially oriented, very small, and separated by profuse connective tissue [12].

Constipation Prescription

Constipation Prescription

Did you ever think feeling angry and irritable could be a symptom of constipation? A horrible fullness and pressing sharp pains against the bladders can’t help but affect your mood. Sometimes you just want everyone to leave you alone and sleep to escape the pain. It is virtually impossible to be constipated and keep a sunny disposition. Follow the steps in this guide to alleviate constipation and lead a happier healthy life.

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